A male medium ground finch (left) next to a female small ground finch (right). It's unclear what the nature of their relationship is—but if the male is small for his kind, love could blossom. Photo Credit: Nick Athanas, flickr

When Looking for Love, Darwin’s Finches Choose Mates Just Like Their Parents

“Wow, you look just like my father” can be a surefire way to bring a date to a screeching halt. Unless, that is, you have hollow bones and live on a remote Pacific island.Today, in the journal PNAS, researchers <a href="http://www.pnas.org/cgi/doi/10.1073/pnas.1813662115">report</a> that the famous finches once studied by Darwin on the Galápagos Islands tend to choose mates that bear an uncanny resemblance to their parents. This strategy helps ensure that birds boost their chances of bunking up with partners of their own species—and may even help solve an age-old evolutionary puzzle.On the small sliver of Earth that Darwin’s finches call home, <a href="https://www.princeton.edu/news/2017/11/27/study-darwins-finches-reveals-new-species-can-develop-little-two-generations">18</a> <a href="http://science.sciencemag.org/content/early/2017/11/20/science.aao4593">species</a> of these birds speckle the skies. Each lineage has its own little quirks. For instance, the shapes and sizes of different finch beaks have been whittled around specific foods: Some more suited to crack open little seeds, while others are particularly adept at pecking at cacti.
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But it was only in the last million years or so that these diverse lineages truly start to crystallize. Even today, their distinguishing features are few and far between—which means breeding season can be a chaotic cross between speed dating and a family reunion. For years, biologists have wondered how so many finch lineages diverged in such close proximity, in such a short amount of time. Without geographical barriers to keep species separate, how did birds avoid interbreeding?Luckily for finches, they’re born with the most reliable role models nature can provide: their parents. By simply paying attention to the subtle features and habits of their primary caretakers, little finches can begin to acquire a sense of their species identity—and which subtle features distinguish one lineage from another. This time-sensitive process of learning is called imprinting.Some imprinted traits are vocal. When a male finch is single and ready to mingle, he can’t be a slouch with song: His winsome, species-specific warble is a big part of what will ultimately woo a female. But young birds aren’t born belting arias. Each male must earn his vocal heritage by studying dutifully under his father’s tutelage during his youth. Because these serenades are sung or recognized on a lineage-by-lineage basis, they help maintain finch fidelity—but imprinting is far from foolproof. If impressionable young males are exposed to the vocal stylings of another species during this period, they may spend the rest of their lives chasing the wrong tail.
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<figcaption>A female cactus finch. When it comes time for her to select a mate, she may trend towards choosing with with a beak shaped like her father's. Photo Credit: Sergey Pisarevskiy, flickr
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Clearly, deciding on a date through one trait alone can be quite the gamble. If a finch is to carry on with its own kind, there’s a good chance it’s imprinting on other cues to tell who’s who in the avian community, reasoned evolutionary biologists <a href="https://eeb.princeton.edu/people/peter-grant">Peter</a> and <a href="https://eeb.princeton.edu/people/b-rosemary-grant">Rosemary</a> Grant of Princeton University. And if the Grants have a theory about Darwin’s finches, it’s probably right.The pair has studied the Galápagos Islands’ winged residents <a href="https://www.pbs.org/wgbh/evolution/library/01/6/l_016_01.html">since 1973</a>, observing evolution in real time as lineages of finches ebbed and flowed through the years. From 1976 to 2012, the Grants tracked thousands of finches on the tiny volcanic outcrop of Daphne Major, meticulously recording their beak and body dimensions and noting each bird’s parentage and mating habits (according to their report, the duo attempted to find every nest and identify the parents in every year of breeding from 1976 to 1998).
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“There is so much power in long-term studies like these,” says <a href="http://www.massey.ac.nz/massey/expertise/profile.cfm?stref=136040">James Dale</a>, an evolutionary biologist who studies bird behavior at Massey University in New Zealand. The Grants, he adds, have “accumulated a vast database that allows them to test so many new theories.”The breadth of the Grants’ body of work is truly staggering. By 2012, the pair had amassed nearly 40 years of anatomical and behavioral data on two species of finch: the <a href="https://neotropical.birds.cornell.edu/Species-Account/nb/species/megfin1/overview">medium ground finch</a> (Geospiza fortis), a puff of a bird with a stubby beak suited for snacking on seeds; and the <a href="https://neotropical.birds.cornell.edu/Species-Account/nb/species/cocfin3">cactus finch</a> (Geospiza scandens), which sports a svelte, spiky beak apt for chowing down on—you guessed it—prickly pear cactus.The Grants quickly discovered each species of finch had a strong tendency to stick to its own kind—and, as they expected, the cues for their choices weren’t limited to song. For instance, in the medium ground finch, body size matched up consistently between mates and parents—especially the father. In the cactus finch, on the other hand, females also went for daddy doppelgängers, but instead prioritized the shape of their fathers’ beaks. Regardless of anatomy, these preferences meant that when most flirty finches were hot to trot, they were recalling the faces and forms they saw most often in their youth.“It was very rewarding to discover that [features] of the mates of both sons and daughters can be predicted from the same characteristics of the parents,” the Grants wrote in an email.Incredibly, because the differences between individual birds are so minor, many finches were selecting mates among anatomical variants of “just millimeters,” says <a href="http://www.utsc.utoronto.ca/~jweir/">Jason Weir</a>, a biologist who studies the evolution of birds at the University of Toronto, but was not involved in the work. Such a subtle pattern, he says, was only detectable through the sheer numbers of birds the Grants tagged and tracked over the years. “They basically know every individual and have every pedigree,” Weir explains.
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<figcaption>A male cactus finch. His pointed beak may appeal to a female looking to couple up—if it resembles her father's, that is. Credit: David Cook, flickr
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Every population, however, is graced by a few renegade romantics. Some medium ground finches ultimately strayed outside their own lineage, blurring the lines between species. But even mismatched lovebirds found common ground. Rather than sticking to their own, some medium ground finches sought the company of <a href="https://neotropical.birds.cornell.edu/Species-Account/nb/species/smgfin1">small ground finches</a> (Geospiza fuliginosa), which are closely related, but, as their name indicates, a bit more diminutive in size. When the Grants examined these oddballs, they realized the medium ground finches most likely to seek mini-mates were often pint-sized themselves.Some large finches took on a similar habit. Over the course of his 14-year life, one lothario from the <a href="https://neotropical.birds.cornell.edu/Species-Account/nb/species/larcaf2/overview">large cactus finch</a> (Geospiza conirostris) population swept no less than six female medium ground finches off their feet—several of whom were particularly big-bodied for their breed. Even in extra-species couplings, like mated with like.Imprinting on parents to find a mate “is a learning process that helps finches to navigate the adult world,” the Grants explain.
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And the findings of this study speak to much more than individual finches’ flights of fancy. By picking partners that parallel their parents, finches increase the likelihood that their mates—and future offspring—will be of their same ilk. Reusing the same imprinting template over and over means that the differences between lineages can get enhanced until they split away entirely. With this mirroring strategy, finches can breed diversity under even the most unexpected of circumstances.Turns out, birds of a feather really do flock together—especially in the Galapagos.

Bringing home fellas that look just like dad might be a red flag to some. But for Galápagos finches, a pairing like this might just do papa proud.

This unfortunate bull came afoul of a few stems of jumping cholla (Cylindropuntia fulgida). The spines of this cactus are decorated with backward-facing barbs that facilitate entry into flesh, but make extraction excruciating. Photo Credit: Lon&Queta, flickr

A Single Spine From This Cactus Can Lift a Half-Pound Slab of Pork

In the arid deserts of the Southwestern United States, there exists a humble-looking cactus by the name of <a href="https://plants.usda.gov/core/profile?symbol=cyfu10">Cylindropuntia fulgida</a>. With its stout stature and knobby tentacles, C. fulgida is a sight to behold—but best admired from afar.At the slightest provocation, the spiny stems of this cactus will detach and lodge themselves in whatever’s nearby, be it fur, clothes, or flesh—earning it the nickname “jumping cholla.” It’s actually a bit of an exaggeration: This plant is not going to physically leap out and attack you. But its trigger-happy strategy is not one to toy with unless you’re eager for a world of hurt.The prickliest part of the problem are the tiny, backward-facing barbs that decorate the tips of jumping cholla spines. These barbs not only enable the spines to impale flesh, but also make extraction excruciating. According to <a href="http://dx.doi.org/10.1098/rspb.2018.2280">a new study</a>, once anchored, the barbs cling so tightly that a single inch-long spine can lift a half-pound slab of pork.
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“This work really brought home for us how tenacious these spines are,” says study author <a href="https://croftss.wordpress.com/">Stephanie Crofts</a>, a biomechanist at the University of Illinois at Urbana-Champaign. “It blew us away.”Thousands of species of cacti exist, spanning all sorts of shapes, sizes, and statures; even their spines (which are actually <a href="https://www.britannica.com/plant/cactus">a type of leaf</a>) are no stranger to diversity. Some spines serve as a blatant deterrent to hungry herbivores, but others, like those from cacti in the Opuntioideae group, which include both the prickly pear and jumping cholla, are thought to play a different role. They help the plant latch onto hapless passersby, giving them a shot at dispersing into new territory like the sharpest of stowaways. To up their staying power, these spines bear ornaments unlike anything you’d ever want to adorn your Christmas tree: microscopic barbs that enable the needly structures to stick and stay stuck. But until now, no one has quantified the difference these embellishments really make.That’s where Crofts and her advisor, <a href="https://www.philipslanderson.com/">Philip Anderson</a>, an evolutionary biomechanist at the University of Illinois at Urbana-Champaign, come in. Crofts’ work focuses on the power of <a href="http://jeb.biologists.org/content/221/22/jeb187294">puncture</a>—work that entails a lot of jabbing, piercing, and perforating. With their naturally prickly arsenal of weaponry, cacti stood out to Crofts as a natural subject for her research.
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<figcaption>Despite its name, the jumping cholla won’t actually leap out and attack you. But brush up against one, a segment of this cactus will happily bury itself in your clothes, hair, or flesh. Good luck getting it out. Photo Credit: L. Brian Stauffer, University of Illinois at Urbana-Champaign
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In Crofts’ and Anderson’s new work, published today in the journal Proceedings of the Royal Society B, they collected five spines each from six species of cacti. Three of them—Pereskia grandifolia, or rose cactus; Echinopsis terscheckii, or Argentine saguaro; and Echinocactus grusonii, or golden barrel cactus—bore unbarbed spines, while the others—Opuntia fragilis, or brittle prickly pear; Opuntia polyacantha, or plains prickly pear; and, of course, Cylindropuntia fulgida, or jumping cholla—had barbed spines.Cactus cuttings encased in newspaper clippings, were shipped to the University of Illinois by the Huntington Botanical Gardens in California. Crofts bravely unwrapped the contents by hand and clipped the spines from each specimen. “It was an exciting few days when we had loose cacti rolling around,” Crofts recalls. “I definitely ended up having to use the tweezers on my Swiss Army knife.” (Crofts graciously thanks the makers of the multitool in the acknowledgments of her publication.)To test the puncturing prowess of each set of spines, Crofts next selected her victims: skinless chicken breasts; raw, skin-on pork shoulders; and a silicon-based polymer. Chicken breast is soft, relatively unprotected muscle—something that might easily mimic the inside of a hungry animal’s mouth, Crofts explains. Pig tissue, on the other hand, would have a similar texture and toughness of human flesh (if anything, pig’s a bit thicker). Lastly, the polymer has an amorphous, elastic texture, and is devoid of the collagen and muscle fibers present in animal flesh. 
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When Crofts got to stabbing, she found that barbed spines were easier to embed into both animal tissue and the polymer. It seems a bit counterintuitive at first, Crofts explains, but the barbs are actually a huge asset to the spines’ otherwise needly structure: Each concentrates pressure onto a small area, reducing the amount of force necessary to grind through its unfortunate target—much like slicing with a serrated knife.Extricating the spines, however, was quite a different story. After barbs deploy within tissue, withdrawal goes against the grain, making the spines a nightmare to extract. And the more fibrous the tissue, the more there is for the barbs to snag on. Because of this, removal of barbed spines from the jello-y polymer made for smooth sailing, but the chicken and pig weren’t quite so lucky.The pork shoulder in particular proved to be astoundingly stubborn. In one trial, Crofts found that a lone, inch-long spine snipped from a jumping cholla was enough to hoist a half-pound cut of pig into the air. Only a couple millimeters of the spine had penetrated the tissue—just enough to clear the layer of skin—but there the pork dangled, as if completely weightless.
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<figcaption>A one-inch spine from a jumping cholla was able to hoist a half-pound slab of pork shoulder into the air by its skin. The researchers had to tug the meat off the spine to free it. Photo Credit: L. Brian Stauffer, University of Illinois at Urbana-Champaign
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“It was this big old chunk of meat just hanging there, a suspended chunk of pig,” Crofts says. “We physically had to pull the pig off the spine.”And after jumping cholla spines like these were wrested from their porcine prisons, Crofts discovered the ordeal had stripped a handful of barbs from their tips. They didn’t penetrate so well after that.Of course, cacti are far from the only plants prone to puncture—or even the only organisms. Throughout their work, the researchers were struck by the resemblance of barbed cactus spines to an unexpected kindred spirit: <a href="http://www.pnas.org/content/109/52/21289">North American porcupine quills</a>. Six years ago, a <a href="http://www.pnas.org/content/109/52/21289">study</a> led by biomedical engineer <a href="http://www.karplab.net/">Jeffrey Karp</a> of Harvard Medical School conducted a similar series of experiments on these quills; when Crofts compared the two sets of results side by side, she was astounded by what she refers to as a “weird cross-kingdom convergence.” Though porcupine quills are much bigger and primarily serve as a defense mechanism, both structures penetrate flesh with ease and stay firmly put—a dangerous double whammy of entry and retention.
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“This is really exciting,” says <a href="http://www.marthamunoz.com/">Martha Muñoz</a>, an evolutionary biologist at Virginia Polytechnic Institute and State University who didn’t participate in the new finding. “Here, we see similarities in the puncture properties in organisms as different as cacti and porcupines. It’s hard to get more distant than that.”However, Crofts and Anderson’s work didn’t directly compare the two structures—something that would probably illuminate the similarities and differences between them more clearly, points out <a href="https://juliemessier.org/">Julie Messier</a>, a plant ecologist at the University of Waterloo who was not involved in the new research. Still, Karp says it’s “wonderful” that this new work has hinted that the benefits of barbs transcend the branches of the tree of life.In fact, shortly after Karp’s group published their initial findings on porcupines, he and his son Joshua also took note of the spiky resemblance between porcupine and cactus—so much so that the younger Karp, then 10 years old, pitted the two head-to-head in his next science fair project, entitled, “The Secret of the Barb: Porcupine Quills vs. Cactus Needles.” In one experiment, Joshua drove quills and spines into raw chicken to see how much flesh the barbs retained upon withdrawal; in another, he pitted the structures tip-to-tip and made them duel to the death.
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<figcaption>North American porcupine quills (top) and jumping cholla cactus spines (bottom) are covered in overlapping, backwards-facing barbs that serve two purposes: aiding penetration into flesh and making extraction extremely difficult. While porcupine quills are much bigger and serve primarily as a mode of defense, there are surprising similarities in structure between the two. White bars represent 100 micrometers in each image. Photo Credit: Jeffrey Karp, Harvard Medical School (top); Stephanie Crofts, University of Illinois at Urbana-Champaign (bottom)
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The results were mixed: The porcupine quill took a bigger chunk out of the chicken—but a cactus spine conclusively skewered a porcupine quill. For now, the jury’s still out on the victor of this jabby joust.Still, one thing’s for sure, according to Crofts: “I do not ever want to brush up against a jumping cholla,” she says. “They do what they do very well. But I don’t want to be what they’re puncturing.”

The spines of the jumping cholla cactus slide in with ease, but they’re grueling to extract. You can thank their barbs.

A new study shows that, in contrast to a longstanding rule in human biology, mitochondrial DNA can be inherited from fathers as well as mothers. Photo Credit: seal1837, Pixabay

Not your mom’s genes: Mitochondrial DNA can come from Dad

The first time <a href="https://www.cincinnatichildrens.org/bio/h/taosheng-huang">Taosheng Huang</a> saw the test results, he was sure there’d been a mistake. Even after a technician repeated the diagnostic, Huang didn’t believe it. “That’s impossible,” he said.Huang, a pediatrician and geneticist at <a href="https://www.cincinnatichildrens.org/">Cincinnati Children’s Hospital Medical Center</a>, asked the patient to come back and provide fresh samples of blood, which Huang then split among several research labs to reduce any chance of error. It was a finding that Huang knew would break a central tenet of human genetics—but time and time again, the outcome was the same.Huang’s patient, a four-year-old boy, was carrying two different sets of <a href="https://www.pbs.org/wgbh/nova/article/hybrid-dolphin-gives-scientists-rare-window-into-evolution/">mitochondrial DNA</a>: one from his mother, as expected—and another, from his father.This was only the beginning. Using modern DNA sequencing technology, Huang and his colleagues have conclusively verified paternally-inherited mitochondrial DNA in 17 individuals spanning three unrelated families. Their <a href="http://www.pnas.org/cgi/doi/10.1073/pnas.1810946115">work</a> appears today in the journal PNAS.“This is a really groundbreaking discovery,” says <a href="http://med.stanford.edu/xinnanwanglab.html">Xinnan Wang</a>, a biologist at Stanford University who studies mitochondria but was not involved in the new findings. “It could open up an entirely new field... and change how we look for the cause of [certain] diseases.”
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Broadly speaking, we’re each a genetic mix of Mom and Dad. In the nucleus, which contains our chromosomes, this holds true. But the nucleus isn’t the only part of the cell that contains DNA. Cells contain power centers called mitochondria that also carry their own sets of DNA—and in nearly all known animals, mitochondrial DNA is <a href="https://www.sciencemag.org/news/2004/05/mitochondrial-dna-mixes-it">inherited exclusively</a> from the mother. This lopsided acquisition is so ingrained that researchers often analyze mitochondrial DNA to trace maternal lineages back in time.Scientists still aren’t completely sure why this process has evolved to be strictly matrilineal, but a few theories have come to the forefront. For one, the mitochondria of sperm are believed to experience <a href="http://www.nrcresearchpress.com/doi/10.1139/gen-2015-0090?url_ver=Z39.88-2003&amp;rfr_id=ori:rid:crossref.org&amp;rfr_dat=cr_pub%3dwww.ncbi.nlm.nih.gov#.W_iwCZNKiLJ">higher rates of mutation</a> than those in eggs, making their input somewhat riskier. Additionally, having only one type of mitochondria makes it easier for the genomes in the nucleus and mitochondria to <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6037425/">coordinate</a>, as both generate the raw materials necessary for proper cellular function, explains <a href="https://bio.umontreal.ca/repertoire-departement/vue/breton-sophie-1/">Sophie Breton</a>, a mitochondrial geneticist at the University of Montreal who did not participate in the study. The addition of another mitochondrial stakeholder, so the theory goes, could muddle this intimate two-way dialogue.
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<figcaption>Cow cells with nuclei stained in blue and mitochondria stained in yellow. Photo Credit: Torsten Wittmann, University of California, San Francisco
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Whatever is driving maternal mitochondrial inheritance, it’s clear our cells have taken quite a few precautions to keep it that way. A formidable bevy of molecular machines stands at the ready to ensure that no unwanted paternal contribution slips through in the process of fertilization. Even though a sperm’s mitochondria help power its odyssey to the egg, its mitochondria aren’t meant to last. Some mitochondria are thought to be lost as sperm develop; the few that make it through the fertilization process are tagged with a chemical marker that allows the egg to <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5282644/">easily recognize and annihilate</a> any rogue paternal interlopers.<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5282644/">A handful of animals</a>, including flies and mice, are known to occasionally <a href="http://www.nrcresearchpress.com/doi/10.1139/gen-2015-0090?url_ver=Z39.88-2003&amp;rfr_id=ori:rid:crossref.org&amp;rfr_dat=cr_pub%3dwww.ncbi.nlm.nih.gov#.W_iwCZNKiLJ">“leak” paternal mitochondrial DNA</a> into offspring alongside maternal input, but humans have historically been excluded from this club. Only a few reports of dad-derived mitochondrial DNA exist, and most have been chalked up to contamination or laboratory sample mix-ups. <a href="https://www.nejm.org/doi/10.1056/NEJMoa020350?url_ver=Z39.88-2003&amp;rfr_id=ori%3Arid%3Acrossref.org&amp;rfr_dat=cr_pub%3Dwww.ncbi.nlm.nih.gov">One case</a>, described in 2002, received considerable attention when it was independently confirmed by another research group. But in the 16 years since, the search for another instance of this genetic anomaly has been unfruitful.
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So even after Huang was convinced that his remarkable four-year-old patient had inherited mitochondrial DNA from both parents, he knew the work was far from done. When the researchers next traced this odd genetic signature back through the boy’s family tree, they found that, across three generations, 10 individuals in the patient’s family appeared to harbor mixed mitochondrial DNA. Now that the researchers knew where to look—and what to look for—as they expanded their search. Not long after, seven additional individuals from two other unrelated families were confirmed to harbor the same condition.Amazingly, all three families seemed to show similar patterns of inheritance of this unusual trait. Not all members were afflicted, meaning that some individuals passed on their genes in the typical fashion, conceiving children that inherited only their mothers’ mitochondrial DNA. But it was clear that the mitochondrial genomes of several men in these families were breaking the biological rules of fatherhood. Rather than being discarded, these paternal packages of genetic information somehow kept pace with their maternal counterparts during fertilization, leaving some of their kids with mixed mitochondria.Daughters with these mixed genomes then went on bequeath their heterogeneous heirlooms onto their own children in carbon copy. This meant their kids inherited mixtures indirectly—even though the children possessed none of their own father’s mitochondrial DNA. Such was the case with Huang’s original four-year-old patient.
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<figcaption>Sheep are one of the few animals in which mitochondrial DNA can occasionally be inherited from both the mother and the father. Photo Credit: herbert2512, Pixabay
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Sons who acquired mixed mitochondrial genomes presented a slightly more complex picture. The ability to pass on paternal DNA seemed to be a dominant trait, or one that requires a gene from only one parent to show itself in offspring. This meant that some of the men who had inherited hybrid mitochondrial genomes were able to transfer them into their children, who displayed even more diversity as they mixed Dad’s genes with Mom’s. But not all men with mixtures retained this ability, leading to occasional dead-ends.Exactly how paternal mitochondrial DNA infiltrates the embryo still isn’t entirely clear. It’s definitely a genetic trait—but not one, ironically, found in the mitochondrial genome itself. In fact, apart from the unusual mode of inheritance, there don’t seem to be any defects present in any of these patients’ mitochondria. Instead, whatever allows dads to deposit their mitochondrial DNA in their offspring is likely a mutation encoded in the nucleus—where things are supposed to come from both mom and dad.<a href="https://icahn.mssm.edu/profiles/florence-marlow">Florence Marlow</a>, a developmental biologist at the Icahn School of Medicine at Mount Sinai who was not involved in the research, theorizes that the anomaly could be at the step where the mitochondria of sperm are normally tagged for destruction. If this labeling step never occurs, she explains, the fertilized egg has no way of identifying and eliminating these paternal interlopers—meaning Dad’s mitochondria get a clear pass to proceed alongside Mom’s.
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Since putting together their initial findings, Huang and his team have already identified several more candidate families with mixed mitochondrial DNA. According to his preliminary estimates, this phenomenon may be present in as many as 1 in every 5,000 people. While Wang stresses that more research by other groups is needed to confirm these numbers, the work from Huang’s team indicates that paternal transmission of mitochondria may be far more widespread than researchers once thought.So how has this been overlooked for so long? “Most people have taken for granted that mitochondrial inheritance is strictly maternal,” Breton explains. “But sequencing techniques are so much more powerful than they were even just a few years ago.”
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<figcaption>Cells taken from the connective tissue of a mouse. The nuclei of the cell are stained in blue and the mitochondria are stained in green. Photo Credit: D. Burnette, J. Lippincott-Schwartz/NICHD
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The possibility of paternal inheritance could also expand our options for assisted reproductive technology in the distant future. Huang’s group previously played a part in the successful conception and delivery of a <a href="https://www.rbmojournal.com/article/S1472-6483(17)30041-X/fulltext">“three-parent” baby</a> in 2016. The child’s mother suffered from a mitochondrial disease, so Huang and his colleagues transferred her nuclear DNA into a donor egg, which had been stripped of its own chromosomes but could supply healthy mitochondria. This hybrid egg, containing the genetic information from two mothers, was then fertilized with the father’s sperm.In theory, Breton says, if paternal mitochondria are viable, they could be added to—or perhaps even replace—defective maternal mitochondria, obviating the need for a third parent.But that’s a big if, especially considering that the mitochondrial DNA of sperm is more prone to mutation. We’re far from understanding the full repercussions of inheriting bi-parental mitochondria under natural circumstances, much less through artificial medical procedures, many of which are likely to fuel <a href="https://www.npr.org/sections/health-shots/2018/06/06/615909572/inside-the-ukrainian-clinic-making-3-parent-babies-for-women-who-are-infertile">continued debates on ethics</a>.For now, maternal inheritance of mitochondrial DNA is still the norm. But simply expanding our view of genetic inheritance opens up countless doors. “These results will change the way we describe mitochondrial inheritance,” Breton says. “I’m 100 percent sure we’ll find more cases like these in the future.”

A new study provides compelling evidence that children can inherit mitochondrial DNA from both their parents.

Humpbacks and many other whales use baleen—the overlapping structures in their mouths where teeth might otherwise be—to filter tiny prey like krill and plankton out of water. But because baleen has the texture of hair or fingernails, it doesn't preserve well in the fossil record. New research now indicates that some whales may have fed by suction in the evolutionary interim between teeth and baleen. Photo Credit: Ari S. Friedlaender/University of California, Santa Cruz under NOAA permit

This 33-Million-Year-Old Whale Sucked Up Food Like A Giant Vacuum

It takes a lot to feed a <a href="http://www.pbs.org/big-blue-live/animals/blue-whale/">blue whale</a>. One of these super-sized swimmers can consume as much as 8,000 pounds of food—about the weight of a Hummer H2—on a daily basis. Which means blue whales need to be exquisitely equipped for some serious eating.The secret to this behemoth’s success? Baleen: a hyper-efficient filter-feeding system found exclusively in whales, composed of a comb-like series of overlapping plates with the texture of fingernails. After taking a massive gulp of ocean water, <a href="http://www.pbs.org/wnet/nature/ocean-giants-whale-fact-sheet/7567/">baleen whales</a>—including blue whales and <a href="https://www.pbs.org/video/life-reef-humpback-whales/">humpbacks</a>—expel unwanted liquid through this sieve, leaving behind a nutrient-dense mouthful of tiny, freshly-strained prey like krill and plankton that are easily swallowed whole.But scientists continue to grapple with the evolutionary origins of nature’s biggest oral colander. Baleen whales’ ancestors had teeth—and many of the details behind the transition to baleen, which preserves poorly in the fossil record, remain mysterious.Now, with the re-examination of a 33-million-year-old fossil, researchers are filling in a piece of this long-standing puzzle. According to <a href="https://www.cell.com/current-biology/fulltext/S0960-9822(18)31414-3">a new study</a> published today in the journal Current Biology, some whales jettisoned their teeth entirely before the acquisition of baleen, and in the interim, adopted a third method of feeding: suctioning up their food like giant vacuum cleaners.
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“If this is right, this fossil fills in a missing spot on the [whale family] tree,” says <a href="https://www.uprovidence.edu/bio/jessica-martin/">Jessica Martin</a>, a paleontologist at the University of Providence who was not involved in the research. “This might change the way we look at the big picture of this transition.”The fossil in question, newly anointed Maiabalaena nesbittae, was first unearthed in Oregon in the 1970s, but spent decades in the collections of the Smithsonian’s National Museum of Natural History still shrouded in rock. “This fossil’s been languishing for years,” says study author <a href="https://paleobiology.si.edu/staff/individuals/pyenson.html">Nicholas Pyenson</a>, the Museum’s curator of fossil marine mammals. “It’s amazing to finally be able to weigh in on it.”At about 33 million years old, M. nesbittae likely swam the seas of the early Oligocene—a period of rapid and vast geologic change. It was around this time that the southern tip of what’s now South America broke free of Antarctica, rejiggering ocean currents and cooling marine temperatures worldwide. Though it’s still unclear exactly how this transition affected ocean biodiversity, if there was a paleontological moment for a lineage of whales to ditch its dentition, it very well could have been this.
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<figcaption>The upper jaw and skull of Maiabalaena nesbittae, an extinct whale species that lived 33 million years ago. M. nesbittae was toothless, and may have been a suction feeder that gave rise to modern baleen whales. Photo Credit: Smithsonian Institution
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Which makes M. nesbittae a potential watershed whale. Only one precious specimen of the species has been uncovered to date, and the fossil is incomplete. Luckily, most of what’s left involves the skull—which is key to studying how this species managed its massive, yet often microscopic, meals. To interrogate this fossil’s feeding forte, a team of scientists led by Carlos Peredo, a palaeobiologist at George Mason University, decided to reopen the 40-year-old case on M. nesbittae.One thing was quickly obvious to Peredo. “I looked for teeth, and it had no teeth,” he explains with a laugh. “That was the easy one.” Teeth, unlike baleen, are hardened and encased in enamel, and thus far more likely to preserve as fossils. Even if its chompers had somehow been lost to time, the roof of M. nesbittae’s mouth also appeared to be structurally inhospitable to sockets that might otherwise house true dentition. This made it unlikely that this whale was munching on large-bodied prey, à la its toothier ancestors.
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Next, the team used CT scanning to more closely analyze the skull’s internal architecture without compromising the fossil. As there was little point in searching for ephemeral baleen, Peredo and his colleagues instead searched for attachment sites from which the baleen might have been suspended like vertical window blinds. They discovered the upper jaw was thin and narrow, without enough surface area to accommodate the gums from which baleen plates might grow. To the researchers, this was a clear strike against filter feeding.The researchers believe M. nesbittae utilized another option entirely on the road to baleen: simply slurping prey out of the surrounding waters through the sheer force of suction. Although the fossil’s facial muscles were long gone, the bones of its throat indicated that the whale was capable of retracting its tongue, creating negative pressure that would have drawn in water and small creatures from the surrounding environment. The toothless whale would have had no means to chew its prey, simply swallowing them whole. But the switch to suction was likely a move that would have averted a significant cost: Teeth are energetically expensive to produce. And it was on this blank canvas, the authors propose, that baleen eventually took shape a few million years later.
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<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/Carlos_Peredo_-_Maiabalaena_Final_Cover.width-990_qCIUmOD.jpg"/>
<figcaption>An artist's imagining of a Maiabalaena nesbittae calf nursing from its mother. Artist Credit: Alex Boersma
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As modern humans can attest, teeth come in handy. They allow creatures to both latch onto their prey and grind them down to swallowable portions, significantly increasing the size and breadth of an animal’s viable menu items. Suction feeding, on the other hand, doesn’t pulverize prey before it goes down the gullet, and favors smaller, softer-bodied meals. But suction feeding’s lack of oral ornamentation means it’s a relatively cheap strategy. Even toothy mammals like humans are no stranger to its efficiency: Suckling is the only way babies can eat before their teeth come in.Some scientists, however, are a little more hesitant to embrace the idea that M. nesbittae was a suction feeder. “I’m not convinced it didn’t have teeth,” says <a href="https://www.rachelracicot.org/">Rachel Racicot</a>, a paleobiologist who studies marine mammals at the Keck Science Center. Critical pieces of the fossil’s jaw are fairly worn or incomplete, she points out, muddying the conclusions that can be drawn about the presence or absence of dentition. Until more fossils from this time period reveal themselves, suction feeding will be difficult to confirm, Racicot adds. Alternatively, evidence of <a href="https://www.frontiersin.org/articles/10.3389/fmars.2017.00067/full">a more stepwise transition</a> from teeth to baleen may exist—which could mean there were once intermediate whale species that harbored both in their mouths. The two theories on this interim aren’t necessarily mutually exclusive.
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Regardless of how these whales fed in the years after they lost their teeth, the ultimate advent of baleen was revolutionary. According to Peredo, “it’s like growing fingernails out of the roof of your mouth.” There’s nothing else quite like baleen in the animal kingdom, and we probably wouldn’t have the sea’s biggest creatures without it. By dispensing of the sea water “filler” in each gulp, filter-feeding ensures that baleen whales can engulf gargantuan amounts of food in one go: A blue whale can take up <a href="http://blogs.discovermagazine.com/notrocketscience/2010/12/09/blue-whales-can-eat-half-a-million-calories-in-a-single-mouthful/">half a million calories</a> in a single mouthful.“If you drink coffee, you’re taking advantage of filter feeding,” Pyenson explains. “It’s just separating fluid out from a substance.” Though in the case of whales, he adds, they’re hungry for the grinds, not the liquid coffee, so to speak. A good filter means that especially tiny forms of prey—which don’t require chewing—can comprise a viable meal.
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<figcaption>Modern baleen whales, like humpbacks, employ a filter-feeding strategy that enormously ups the nutritional value of each mouthful compared to suction feeding. This has enabled many whale species to grow to gargantuan sizes. Photo Credit: Ari S. Friedlaender/University of California, Santa Cruz under NOAA permit
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Chomping and suction feeding still have their perks, though. All three feeding strategies exist in modern whales, helping many species comfortably share the seas. And adopting one tactic doesn’t preclude a whale from utilizing another. While some toothed whales do chew on their prey, many like orcas or beluga whales simply use their teeth as graspers before swallowing food whole. Others like the beaked whale or narwhal have only rudimentary tooth structures, and must rely on suction to more or less inhale their prey. Though gray whales are equipped with baleen, they happily suction feed as well.This sort of flexibility may become increasingly important as the planet continues to change under the influence of human intervention. “Maiabalaena is like its own little case study,” Peredo says. “The oceans changed dramatically, and this is what happened—whales changed what they ate, the physical structures of their mouths.”But whales aren’t infinitely adaptable; even baleen took millions of years to come into being. It remains unclear how sea life will fare in an age of <a href="https://www.pbs.org/wgbh/nova/article/climate-change-plants-high-temperature/">rising temperatures</a> and increasingly pervasive <a href="https://www.pbs.org/wgbh/nova/article/ships-noises-humpback-whale-song/">acoustic pollution</a>. For now, researchers are keeping an eye on the past. The search for baleen’s origins may yet reveal the climactic changes that triggered its rise—and how whales weathered the storm with their mysterious shuttered smiles.

After discarding their teeth, the ancestors of today’s filter-feeding whales may have fed by suction before evolving baleen.

The size and shape of rodent sperm varies drastically across the many species that inhabit South American forests. Photo Credit: Júlio Vilela, The Field Museum of Natural History

What Rodent Sperm Can Teach Us About Biodiversity

In the summer of 2008, waylaid on a trip to Buenos Aires, <a href="https://www.researchgate.net/profile/Noe_De_La_Sancha">Noé de la Sancha</a> found himself face-to-face for the first time with his future in-laws, struggling to explain the hundreds of pickled testicles slung over his shoulder.But de la Sancha, a biologist at the Field Museum of Natural History and Chicago State University, had good reason for toting around such precious cargo: The package was for a friend (really, it was). Awaiting him at the end of his journey was a team of researchers at the University of Buenos Aires, ready to extract hordes of formalin-preserved sperm of all shapes and sizes from the rodent reproductive tracts that de la Sancha had collected during his field studies.The team’s work, <a href="https://doi.org/10.1093/jmammal/gyy146">published today</a> in the Journal of Mammalogy, showcases the surprising extent of diversity in South American rodents. In some cases, the physical characteristics of sperm might even help researchers distinguish between the many closely-related species that share this rich landscape.
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From the meek house mouse to the bizarrely behemoth capybara, the breadth of rodents in our modern world is vast: About <a href="http://www.ucmp.berkeley.edu/mammal/rodentia/rodentia.html">1,500 species</a> scurry the globe. Some, like chipmunks or voles, are capable of carrying deadly diseases that can be passed to people; others, like <a href="https://www.iucnredlist.org/species/4651/22191157">the uber-fluffy chinchilla</a>, have been driven to the brink of extinction by human hunters. Despite this diversity, it’s sometimes impossible to tell these cuddly critters apart by looks alone—especially when related species scamper around in close proximity. So de la Sancha and his colleagues, led by biologist <a href="https://www.researchgate.net/scientific-contributions/58645939_Luis_F_Rossi">Luis Rossi</a> of the University of Buenos Aires, decided to go microscopic.“Sperm are some of the most biologically diverse cell types in nature,” explains <a href="http://biology.umd.edu/heidi-fisher.html">Heidi Fisher</a>, who studies the sexual evolution of rodents at the University of Maryland, College Park but did not contribute to the new research. Eyeballing sperm might just help researchers pick apart subtle differences between species.To test this possibility, de la Sancha, Rossi, and their colleagues sampled the sperm of 58 individual rodents spanning 18 different species native to South America. All the specimens had been chemically preserved during de la Sancha’s travels, enabling the researchers to extract the contents of the rodents’ epididymides—the long, highly convoluted tubes that cradle the testes.
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<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/IMG-6259.width-990_TJ5g6TP.jpg"/>
<figcaption>These taxidermied rodents showcase several of the species that romp the forests of South America. While many of these rodents are only distantly related, they look very similar, making species distinctions tough in the field. Photo Credit: Field Museum of Natural History
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When the researchers peered at their pickled prizes under the microscope, they were surprised to find just how much sperm differed in physique. Sperm are sculpted a bit like balloons: Each has a head, a slight neck, a midpiece, and a tail. The head contains the sperm’s genetic payload—what will ultimately combine with the DNA of the female egg—while the midpiece is chock full of energy-generating machinery to power the whiplike movements of the sperm’s tail. But beyond these general motifs, the similarities end.Two of the species surveyed—<a href="https://www.iucnredlist.org/species/3613/115065791">Calomys laucha</a>, or the small vesper mouse, and <a href="https://www.iucnredlist.org/species/750/22333428">Thaptomys nigrita</a>, or the blackish grass mouse—appeared to produce sperm with oval heads, not unlike the ones found on human variants. The other 16 species, however, harbored sperm with “hooked” heads, whose tops resembled the curved peaks of meringues; of these, some were rounded at the junction with the neck, while others were more square-jawed. Additionally, some species’ sperm swam with highly elongated midpieces and tails, while others were substantially more compact.
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Unexpectedly, more closely related species did not tend to have more similar looking sperm. Divergences in sperm size and shape could help researchers quickly and efficiently sort rodents in the future, de la Sancha says—especially in cases where rodent populations look alike and occupy overlapping habitats. Many researchers rely on DNA sequencing, which has gotten faster and cheaper in recent years, to sort species, but genetic information hasn’t been banked for all rodents. Even when the sequence of a genome is known, the boundaries between populations often blur. In these cases, starkly divergent sperm could offer some identifying clues.“There isn’t one magic bullet for distinguishing species,” de la Sancha says. “To describe a new species, you have to use as many types of evidence as you can.”In fact, there’s precedent for the utility of scrutinizing rodent sperm for exactly this purpose, says <a href="https://www.adelaide.edu.au/directory/bill.breed">William Breed</a>, a reproductive biologist at the University of Adelaide who has dedicated his career to the study of sperm in Australia’s rodents and marsupials. Breed’s previous work has identified at least <a href="https://drmc.library.adelaide.edu.au/dspace/handle/2440/5329">two</a> <a href="https://www.researchgate.net/publication/230191076_Spermatozoa_of_murid_rodents_from_Africa_morphological_diversity_and_evolutionary_trends">cases</a> in which very different forms of sperm appear in males supposedly of the same mouse or rat species, hinting at the possibility that some subpopulations have begun to drift apart genetically.
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<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/Figure_2.width-990_7UB2iR6.jpg"/>
<figcaption>The heads of sperm of vesper mice in the Calomys genus come in different shapes. The tops of some are hooked, while others are oval; at the base, some are polygonal, while others are entirely round. Credit: Rossi et al. 2018
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But the system is far from perfect. For one thing, sperm are finicky, and there can be quite a bit of variation in how they develop. Within a single species—or even a single individual—not all sperm are identical. Sperm are churned out en masse throughout a male’s lifetime, and suffer high rates of DNA mutation. These sensitive swimmers are also prone to shapeshifting in response to changing environments, which can introduce stress or toxins, Fisher explains. Little tweaks in sperm anatomy can even occur over time as an individual ages, or when he comes into heat during the mating season, Breed adds.With such caveats in mind, Breed says, researchers leading future studies will need to be conscious of changing conditions to get an accurate picture of these eccentric ejaculates. What’s more, several of the species surveyed in the current study were represented by only one individual—and though the researchers collected at least 50 sperm from each species, the build of one rodent’s sperm isn’t necessarily representative of his brethren.
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Regardless of its utility for species identification, this study highlights the incredible range of sperm shapes and sizes—even within a single group of rodents, says <a href="https://hoekstra.oeb.harvard.edu/">Hopi Hoekstra</a>, an evolutionary biologist at Harvard University who did not participate in the research. And in the future, this kind of diversity could speak volumes about these and other understudied species.Sperm <a href="http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.531.1032&amp;rep=rep1&amp;type=pdf">vary immensely</a> <a href="https://www.sciencedirect.com/science/article/pii/B9780123725684000033">across the tree of life</a>. Fruit flies, for instance, can produce sperm cells that run <a href="https://www.washingtonpost.com/news/speaking-of-science/wp/2016/05/25/fruit-fly-sperm-is-monstrously-long-and-we-might-finally-know-why/?utm_term=.4e307bb8d9b2">about 20 times the length</a> of their own bodies. But rodents in particular—which, by species count, make up about 40 percent of mammals—seem to <a href="https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.1992.tb01193.x">really run the gamut</a> of this manly morphology.And the heterogeneity isn’t just for kicks. Sperm architecture, for instance, might be intricately linked to sexual behavior. Sperm with <a href="https://hoekstra.oeb.harvard.edu/files/hoekstra/files/fisheretal2016.pdf">longer midpieces</a>, which supply the power necessary to weather the labyrinthine journey to the egg, tend to swim faster, and are often found in rodent species with more promiscuous males.
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<figcaption>This montane grass mouse (Akodon montensis) has sperm with a pyriform head shape: The tops look like the tips of meringues, and the base is rounded.
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Hooked heads—which are <a href="https://onlinelibrary.wiley.com/doi/full/10.1111/j.1420-9101.2009.01867.x">especially common</a> in rodents—have also been the source of much study, though the impetus behind their clawed appearance remains mostly mysterious. Some scientists theorize that hooks could come in handy as sperm snake eggward, acting as anchors that allows these stalwart swimmers to <a href="https://onlinelibrary.wiley.com/doi/full/10.1111/j.1420-9101.2009.01867.x">lash themselves</a> to the sides of the <a href="https://onlinelibrary.wiley.com/doi/full/10.1111/jeb.12857">female reproductive tract</a>. Especially long and curvy hooks might also help sperm <a href="https://watermark.silverchair.com/biolreprod0025.pdf?token=AQECAHi208BE49Ooan9kkhW_Ercy7Dm3ZL_9Cf3qfKAc485ysgAAAmEwggJdBgkqhkiG9w0BBwagggJOMIICSgIBADCCAkMGCSqGSIb3DQEHATAeBglghkgBZQMEAS4wEQQMVuX8B5rk1RbpZfG8AgEQgIICFOocqyjcAA1S-Ui5GZfqK4mAUMiZF9_nHeTTuMXi6E8BvaMDtdd9KLrRc0Bsw2fAPdZ-7g4e8Y88frZX6WbvB8JZddQ3nQbWIXVhjav_9-U8c91Fl_RFRuYlpP3AN0DpjUNKZ4-Uyayy1CDXS2-_pmV7RBqm7-_wDFLN3vstVrPJBuR2YQa9nZFGHaUnboW0gH2K9YhKifbU0SojbFo7gnjSLWu_idYyCR8gs2VnnkgrHhnYwWcNLOZTrvOylHTVhxUoUu9o1_WGh1O1-1M_j4WkogR0Qq64nDxTTy7hEmIT16eCYgm7iAprCmXHTElSUWQZYYhwSRdqVIC5RMxSixNxsjDh7JeZrNd2Bc102cKuQueStFk0X-GUXLTJEZJchqI3Yt3CpHBFlxNzY_AnWPM7V-ZY8YlXiviF2f5xqQOw1_DXAPQGu2COh7BZ1uzyQ0GoBv9ahBa_P0xU1B07kWEYwD2oQZunIvIvVMW0Rpfupbm1XeV4cV0v6uaa9xsFBwlSaFmRvA47E_-BsYRrYCOXgGWEbRU8MH4SOSIAGkqctF65bcLCO8DrV_td0PaWAVY8ipeR9Frkc6D6Za7_UQQ7vDYa3xZw1lbzcHNdZC39BaRzQt2J-Opl1eNR5tBpw57dPMi9gQoADizbs1bTV6QhVVPlyCGKteMa4yZ9CKNQVs12McXGsmtzuOD4ZvmT_rq8jW8">speed along</a> by cutting through the current of the vaginal milieu like the bow of a boat.Currently, too little is known about the South American rodents in question to determine which of these theories will pan out. But de la Sancha hopes studies like his will renew interest in conservation, especially in an era of such rapid species loss and habitat destruction. The clock is ticking—and every new bit of information counts.“If these species go extinct before we can understand them,” Fisher says, “we won’t know what we’re losing. There’s inherent value in looking at diversity.”

When it comes to understanding the diversity of South American rodents, sperm might have a lot to say—if you’re willing to listen.

A month of space flight can take its toll on the antibody-producing cells of mice. The trip back to Earth may not be enough to repair the damage. Photo Credit: National Cancer Institute, Wikimedia Commons

These Mice Weathered Space. Here’s What Happened to Their Immune Systems.

Humans weren’t built for space.The vast emptiness beyond Earth’s cozy atmosphere is an unforgiving place: Without the pull of gravity, the body can literally begin to unravel. Bones lose mass. Muscles deteriorate. Organs and blood vessels slosh and swell. And the immune system is particularly <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6038331/">sensitive</a> to environmental flux; astronauts experience <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5098747/">an uptick in infections</a> while aboard the International Space Station.Now, scientists report that even a brief sojourn into space can discombobulate immune factors necessary to fight off disease—and the effects may persist or worsen after the return to Earth. According to <a href="https://www.fasebj.org/doi/10.1096/fj.201801463R">a study</a> published today in The FASEB Journal, mice ferried through space by the <a href="https://www.nasa.gov/ames/research/space-biosciences/bion-m1">Bion-M1 biosatellite</a> suffered a prolonged drop in the production of B cells, the white blood cells responsible for antibody production. If the results hold true in humans, the new research might explain some of the biggest and most lasting pitfalls of our weightless wanderings in space.“This study is very valuable, and an important piece in this puzzle,” says <a href="https://www.topicalteam-stressinspace.eu/index.php/chouk%C3%A8r.html">Alexander Choukér</a>, who studies space medicine at the University of Munich in Germany but did not participate in the study.
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So far, only a few studies have investigated the effects of <a href="https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-microgravity-58.html">microgravity</a> on animal bodies. Jettisoning organisms into space is no trivial feat—and the handful of humans who have made the trip are few in number, leaving many of the most pressing questions in extraterrestrial travel unanswered.In an attempt to fill in some of these gaps, NASA and the Russian Institute of Biomedical Problems launched the <a href="https://www.nasa.gov/centers/ames/news/2013/13-34AR-bion-space-biology-mission.html">Bion-M1 biosatellite</a>—a veritable spacefaring <a href="https://www.nasa.gov/ames/research/space-biosciences/bion-m1/">menagerie</a> teeming with mice, geckos, gerbils, fish, snails, and microbes––in 2013. The story quickly turned grim: During the month-long orbit, a series of equipment failures prompted the demise of all the spacecraft’s gerbils and fish, along with about two-thirds of the mice on board. When the spacecraft touched back down on Earth, the researchers found that only the geckos, snails, and 16 disgruntled mice had survived the trip.
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<figcaption>The Bion-M1 biosatellite, contained within the Sojuz in this photo, launched in April 2013, was a veritable Noah's ark of space exploration, with gerbils, mice, fish, snails, geckos, and more aboard its capsule. Due to equipment failures, many of its passengers perished during the journey. Photo Credit: DLR photo, Wikimedia Commons
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Undeterred, scientists around the world divvied up the remaining animals for further study. Several mice came into the hands of a team led by <a href="https://www.biochem2.com/index.php/22-ibcii/pqc/130-frontpage-pqc#georgtascher">Georg Tascher</a>, an analytical chemist at the University of Frankfurt in Germany, and <a href="https://www.researchgate.net/profile/Fabrice_Bertile">Fabrice Bertile</a>, a biologist at the Hubert Curien Multi-disciplinary Institute in France.Curious to see how mouse immunity had fared on the final frontier, the researchers split the newly-arrived mice into two groups. The scientists analyzed the first group of mice within a day of landing on Earth, and the second after the mice had spent a week recovering—which translates to approximately a couple months for a human. (A third control group of mice was housed on Earth.) Tascher, Bertile, and their colleagues then extracted the rodents’ spleens and leg bones—including the bone marrow, where white blood cells are manufactured—and inspected their contents.As Tascher recalls, due to the precious nature of the samples, it was a high-stakes job. “I was sweating a lot during sample preparation,” he explains. “If something goes wrong, you can’t just send the mice into space again.”
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The researchers discovered that, compared to the earthbound rodents, the mice profiled immediately after orbit experienced a small dip in several immune-related proteins.But the worst was yet to come. Even after a week under normal gravity conditions, many known protective pathways were still on the fritz, and the levels of some proteins had actually decreased further. Upon touchdown, the number of B cells in the spleens and bone marrow of wayfaring mice was mostly on par with rodents who hadn’t left Earth. Just a few days later, however, about half the cells had disappeared. The month abroad, it seems, had taken a serious toll on these antibody factories—even after re-acclimating to Earth.“You always think, what happens to a person when they go to space? It’s a big change: There’s no gravity, and cells are working differently,” Choukér says. “But the return is a big change, too.”
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<figcaption>A view of Earth’s horizon from the International Space Station (ISS). Many unknown variables remain in the science of space flight—but it's already clear that microgravity and radiation from the sun negatively impact health. Photo Credit: NASA's Marshall Space Flight Center, flickr
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This seems in keeping with some preliminary results from NASA’s ongoing <a href="https://www.nasa.gov/mission_pages/station/research/experiments/2099.html">Twins Study</a>, which is profiling the differences between identical twins Mark and Scott Kelly, the latter of whom recently spent a year living on the International Space Station. Researchers on this project have already found that while some parts of Scott’s immune system <a href="https://www.nasa.gov/mission_pages/station/research/experiments/2099.html">remained intact</a> in space, several genes important for fighting off disease were <a href="https://www.nasa.gov/mission_pages/station/research/experiments/2099.html">expressed differently</a> during and <a href="https://www.nasa.gov/feature/nasa-twins-study-confirms-preliminary-findings">after</a> space flight.Even in mouse terms, however, a week isn’t that long. Given more time off duty, it’s possible the mice used in the study may have more or less returned to baseline. Previous studies have hinted at the <a href="https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070030101.pdf">resilience</a> of human astronauts, explains <a href="https://surgery.ucsf.edu/faculty/general-surgery/tammy-t-chang,-md,-phd.aspx">Tammy Chang</a>, a biologist and physician at the University of California, San Francisco, who was not involved in the current study. Similarly, Scott Kelly’s initial follow-up was at only six months post landing, leaving plenty of room for further developments down the line.As things stand, most of the long-term effects of these journeys remain mysterious. Future experiments are needed to address these effects, as well as the question of how even longer odysseys in space might rejigger the road to recovery.
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And proceeding with caution is essential. It’s still not clear what aspect of space flight is actually causing the immune systems of these mice to go haywire. <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3501893/">Microgravity</a> is sure to weigh heavy on the mind, but space travel is rife with hardship. Another problem is the high intensity of radiation from the sun, which, in space, is no longer filtered through Earth’s protective atmosphere. To make matters worse, there’s evidence that some infectious microbes don’t just survive the altered conditions of space. They <a href="https://www.washingtonpost.com/news/to-your-health/wp/2016/10/25/bacteria-get-dangerously-weird-in-space/?utm_term=.73d2b2746882">thrive</a> in it.“There’s still a long way to go to really understand the biology of space flight,” says <a href="https://www.researchgate.net/profile/Rasha_Hammamieh">Rasha Hammamieh</a>, director of the Integrative Systems Biology Program at the U.S. Army Center for Environmental Health Research. “This is just the tip of the iceberg.”
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<figcaption>Sojuz 2.1A, which carried Bion-M1 as well as some small satellites, on its way to the launch pad, in April 2013. A Bion-M2 is already in the works, which will build on Bion-M1's findings and correct the unexpected issues that occurred during the initial trip. Photo Credit: DLR photo, Wikimedia Commons
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Of course, mice aren’t humans. Any work done with these pint-sized cosmonauts needs to be taken with a grain of salt. But the perils of space make for a <a href="https://www.businessinsider.com/nasa-launches-mice-to-space-station-for-twin-study-2018-6">pretty great equalizer</a>: <a href="https://www.physiology.org/doi/full/10.1152/japplphysiol.01052.2002">Across</a> <a href="https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070030101.pdf">species</a>, space has a way of compromising the immune response, Hammamieh explains.Scientists are already brainstorming ways to minimize the drawbacks of future expeditions. Interventions to bolster astronaut immunity could take the form of anything from gut microbiome probiotics to anti-inflammatory drugs. And plans for a Bion-M2 mission, set to launch within the next couple years, are already underway, study author Bertile explains. Some of the aforementioned countermeasures may even be put to the test on the new satellite.Riding on the heels of unfortunate mishaps from the first endeavor, scientists are eager for a second shot at success. When the new Bion-M2 re-enters Planet Earth’s stratosphere, it will hopefully carry a capsule full of cargo that’s still alive and kicking—and full of stories to tell about the rigors of space travel.

A month of space flight might compromise the ability of mice to make disease-fighting antibodies—even after their return to Earth.

Exposing a developing embryo to Prozac could affects its hormones and behavior later in life—and these effects may even pass on to future generations. Photo Credit: Vasyl Dolmatov, iStock

Parents Might Pass the Effects of Prozac on to Future Generations

There are few relationships more biologically intimate than the one between a mammalian mother-to-be and her growing fetus. But the dialogue of <a href="https://www.pbs.org/wgbh/nova/video/lifes-greatest-miracle/">pregnancy</a> can be a double-edged sword.All sorts of chemicals easily traverse the placental barrier, and antidepressants are no exception. While the use of drugs like Prozac during pregnancy has been linked to <a href="https://bpspubs.onlinelibrary.wiley.com/doi/full/10.1111/bcp.13321">cardiac defects</a> and <a href="https://www.nature.com/articles/pr201577">behavioral changes</a> in children, none of the research done in humans can ethically test for cause and effect. But with <a href="https://www.cdc.gov/nchs/products/databriefs/db283.htm">more than 16 percent</a> of American women currently taking antidepressants, the need to fully suss out the long-term repercussions of these early chemical exposures is ever-growing.Now, researchers have unveiled an unexpected ally in their search for understanding: a striped, diminutive minnow called a zebrafish. According to <a href="https://www.pnas.org/cgi/doi/10.1073/pnas.1811695115">a new study</a> published today in the journal PNAS, developing zebrafish embryos exposed to the active ingredient of Prozac are less equipped to cope with stress later in life—and these effects are passed on through future generations. While the results have yet to be repeated in human populations, they may prompt a shift in our outlook on the prescription of antidepressants during this vulnerable period.
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In spite of their stripes, zebrafish are far more fish than zebra. But we have more in common with these mighty minnows than you might think: <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3703927/">About 70 percent</a> of human genes have an obvious zebrafish counterpart. Like us, zebrafish respond negatively to stressful events and release many of the same hormones—some of which are even more comparable to those in humans than those produced by <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4336318/">rodents</a>.“The overwhelming evidence today supports the value of zebrafish for modeling complex disorders, including schizophrenia, depression, and anxiety,” explains <a href="http://pharmacy.swu.edu.cn/viscms/pharidex/bumen21044/20130729/392494.html">Allan Kalueff</a>, a neuropharmacologist who studies zebrafish at Southwest University in China, but did not participate in the new research. “There is a striking similarity when it comes to drug responses.”To mimic the medicinal milieu of the human fetal environment, study author <a href="https://teamendo.ca/?page_id=283">Marilyn Vera-Chang</a>, a biologist at the University of Ottawa in Canada, first exposed broods of newly-fertilized zebrafish eggs to fluoxetine, an antidepressant commonly marketed under the brand name Prozac. She then waited patiently as the eggs hatched into juveniles and matured into adults—a six-month process—and measured their chemical and behavioral responses to stress.
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<figcaption>Zebrafish embryos are transparent, making it easy to observe their development. Mothers can lay up to 200 eggs a week. Photo Credit: Despoina Chrysostomou, Wikimedia Commons
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Measuring stress in zebrafish <a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0036931">isn’t trivial</a>: These little fish aren’t particularly chatty about their feelings. Without saying a word, however, zebrafish can still speak volumes—chemically, that is. Just like humans, zebrafish produce the stress hormone cortisol in response to taxing situations. Cortisol is a body’s way of signaling that something’s amiss, so resources can be diverted to weather the storm. Like many other chemicals, cortisol operates on a Goldilocks principle. Too much cortisol puts the body on constant high alert—a state that’s been <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4451064/">linked to depression</a>. Too little, however, leaves an animal ill equipped to process stress, and can even lead to <a href="https://www.mayoclinic.org/diseases-conditions/addisons-disease/symptoms-causes/syc-20350293">diseases</a> characterized by extreme fatigue.When Vera-Chang measured the zebrafish’s cortisol levels after they’d had a brief, nerve-wracking experience—being suspended out of water—she found that fish exposed to fluoxetine as eggs produced less cortisol compared to their untreated counterparts. Even when left to mind their own business, treated fish had less of the hormone coursing through their bodies.It was a striking result. The adult zebrafish had spent the past five-plus months Prozac-free—and yet, the one-time exposure had blunted their stress response systems in a lasting way.
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To see how this hormonal flux affected behavior, Vera-Chang next plopped individual zebrafish into a strange new environment—a fresh tank of water—to see how they’d react. Normally, in this situation, zebrafish spend a minute or two zipping around the bottom of their cage, searching for an exit. After the nerves have worn off a bit, they’ll tentatively begin to venture into the top half of the tank.And that’s exactly what Vera-Chang saw—at least, in the fish that had spent their early development Prozac-free. Those who had been exposed to fluoxetine, on the other hand, remained stalwart bottom-dwellers. “It’s like they didn’t have energy to keep exploring,” Vera-Chang explains, “as if they were experiencing burnout.”But the most surprising results were still to come. When the fluoxetine-treated fish had offspring of their own, they, too, produced less cortisol and were less eager to explore their tanks—despite never having seen the Prozac themselves. And, amazingly, when these zebrafish gave rise to another generation, the effect was still there. Somehow, the single exposure the first set of zebrafish had experienced during development was being passed down through the generations like an insidious inheritance.
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Eventually, three generations down the line, the behavioral effects faded. But even at this juncture, cortisol remained low. “This is like your grandparents influencing your behavior, and your great-grandparents influencing your hormone levels,” explains study author <a href="https://teamendo.ca/?page_id=2">Vance Trudeau</a>, a neuroendocrinologist at the University of Ottawa.It’s still unclear what these differences mean, but the fact that the changes were inherited from generation to generation at all might spark concern. While more work is needed, the researchers theorize this may be the result of epigenetics—a phenomenon in which chemical markers are added to DNA, altering the expression of genes without affecting the underlying sequence. In recent years, <a href="https://www.smithsonianmag.com/science-nature/dads-pass-more-genetics-their-sperm-180969760/">several</a> <a href="https://www.smithsonianmag.com/science-nature/how-dads-genes-can-prepare-mom-parenthood-180969793/">studies</a> have <a href="https://www.pbs.org/wgbh/nova/article/epigenetics-abuse/">indicated</a> that epigenetic modifications can be <a href="https://www.pbs.org/wgbh/nova/article/anxiety-may-be-passed-down-generation-to-generation/">passed</a> on to future generations, and this new study may be yet another example.“We have to think about how we’re living our lives,” Vera-Chang says. “It’s not just us that are being affected by our actions.”
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<figcaption>About 10 percent of Americans, and over 16 percent of American women, currently use antidepressants. Fluoxetine, or Prozac, is an especially common selective serotonin reuptake inhibitor, or SSRI. Photo Credit: Tom Varco, Wikimedia Commons
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However, researchers are quick to caution that it’s too early to jump to conclusions about human populations. Working with zebrafish makes it possible to conduct in-depth experiments that can’t be done in people—but analogs between the two species have their limits.For one thing, humans who take antidepressants do so for a reason: to treat mental illness. All the fish in this study were healthy, making it tough to evaluate how a drug might traverse the barriers between a depressed mother and her child, explains <a href="https://www.ru.nl/english/people/homberg-j/">Judith Homberg</a>, a behavioral neurogeneticist at Radboud University in the Netherlands who did not participate in the research.“We can replicate anxiety and depression to an extent in animal models,” explains <a href="https://www.bcchr.ca/toberlander">Tim Oberlander</a>, a developmental pediatrician at BC Children’s Hospital Research Institute in Canada who did not participate in the new research. “But ultimately, the human experience needs to be recognized as a unique thing.”
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Additionally, incubating fish eggs in a petri dish is only a partial proxy for the placenta, says Homberg. Placental function itself could be affected by fluoxetine, meaning there could be other, indirect effects on a human fetus.Still, a Prozac bath may not be terribly far off the mark. The uterine environment isn’t the only source of indirect exposure to antidepressants: Fluoxetine is one of many chemicals that have been found in water surrounding urban centers. “The bioecological impact of this study is very important,” Kalueff, the Southwest University neuropharmacologist, explains. “This tells us what could happen to children who drink fluoxetine-enriched water in big cities.”Unfortunately, we’re still waiting on some of the most critical, human-centric data. Prozac first hit the market in the 1980s; as such, we’re only about one-and-a-half generations into use, Trudeau explains. As things stand, there’s some evidence that children of mothers taking fluoxetine during pregnancy are <a href="https://www.nature.com/articles/pr201577">more withdrawn or anxious</a>, but there isn’t yet data on whether the effects persist into subsequent generations.
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<figcaption>A young zebrafish. While zebrafish don't physically resemble humans, the two species are very genetically similar, and both produce cortisol in response to stressful events. Photo Credit: Nikita Tsyba and Azamat Bashabayev, Wikimedia Commons
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In the meantime, studies like these are essential for generating awareness, says <a href="https://www.mountsinai.org/profiles/veerle-bergink">Veerle Bergink</a>, a psychiatrist and obstetrician-gynecologist at the Icahn School of Medicine at Mount Sinai who was not involved in the new find. “These findings show clearly that this is a topic that warrants further investigation in humans,” she adds. “We need to have these discussions. It’s a medical issue, but it’s also a societal issue.”Ultimately, the researchers emphasize that results of this study aren’t prescriptive. While antidepressants may have their downsides, depression during pregnancy is also <a href="https://journals.sagepub.com/doi/abs/10.1177/070674370404901103">potentially</a> <a href="https://www.psyneuen-journal.com/article/S0306-4530(15)30006-8/fulltext">damaging</a> to a growing fetus. Women faced with this choice often find themselves saddled with a difficult decision—but every case is different. Concerned mothers-to-be should always engage in a careful risk and benefit evaluation with their physicians.“There are many women who <a href="https://www.ncbi.nlm.nih.gov/books/NBK361016/">really need this medication</a> to maintain mood stability,” Bergink advises. “For some women, it may be the best option to continue medication. However, there are also women for whom it may be a good option to see if they can taper, or seek alternate treatment, such as psychotherapy, cognitive behavioral therapy, mindfulness based therapy, or group support, among others.”Oberlander agrees: There’s not going to be a one-size-fits all solution. “Is the treatment better or worse than the condition itself?” he says. “That’s the core question that we need to figure out urgently.”

Treating zebrafish embryos with the active ingredient in Prozac can compromise the stress responses of generations to come.

Katherine J. Wu

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