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What Rodent Sperm Can Teach Us About Biodiversity

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

ByKatherine J. WuNOVA NextNOVA Next
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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

In the summer of 2008, waylaid on a trip to Buenos Aires, Noé de la Sancha 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, published today 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 1,500 species scurry the globe. Some, like chipmunks or voles, are capable of carrying deadly diseases that can be passed to people; others, like the uber-fluffy chinchilla, 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 Luis Rossi of the University of Buenos Aires, decided to go microscopic.

“Sperm are some of the most biologically diverse cell types in nature,” explains Heidi Fisher, 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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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

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—Calomys laucha, or the small vesper mouse, and Thaptomys nigrita, 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.

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 William Breed, 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 two cases 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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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

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 Hopi Hoekstra, 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 vary immensely across the tree of life. Fruit flies, for instance, can produce sperm cells that run about 20 times the length of their own bodies. But rodents in particular—which, by species count, make up about 40 percent of mammals—seem to really run the gamut 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 longer midpieces, 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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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.

Hooked heads—which are especially common 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 lash themselves to the sides of the female reproductive tract. Especially long and curvy hooks might also help sperm speed along 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.”