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Who needs Tinder when you can reproduce on your own? Not female smalltooth sawfish. These shark-like creatures can make babies asexually, according to a new study in Current Biology. The find marks the first observation of a free-living vertebrate animal that successfully switched from sexual to asexual breeding — a phenomenon known as facultative parthenogenesis — and yielded viable offspring in the wild.
Smalltooth sawfish, a species of ray, join a surprising collection of vertebrates that have made the switch. The earliest accounts of the facultative parthenogenesis involved birds: farmed chickens in 1872, pet pigeons in 1924 and domesticated turkeys in the 1954. “Self-loving” female sharks produced viable offspring at zoos in Omaha, Nebraska and Detroit in the last decade. Female Komodo dragons prefer coitus in the wild, but put them in a pen, and they’ll get down asexually.
“I think that facultative parthenogenesis is a more common occurrence than people would ever expect, said evolutionary biologist Warren Booth of the University of Tulsa in Oklahoma who wasn’t involved in the study. “In the last 5 years, a whole suite of studies have come out documenting the phenomenon with animals in captivity.”
Smalltooth sawfish (Pristis pectinata) are named for their sawlike beaks. The beaks act as hunting tools that track weak electric fields emitted by fish prey and then serve as the perfect weapon for slashing their meal into oblivion (see video).
This movie shows four different feeding behaviors of sawfish. Video by Wueringer BE et al 2012.
But due to overfishing and habitat loss, smalltooth sawfish are critically endangered, with estimates suggesting that the global species lost between 95 to 99 percent of its members over the last century. One of their final homesteads exists along Florida’s southern tip, where scientists keep constant watch on a population that has shown indications of stability in recent years.
“We do regular monitoring of their genetics to see how the sawfish in Florida are coping with things like global warming and ocean acidification,” said study co-author and ecologist Andrew Fields of Stony Brook University in New York. Their broad census consisted of tissue samples from 190 sawfish that were caught and released between 2004 and 2011. But when romantic partners become scarce, fish sometimes resort to mating with siblings, so part of the team’s objective also involved scanning the DNA samples for signs of inbreeding.
That’s when the team happened across seven peculiar females. They were using a computer program named STORM to decode the gene sequences and calculate their “internal relatedness” (IR) — a numeric value that determines whether an individual’s parents were each other’s siblings. An IR reading near zero means both parents came from different families, while a readout between 25 to 50 percent indicates that an organism was conceived by half or full siblings, respectively.
The “sensational seven,” as these sawfish came to be known, had IR values ranging from 84 to 100 percent, suggesting that they were birthed via facultative parthenogenesis — where a formerly sexual creature becomes asexual.
Such observations among free-living organisms have evaded scientists because spotting an act of asexuality in the wild is tricky. Booth’s team came closest in 2012 when they collected two pit vipers from the wild, which subsequently birthed asexually in his lab. As demonstrated in that case, you typically need the DNA of both the parents and the kids to prove parthenogenesis, but this new study on sawfish shows that scientists can accomplish the task with a genetic survey of a large population.
“We now have the genetic tools to easily and inexpensively address these questions on a wide scale,” Booth said. “It’s a really valuable study — not just for the field of parthenogenesis but for shark biology and ray biology.”
For instance, the genetic analysis argues that the sawfish performed a brand of parthenogenesis called automixis: wherein an unfertilized egg fuses with a sister cell called a polar body. The result is a highly inbred offspring with half the genetic diversity of the mother. This process differs from “obligate” asexuality seen with some frogs, salamanders, fish and lizards, which creates an exact clone of the mother. Humans and other mammals can’t reproduce asexually thanks to a failsafe system — called genomic imprinting — that won’t let an egg develop into an embryo without both male and female input.
It remains unknown if parthenogenesis is ultimately good or bad for the smalltooth sawfish. Sure, asexual breeding allows females to continue the species during a time when males might be scarce, but it isn’t the most ideal solution in the long term.
“It will skew the sex ratio because their offspring will always be female, so you’d never have sexual reproduction again, once all the males died off,” Fields said. “Plus their offspring live with reduced genetic diversity, meaning they may not be able to adapt to changing environmental conditions around them, such as global warming or mutating bacteria.”
Editor’s note: The headline of this post was updated to change the word “man” to “male.”
Nsikan Akpan is the digital science producer for PBS NewsHour and co-creator of the award-winning, NewsHour digital series ScienceScope.
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