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Virgin Sawfish Can Still Be Moms

Talking about sex might be a bit taboo, but frankly, human copulation is pretty tame. The spectrum of reproduction contains way weirder acts than anything you’d find in Fifty Shades of Grey. If you’re a sawfish, you don’t even need a partner.

Reporting today in the journal Current Biology, Andrew Fields, a geneticist at Stony Brook University in New York, and his team have found genetic evidence that the smalltooth sawfish — closely related to sharks and rays — is the first vertebrate known to produce offspring via virgin birth in the wild.

The researchers didn’t witness this in a manger with a star overhead, mind you, but found evidence of suspiciously close genetic relations between sawfish in the same population. This suggested that some fish were produced asexually in a process called facultative parthenogenesis — when an organism that reproduces sexually can also sometimes produce offspring asexually.

Typically, sexual reproduction occurs in sawfish as you may have learned in school: a male sperm fertilizes a female egg and so on. But in parthenogenesis, a female fertilizes her own egg with a cell called a polar body. The resulting offspring is much more related to the mother genetically. This is how Fields and his team picked out the virgin-birthed fish from the rest.

“We were conducting routine DNA fingerprinting of the sawfish found in this area in order to see if relatives were often reproducing with relatives because of their small population size,” said Fields in a press release. “What the DNA fingerprints told us was altogether more surprising; female sawfish are sometimes reproducing without even mating.”

Of the 190 sawfish sampled in the Florida estuary, around three percent appeared so genetically similar to others sampled that it suggested parthenogenesis.

SawVirgin_GIFA sawfish in action.

Parthenogenesis is common in invertebrates like insects, but rare among the animals with backbones. Of those, the only examples had been seen in captivity among some birds, reptiles like snakes and Komodo dragons, and sharks. Smalltooth sawfish are the first vertebrates known to asexually produce viable offspring in the wild.

But if these sawfish have the ability to reproduce sexually, why reduce genetic variability with asexual reproduction? It could have something to do with environmental pressures like overfishing. Once present in over 90 countries, thanks to our efforts, sawfish are now found only in Australia and Florida. Facultative parthenogenesis could then be an occasional way for some vertebrates to reproduce when there aren’t many viable partners.

“It is possible that parthenogenesis is most often expressed in wild vertebrates when the population is at very low levels and the animals have difficulty finding one another,” said Fields.

Seeing as many vertebrates face the same pressures, the study authors are suggesting that other scientists double-check their data for all kinds of animals. It could be that virgin births are much less miraculous than we thought.

STUDY: Facultative parthenogenesis in a critically endangered wild vertebrate

IMAGE: What a saw by Lorenzo Blangiardi

HT: Nature

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