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Endangered Sawfish Are Cloning Themselves to Stave Off Extinction

ByAbbey InterranteNOVA NextNOVA Next

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Recently, off southwest Florida, researchers found evidence of virgin births of an unlikely sort—those of a sawfish. They’re the first known cases of vertebrates reproducing asexually in the wild.

Asexual reproduction isn’t unusual in nature—bacteria and many invertebrates employ it—but it isn’t often seen in species with a backbone. Some captive animals, including snakes and sharks, have been known to reproduce asexually, but never in the wild until now.

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sawfish
Sawfish are cartilaginous fish with a large rostrum that's laced with electrosensitive pores.

Smalltooth sawfish are critically endangered, with their population hovering at only 1–5% of its original size. As a result, their population density is significantly lower, meaning it’s harder for females to find males to mate with, which could force females to undergo what’s called facultative parthenogenesis. (Parthenogenesis is reproduction without a fertilized egg, while facultative means they’re not required to reproduce sexually, but can if the circumstances arise.)

For the endangered sawfish, asexual reproduction could help prop up the species, but only if the parthenogens—the term for the asexually produced offspring—are fertile, something we won’t know until they’re a bit older.

Researches captured 190 sawfish over nine years, according to a study published on this week in Current Biology. Thanks to DNA evidence, there is little doubt that some of these sawfish are clones of their mothers. The scientists compared microsatellites, or the genetic fingerprints, of the captured sawfish and found that the microsatellites of sawfish siblings and half siblings matched between 25% and 50% of the time. In the sawfish that weren’t related, less than 10% of their microsatellites matched. However, in seven sawfish, it was a different story.

Cathleen O’Grady, writing at Ars Technica:

The seven outliers, though, had matches of 84 to 100 percent. Two of the seven outlier sawfish were unrelated to any of the other five, suggesting they each had a different mother. Five of them, however, all seem to have shared a single mother, meaning they were possibly from a single brood. All seven were female—parthenogens can’t have a different sex from their mothers.

It is technically possible for matches to be so high in individuals produced by normal sexual reproduction, but the chances are extremely low. For the sawfish with a match on 84 percent of the examined microsatellites, it’s one in 437 billion. For those with a 100 percent match, it’s one in 239 trillion.

Given smalltooth sawfish are not the only endangered cartilaginous fish, scientists think that parthenogenesis might occur in other species in the wild, too. Fortunately, other teams won’t necessarily have to scour the oceans for evidence. The researchers are encouraging their colleagues to check existing DNA databases for other similar genetic patterns, perhaps among similarly endangered species.