Endangered Sawfish Reproduce Through "Virgin Birth"

Diliff via Wikimedia Commons // CC BY 2.5

You know the drill: When a male and a female love each other very much—or are compelled by nature—they make a baby together. But sometimes a female becomes pregnant without any input from a male. So-called “virgin births” may seem miraculous, but they’re actually the product of parthenogenesis, a form of asexual reproduction in which an embryo grows and develops without fertilization.  

Parthenogenesis has been observed in some invertebrates like honeybees and scorpions, and very rarely among captive vertebrates like snakes, lizards, and birds. Now researchers at Stony Brook University say they’ve discovered the first known example of parthenogenesis in wild vertebrates—and the phenomenon could be more common than we think.

Named for what looks like a massive chainsaw blade protruding out of its face, the smalltooth sawfish, which lives in southern Florida, was first listed as an endangered species in 2003. Its numbers have been drastically reduced thanks to overfishing and habitat loss. Andrew Fields, a Ph.D. candidate at Stony Brook's School of Marine and Atmospheric Science, wanted to get a sense of exactly how many sawfish still exist.

“We wanted to be ready to say, ‘This is happening, and we need to do something now before they go extinct,’” Fields told mental_floss.

They may have set out to do a population count, but they found much more. As they document in Current Biology, Fields and his team examined sawfish DNA for signs of inbreeding, which can indicate a dwindling number of individuals in a species. In the process they discovered that about 3 percent of the sawfish they studied were solely the offspring of their mothers and had no genetic contribution from a male.

In normal sexual reproduction, the female egg ejects half its chromosomes through a series of cell divisions after it matures, making up the difference by combining with the chromosomes from the male sperm. In parthenogenesis, another female cell known as the polar body provides the second half of chromosomes. Although the offspring will have two sets of chromosomes, they will be identical.

Perhaps even more surprising is that the fatherless sawfish seem relatively normal. “We don’t have any physical proof they’re any different, which is strange because the general theory right now is [that] parthenogenetic offspring are not fit, and so they don’t survive in the wild,” Fields says. “So this is kind of a big step.”

But why do “virgin births” happen? Until now, sexless reproduction in vertebrates had only been observed in captivity, so the standing theory has been that the special stresses of captivity are the cause, Fields says. In 2001, for example, a hammerhead shark at a Nebraska zoo gave birth despite not having contact with a male for three years.

But these new findings suggest that maybe it’s not the stress of being in captivity that prompts parthenogenesis, but a lack of males. Perhaps a virgin birth is a dwindling species’ last-ditch effort to avoid extinction.

Unfortunately for the sawfish, parthenogenesis probably won’t help keep the population alive. While the parthenogenetic fish seem normal, they are all female, which could exacerbate the problem of male scarcity. “If the sex ratio flips enough, it could help with the extinction,” Fields says.

Fields and his team are urging other researchers to join the hunt for examples of virgin births in wild vertebrates by reexamining existing DNA evidence. “A lot of shark populations have been fished for the shark fin trade, so it’s possible we could start looking at a number of shark species,” Fields says. “It may be the data is out there, but nobody has looked for it because we think it doesn’t happen.”

[h/t The Guardian]