It seems that males may not be necessary for reproduction after all. Scientists have discovered that around three percent of smalltooth sawfish living in a Florida estuary are the product of "virgin births" - the first case ever seen in vertebrates in the wild.
A "virgin birth" occurs as the result of a process called parthenogenesis, which is a type of reproduction that doesn't involve sex. This phenomenon is common in invertebrates but is relatively rare in vertebrates. It has been seen in female birds, reptiles, sharks and even rays living in captivity; however, whether it was possible in wild populations remained a mystery.
That is, until scientists at Stony Brook University, Florida Fish and Wildlife Conservation Commission (FWC), and The Field Museum conducted DNA analyses on Florida sawfish living in the Caloosahatchee and Peace rivers and stumbled upon evidence of virgin births.
"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," lead author Andrew Fields, a Ph.D. candidate at the Stony Brook University's School of Marine and Atmospheric Science, said in a press release provided via email. "What the DNA fingerprints told us was altogether more surprising; female sawfish are sometimes reproducing without even mating."
This means "parthenogens" are living alongside other sawfish produced via normal sexual reproduction, and that virgin births may be more common in the wild than scientists previously thought.
Parthenogenesis is thought to be triggered by an unfertilized egg absorbing a sister cell called the polar body that is nearly genetically identical to the egg. This results in an offspring that has roughly half the genetic diversity of its mother. In many cases these offspring are malformed or die early, but that may not be true for these sawfish.
"There was a general feeling that vertebrate parthenogenesis was a curiosity that didn't usually lead to viable offspring," said Dr. Gregg Poulakis of the FWC, who led field collections of the sawfish. "The seven parthenogens we found looked to be in perfect health and were normal size for their age. This suggests parthenogenesis is not a reproductive dead end, assuming they grow to maturity and reproduce."
This discovery may be good news for the sawfish (Pristis pectinata), which is listed as a critically endangered species on the IUCN Red List. When once they inhabited waters in most of the Atlantic, now overfishing and coastal habitat loss has confined them to just certain areas of southern Florida. But by being able to reproduce without a mate, this species may save itself from the brink of extinction.
However, there is a downside to the method of parthenogenesis.
"It could help increase their numbers possibly, [but] on the other hand it reduces their genetic diversity. It could make their population more susceptible to diseases or other factors," Fields told Nature World News (NWN).
So why exactly is this happening? The team of scientists speculates that because numbers of these ocean giants are few, females may be struggling to find a male during mating season. And when that happens, it may be up to her alone to get the job done.
"It's possible that this could be in many vertebrate species and the sawfish are just at the right conditions that we [are able to] see it in the wild," Fields noted. "It could be that because of their low density we see it, whereas in certain species they're not at a low enough density to see it because sexual reproduction is the main mode of reproduction, so you don't see the parthenogenesis. It also could be that this is not found as commonly in vertebrates as it may be."
"What it comes down to at the end of the day," he continued, "is we don't know if this happens in several vertebrate species or if it's just unique to sawfish. That's our next step, to start scanning databases to see if we can figure that out."
The findings were published in the journal Current Biology.
For more information on the plight of sawfish and efforts to save them, click here.
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