A small number of sawfish that live in a Florida estuary appear to reproduce without sex.
The outwardly normal fish live alongside other sawfish produced through normal sexual reproduction, suggesting that these occasional virgin births may be more common in natural populations than ever suspected.
Female birds, reptiles, and sharks living in captivity have sometimes surprised their keepers by giving birth even though, as far as anyone can remember, they have never been housed with a male.
Scientists used DNA analysis to solve this mystery some time ago, showing the babies were produced by asexual reproduction, a process called parthenogenesis. But until now, it has been unclear whether the phenomenon ever happened in wild populations, too.
Save the sawfish
The smalltooth sawfish (Pristis pectinata) is one of five species of sawfish, a group of large rays known for their long, tooth-studded rostrum that is used to subdue small fish.
Sawfish may be the first entire family of marine animals to become extinct because they are all critically endangered as a result of overfishing and coastal habitat loss.
Researchers say the race is on to save the species, which has disappeared from most of the places in the Atlantic where it was common a century ago but is now listed as endangered under the US Endangered Species Act.
Smalltooth sawfish are mainly found in a handful of locations in southern Florida, including the Caloosahatchee and Peace rivers. It was here that scientists have discovered that these critically endangered ocean giants are sometimes breeding without sex.
“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,” says Andrew Fields, a PhD candidate at Stony Brook University’s School of Marine and Atmospheric Science and lead author of the study that is published in Current Biology.
“What the DNA fingerprints told us was altogether more surprising; female sawfish are sometimes reproducing without even mating.”
A reproductive dead end?
Parthenogenesis is common in invertebrates but relatively rare in vertebrates. Among the birds, reptiles, sharks, and now rays, 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—but in many cases these offspring are malformed or die early.
“There was a general feeling that vertebrate parthenogenesis was a curiosity that didn’t usually lead to viable offspring,” says Gregg Poulakis of the Florida Fish and Wildlife Conservation Commission, 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.”
The parthenogen sawfish were all tagged and released back into the wild as part of an ongoing study of sawfish biology and ecology.
The researchers are now encouraging others conducting similar studies to take a few moments to screen their DNA databases from other animals to see if there are hidden parthenogens living in other wild populations.
“This could rewrite the biology textbooks,” says Kevin Feldheim of the Pritzker Laboratory at The Field Museum in Chicago, where the DNA fingerprinting was conducted. “Occasional parthenogenesis may be much more routine in the wild than previously thought.”
Since smalltooth sawfish are so rare, females might sometimes fail to find a male during the mating season, inducing the parthenogenetic process, researchers say.
“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,” Fields says. “Parthenogenesis could help endangered species like sawfish dodge extinction for a little while, but it should also serve as a wake up call that we need serious global efforts to save these animals.”
Source: Stony Brook University