Facultative parthenogenesis is a term used to describe what happens when a species that normally reproduces sexually undergoes asexual reproduction. This is different from obligate parthenogenesis, where females normally reproduce without the benefit of a mate.
It is believed that facultative parthenogenesis occurs when viable mates are few and far between. And this appears to be the case with the smalltooth sawfish, Pristis pectinata. The smalltooth sawfish is sometimes known as the wide sawfish, and is a member of the ray family.
Found mainly in a few locations in Southern Florida, including the Caloosahatchee and Peace Rivers, this particular family of marine animals may be the first to be driven to extinction due to over-fishing and habitat loss.
Demian Chapman of Stony Brook University in New York and scientists from the Pritzker Laboratory at the Field Museum of Chicago and Florida Fish and Wildlife Conservation Commission discovered the parthenogenesis in sawfish while doing routine DNA fingerprinting of the fish.
According to Andrew Fields, a PhD candidate at the Stony Brook University’s School of Marine and Atmospheric Science, “We were conducting routine DNA fingerprinting of the sawfish found in this area in order to see if relatives were often reproducing with relatives due to their small population size. What the DNA fingerprints told us was altogether more surprising: female sawfish were sometimes reproducing without even mating.”
“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.
And Poulakis is dead-on in this observation. When parthenogenesis does occur, it is thought that an unfertilized egg absorbs a genetically identical sister cell. The resulting offspring has only about half the mother’s DNA, and usually dies soon after birth.
The one thing different about the seven offspring found by the researchers all appeared to be well and healthy. All of them were tagged for future study purposes and released back into the wild. Kevin Feldman, with Pritzker Laboratory where the DNA fingerprinting was done, said, “Occasional parthenogenesis may be much more routine in wild animal populations than we ever thought.”
While the sawfish might be helping to sustain a critically endangered species, all the offspring from this type of parthenogenesis are females. Fields points out, “It will skew the sex ratio because their offspring will always be female, so you’d never have sexual reproduction again, once all the males died off.” He adds that genetic diversity is reduced, in turn impacting on the animal’s ability to adapt to changing environmental conditions around them.
This study was published in the online journal Current Biology on June 1, 2015. The title is: “Facultative parthenogenesis in a critically endangered wild vertebrate”
