Posted by Jayme Blaschke
University News Service
August 20, 2013
A new study by Texas State researchers published in Proceedings of the National Academy of Sciences (PNAS) sheds new light on the origins of an intriguing species of unisexual fish known as Amazon mollies.
The study, "Population genomics reveals a possible history of backcrossing and recombination in the gynogenetic fish Poecilia formosa," was authored by Laura Alberici da Barbiano, Zachariah Gompert, Andrea Aspbury, Caitlin Gabor and Chris Nice, all members of the Texas State Department of Biology.
The Amazon molly is a clonal species with only one sex--female. Found only in brackish waters along the coast of South Texas and Northern Mexico, Amazon mollies reproduce by mating with male sailfin mollies or Atlantic mollies, but the eggs develop without incorporating any genetic material from the male.
Up until the Texas State study, the Amazon molly species was generally believed to have arisen through a hybrid cross of closely-related molly species nearly 100,000 years ago, said Gabor. What the researchers discovered instead was that the hybrid fish interbred with the parent species for multiple generations before establishing itself as a clonal species, thus ensuring it had far greater genetic diversity than would be expected if the species had arisen from a single, first-generation hybrid.
"This is not first time this hypothesis has been proposed, but this study has more specific data supporting the idea," Gabor said. "It explains why people can make hybrids from the Amazon molly's parent species, but not make Amazons. That indicates why they have more genetic variation."
That genetic variation is key to the persistence of the species. Unisexual species are very rare among vertebrates, and because of the limited genetic diversity, are expected to go extinct fairly quickly. Unisexual reproduction has short-term advantages for a species--less energy is expended for reproduction--but sexual species are more resilient long-term, better able to repair genetic damage because they've inherited DNA from both parents. The surprisingly diverse genetic reservoir of the Amazon molly has served as a sort of buffer, protecting the species from expected chromosomal decay over time.
"Something is going on with the genetic material," Gabor said. "Instead of being strictly clonal, there's more variation among the population. There's some type of recombination in the background. When DNA gets repaired in a sexual species, there are two strands--one from mom and one from dad--that can be used to repair the damage. With a clonal species, when DNA is repaired there's only one strand, so the outcome is different.
"The door you can open up with this are questions about sexual versus asexual reproduction, and about the maintenance of sex," she said. "The Amazons have the asexual way, and they've been able to survive a long time."