The evolution of sex: Biologists study Texas fish that reproduces asexually

Two small fish

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No species is immune from the suffering of unrequited love, but scientists expect to learn volumes about the biological basis of sex from the newly sequenced genome of an all-female, asexual Texas native—the Amazon molly—that has thrived as a master of male manipulation over millennia.

The fresh waters along the Texas-Mexico border serve as home to this evolutionary anomaly — a fish that has flourished by defying nature’s odds to reproduce asexually through a natural form known as parthenogenesis in which growth and development of embryos occurs without fertilization, resulting only in daughters that are true clones of their mothers.

Texas A&M University Hagler Institute for Advanced Study Faculty Fellow Manfred Schartl led the international team that recently sequenced the first Amazon molly genome and the genomes of the original parental species that created this unique fish in an effort to better understand how its reproduction deviates from the male-female sexual norm and why the Amazon molly as a species has fared so well in the process.

The findings from their National Institutes of Health-funded research are published online Feb. 12 in the Nature research journal Nature Ecology & Evolution.

“The existence of two sexes, male and female, is one of the oldest and most widespread phenomena in biology,” says Schartl, a world leader in cellular and molecular biology of Xiphophorus model systems including platyfish and swordtails. “Studies on the exceptional case of asexuality helps us to better understand the biological meaning and evolution of sex.”

Animals that reproduce asexually are rare, compared to the overwhelming majority of species that exist as males and females and reproduce sexually. Because it was long thought that vertebrates would not be able to exist in such a way, Schartl says it was a sensation when the Amazon molly was the first asexual vertebrate discovered in 1932.

But even the most independent females occasionally need a male — in the Amazon molly’s case, to kick-start the parthenogenensis process. They seduce males from related sexual species for this service, which Schartl notes lacks the regular benefit for these males, which do not contribute their genes to the next generation.

“In essence, mollies repeatedly clone themselves by duping the male fish of another species to waste their germplasm,” Schartl says. “This reminds one of the tribe of female warriors in the Greek mythology, from which their name is derived.”

The team’s research traces the existence of Amazon mollies back anywhere from 100,000 to 200,000 years ago to a sexual reproduction event involving two different species of fish, an Atlantic molly and a Sailfin molly.

“That’s about 500,000 generations if you calculate it out to the present day, which makes them genetically older than humans,” Schartl says. “This is unexpected because asexuals are expected to be at disadvantage compared to their sexual counterparts.”

Schartl notes that one of the theories as to why asexual reproduction is incompatible with a species’ sustainability is the idea that if no new DNA is introduced during reproduction, then harmful gene mutations can accumulate over successive generations, leading to eventual extinction. Another hypothesis states that asexual reproduction is not like sexual reproduction, where the different genomes of the two parents are newly combined and create new genomes with every offspring. Because the absence of recombination in asexuals limits genetic diversity within a species, he says it gets more and more difficult to adapt to changes in the environment.

“Unexpectedly, we did not find the signs of genomic decay as predicted,” Schartl adds. “Our findings suggest that the molly’s thriving existence can be explained by the fact that the fish has a hardy genetic makeup that is often rare in nature and gives the animals some survival benefits.”

Schartl, professor and head of the Department of Physiological Chemistry at the University of Würzburg, Germany, is a visiting professor in the Texas A&M Department of Biology, where he continues to collaborate with faculty, research scientists and graduate students in the Health Science Center and the College of Veterinary Medicine and Biomedical Sciences.

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