An Amazon molly (right), caught in action while seducing a male Sailfin molly to steal sperm. (Dr. Manfred Schartl.)
Thriving by cloning
“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 says the hybridization of the Atlantic and Sailfin mollies’ two different species genomes into a new one created a situation well known in the animal and plant breeding world — an artificial hybrid that is bigger, more colorful and capable of generating more and better products than the purebred parents, a phenomenon known as hybrid vigor.
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The key to successful hybridization
“We found the signature of such hybrid vigor in the genome of the Amazon molly,” Schartl explains. “Moreover, in studying the genomes of individual Amazon mollies from all over their natural range, we detected a considerable genetic variability of clones that have diverged since the single species cross that produced the so-called ‘Eva of the Amazons.’ This genetic diversity was particularly obvious when we looked at the genes that make up the immune system and are required to fight diseases and parasites.”
As impressive as the Amazon molly’s sexual self-reliance is, Schartl cautions that the team’s research leads them to believe it’s less probable than the odds of finding true love.
“The successful hybridization of two genomes from different species appears to be a random event that would require nearly perfect compatible genomic elements to bypass sexual reproduction,” he explains. “The probability that such an event happens is rare, to the extent that successful asexuals emerge only once in thousands of vertebrate species.”
While the Amazon molly adopted an atypical mode of reproduction in borrowing sperm from males of a related species, Schartl says there is another asexual fish that goes one step further.
“These females steal the entire genome of their host males, keep it for one generation and then throw it out again,” he says. “The genome of this fish is target of a collaboration that I started with Texas A&M biologists Mariana Mateos and Gil Rosenthal.”
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 Rosenthal, along with faculty, research scientists and graduate students in the Texas A&M Health Science Center and Texas A&M College of Veterinary Medicine and Biomedical Sciences.
A member of the National Academy of Sciences of Germany and a recipient of the Heisenberg Award from the German Research Foundation, Schartl is globally renowned for his career work explaining the molecular-genetic basis of cancer formation using non-mammalian models and for translating basic evolutionary research into discoveries with clear and direct impacts on human health. He has elucidated key mechanisms in swordtail fish whereby genetic variation generates variation in physical traits and also addressed important questions surrounding the development of melanoma in interspecific hybrids of these fish while taking a lead role in sequencing the first genome in the genus.
The team’s paper, Clonal polymorphism and high heterozygosity in the celibate genome of the Amazon molly, can be viewed online along with related figures and captions.
Media contact: Shana K. Hutchins, (979) 862-1237 or firstname.lastname@example.org or Dr. Manfred Schartl, 931-31-84148 or email@example.com.