University of Missouri researchers have shown how an all-female fish species called the Amazon molly defies the long-held belief that asexual reproduction is an evolutionary dead end.
The key is gene conversion — a process in which one copy of a gene overwrites the other one. Using a technique known as long-read sequencing, Mizzou researchers Wes Warren and Edward Ricemeyer were able to document this process at the genetic level in the Amazon molly for the first time.
Animals that reproduce asexually by cloning themselves have genetic disadvantages. Harmful mutations are expected to accumulate over time, and limited genetic diversity can reduce their ability to adapt — often putting them on a fast track to extinction.
The Amazon molly shatters that assumption. It isn't merely surviving; it's thriving. Now, Mizzou researchers have uncovered the secret behind its evolutionary success.
A blast from the past
The Amazon molly fish first emerged over 100,000 years ago, born from a rare hybrid pairing between two different fish species: a male Poecilia latipinna fish and a female Poecilia mexicana fish. The hybrid has been cloning itself ever since. In 1932, the Amazon molly became the first vertebrate confirmed to be capable of asexual reproduction — a list that today includes about 100 species.
Even then, the discovery raised eyebrows. Prediction models suggested the species should not have survived beyond 10,000 years. So how is it still thriving genetically more than 100,000 years later?
Warren and Ricemeyer have spent more than a decade trying to find out.
In 2018, Warren — now a Curators' Distinguished Professor in the College of Agriculture, Food and Natural Resources and School of Medicine — mapped out the Amazon molly's full genome for the first time, expecting to find the genetic damage left behind by millennia of cloning. Instead, the DNA looked healthy, similar to what scientists would expect to see in a species that reproduces sexually.
Warren speculated that gene conversion was responsible. The process allowed the Amazon molly to preserve and repair DNA inherited from both original parent species, even after tens of thousands of years.
Until recently, however, there was no way to prove his theory.
That changed with the advent of long-read sequencing, which allowed Warren and Ricemeyer to accurately compare the DNA sequences from both of the Amazon molly's parents to accurately measure how they have evolved. They found that the two genomes of the parents were mutating at different rates, with one side mutating faster than the other.
"This was shocking because it goes against everything scientists thought we understood about mutation rates," Ricemeyer, a computational biologist, said. "Normally, mutations are based on what is happening externally to the fish, whether that is changes in environment or population size, so we assumed mutations from both genome sets are occurring at the same rate. To have two genomes be present inside the same cells of the same fish doing two very different things in terms of mutation rates was shocking. When we submitted our work to the journal, the reviewers didn't believe us at first. They were just as surprised as we were, and asked us to provide much more evidence."
Gene conversion seemed to be happening at an optimal rate. Too much gene conversion would limit genetic diversity while too little would allow bad mutations to accumulate. The good genes seemed to be spreading more while the bad genes were being weeded out over time — a process that normally only happens through sexual reproduction.
"If a genome is supposed to decay and it doesn't, why?" Warren, a principal investigator in the Bond Life Sciences Center , said. "As curious researchers, we were excited to find out. This fish seems to have the best of both worlds — the genetic health that normally comes from sexual reproduction while not needing a male's DNA to reproduce."
How we got here
The research reshapes how scientists understand the evolutionary potential of asexual reproduction. While fish are ideal for laboratory study, the discovery could spark future work to see whether other animals that reproduce asexually, such as Komodo dragons and New Mexico whiptail lizards, rely on gene conversion in a similar way.
Advances in genome evolution research have already improved plant and animal breeding. They've helped scientists better understand the causes of genetic diseases, including how genes mutate and repair themselves — work that plays a critical role in cancer treatment.
"Better understanding the different ways that reproduction happens helps us better understand ourselves," Ricemeyer said. "How we got here, and where we may be headed."
The study, "Gene conversion empowers natural selection in a clonal fish species," was published in Nature.
