Amazon Mollies: The Asexual Fish That Defies Evolutionary Theory

When an Amazon molly says she doesn't need a man, believe her. Every single one of these small fish, found in freshwater streams in Mexico and Southern Texas, is female, and reproduces exclusively asexually—birthing clones of itself.

As enviable as that is, the fish presents an evolutionary paradox. According to evolutionary theory, at least some sexual reproduction is necessary to keep a species genetically diverse, and therefore less likely to go extinct. So how have these fish managed to thrive for millennia?

According to a new study published in the journal Nature, Amazon mollies have not suffered the predicted negative effects of asexual reproduction. What's more, they seem to have a secret weapon to counteract the disadvantages of asexuality. The findings shed new light on how Amazon mollies and other asexual species avoid extinction.

Amazon mollies are small, round-finned fish no bigger than a thumb. Named after the female warrior race from Greek mythology, these fish first appeared around 100,000 years ago after a female Atlantic molly took a liking to a male Sailfin molly. While couplings between members of different species usually produce infertile offspring, this one gave rise to a species capable of giving live birth to carbon copies of itself. These fish do need to have sex with male mollies of other species to trigger their self-cloning, a process known as gynogenesis, but their offspring never carry the DNA of those males.

When Amazon mollies were discovered in 1932, they were the first vertebrates known to be capable of asexual reproduction. While dozens of vertebrates have since been discovered to possess the same power, including Komodo dragons and hammerhead sharks, Amazon mollies are one of the only vertebrates to do so exclusively.

How they’ve managed to do so has long been a mystery. According to current models of how genetic mutations accumulate over time in asexual reproduction, Amazon mollies “should have gone extinct after 10,000 years or so,” says Edward Ricemeyer, computational biologist at the Ludwig Maximilian University of Munich and co-author of the new study. “The fact that they have been around for much longer than that presents this paradox."

Ricemeyer, who was a research scientist at the University of Missouri when he began studying Amazon mollies in 2019, says scientists still don’t know how asexual species, especially complex ones like vertebrates, don’t rack up harmful mutations, given that natural selection isn’t constantly weeding them out.

To find out, Ricemeyer and his colleagues examined the genomes of several Amazon mollies and discovered that they’ve been doing their own genetic housekeeping for tens of thousands of years.

According to their genetic analysis, harmful mutations pop up in Amazon mollies just as much as they do in their sexually reproducing cousins. However, Amazon mollies seem to be making use of a little known genetic process to ensure such mutations are purged or corrected.

This process, known as gene conversion, works by replacing a segment of chromosomal DNA with a corresponding sequence copied from a similar sequence on another chromosome. Mammals, including humans, have this capability, primarily to repair DNA damage.

But for Amazon mollies, gene conversion seems to serve the same role as a phenomenon called crossing-over recombination, the method by which genes from a mother and father are mixed in sexually reproducing species. Gene conversion, like crossing-over recombination, creates genetic variability in Amazon mollies that natural selection can act upon, resulting in the removal and repair of undesirable mutations.

Scientists suspected that asexual species might have such a workaround, “but this is the first time it has actually been shown to be happening,” says Ricemeyer.

What Ricemeyer and his colleagues found in the genomes of Amazon mollies “is probably what's going on [in other asexual species], they just haven't shown it yet,” says Micah Dunthorn, a microbiologist and professor at the University of Oslo.

Dunthorn, who was not involved with the study, says he’d like to see similar studies done on other asexual species. “It would be interesting to see how widespread this is within other animals, plants, and fungi, and then also if it's happening in microbial eukaryotes or protists.”

It’s unclear if all asexual species utilize similar genetic tools, and both Ricemeyer and Dunthorn hope future research will answer that question. Of the thousands of species known to reproduce through cloning, scientists have only conducted detailed studies of a handful at the level of the genome. It’s possible, Ricemeyer says, that nature has come up with multiple ways to deal with the costs of asexual reproduction, but we won’t know what they are until we take the time to study them.

Understanding these genetic forces “could have many applications,” says Ricemeyer, from genetically modifying crops to treating. Cancer, Ricemeyer notes, “is a disease in which a clonal lineage of cells accumulates mutations that cause it to outgrow and outcompete unmutated lineages.” While Amazon mollies are fascinating in their own right, their cloning prowess may teach us something about combatting a key human health threat.