All Female Fish Defies Evolution by Cloning Itself for 100000 Years

A Genetic Miracle in Texas Waters

Deep in the warm rivers along the Mexico-Texas border lives a fish that shouldn't exist. The Amazon molly resulted from a sexual reproduction event involving two different species of fish, when an Atlantic molly first mated with a Sailfin molly 100,000 to 200,000 years ago. The offspring of this cross-species coupling ought to have been sterile, like a mule. But this particular hybrid went on to birth a brood of daughters—all of which were genetic clones of their mother.

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. That's about 500,000 generations if you calculate it out to the present day. The expectation is that many harmful mutations would accumulate in that time, but that's not what we found.

The Amazon molly reproduces by "mating" with a male fish of a related species. But the male's DNA is not incorporated into the offspring. Instead, mating with the male fish triggers the replication of the entire maternal genome. In essence, mollies clone themselves.

The DNA Copy-Paste Solution

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. This discovery launched scientists on a quest to understand how these fish avoided genetic doom.

Using long-read sequencing, researchers identified a process called gene conversion, in which one copy of a gene can overwrite another, helping to repair DNA as the fish reproduces. The findings, published in Nature, offer an explanation for how this species has persisted longer than expected. It uses a copy-and-paste DNA repair mechanism called gene conversion, in which small stretches of genetic code are copied from one chromosome to another. These genetic handoffs can erase unfavorable mutations and sometimes spread beneficial ones, allowing natural selection to keep operating.

Researchers also discovered that the gene conversion was 10.6 times more likely to revert a new mutation back to the ancestral state than to spread it, thereby efficiently purging the harmful impacts. If a harmful mutation occurs on a chromosome, the fish often rewrites it with a healthy version from its other chromosome.

Rewriting Evolution's Rules

The results provide the first evidence that gene conversion can counteract mutation buildup in an asexual vertebrate — that is, one whose offspring are essentially genetic copies, or clones, of the parent. This discovery challenges fundamental assumptions about why sexual reproduction evolved and dominates life on Earth.

Even with this sophisticated DNA repair strategy, the Amazon molly does not escape all evolutionary challenges. The species still accumulates mutations faster than animals that reproduce sexually. And gene conversion cannot generate the vast number of new gene combinations that arise when two parents mix their DNA. Yet the fish has found a remarkable workaround that keeps its genome stable enough to thrive.

Beyond Fish Biology

If gene conversion can help preserve genetic health in one long-lived clonal vertebrate, it raises a natural question: do other asexual (or occasionally asexual) animals use similar tricks? The researchers point to species like Komodo dragons and New Mexico whiptail lizards as examples where future work could test whether gene conversion plays a similar role.

Discovering these mechanisms could have implications beyond fish biology, potentially informing human genetics and our understanding of diseases like cancer, which arise from accumulated mutations. 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. The Amazon molly proves that evolution's toolkit contains more surprises than scientists previously imagined, opening new avenues for understanding how life maintains itself against genetic decay.