These all-female fish should have gone extinct millennia ago, but genetics had other plans

A biological anomaly that defies mathematical models

In the freshwater streams of Texas and Mexico swims a small creature that is upending the established principles of evolutionary biology. The Amazon molly, known by its scientific name Poecilia formosa, forms a school of fish composed exclusively of female clones. According to genetic prediction models, this species should have gone extinct long ago.

Theoretical calculations estimated that this vertebrate should not have survived beyond 10,000 years. However, observations show that these fish have been thriving for over 100,000 years in their natural environment. This exceptional longevity has prompted researchers to reexamine the fundamental mechanisms of animal survival in the absence of genetic diversity.

The findings of this research, published in the scientific journal Nature, reveal an internal mechanism of remarkable efficiency. The research team, led primarily by scientists from the University of Missouri, had to dissect the animal’s genome to understand how such a lineage could persist without the DNA renewal inherent in conventional reproduction.

Gynogenesis, or the art of doing without males

The Amazon molly’s prolonged survival relies on a very specific reproductive process called gynogenesis. Unlike species that rely on standard sexual reproduction, females of this species do not need to incorporate genetic material from a male to produce a new generation. They produce offspring that are exact clones of themselves.

However, the process still requires interaction with males from distinct but closely related species. The females use the sperm from these males solely as a physical trigger to initiate egg production. Once this stimulation has taken place, the male DNA is simply expelled and destroyed by the mother’s body.

Wes Warren, a principal investigator at the Bond Life Sciences Center at the University of Missouri, commented on this dynamic in a press release from the institution. "If a genome is supposed to degrade and it doesn’t, why not? As curious researchers, we were eager 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 male DNA to reproduce."

Muller’s Ratchet and the Threat of Mutations

In theory, the permanent cloning system should function for only a limited period. The absence of new DNA derived from sexual reproduction exposes the species to an evolutionary phenomenon known as “Muller’s ratchet.” This principle posits that, without genetic mixing, harmful mutations inevitably accumulate in an organism’s genome from one generation to the next.

This accumulation of genetic defects should logically undermine the population’s viability and ultimately lead to the species’ total extinction. Scientists who analyzed the Amazon molly did indeed find that the expected mutations had indeed accumulated in the genome of these fish. The paradox of their survival therefore remained intact in the face of this measurable deterioration.

To solve this puzzle, the team turned its attention to internal cellular repair processes. As reported by the British media outlet BBC Future, Edward Ricemeyer, the study’s lead author, highlighted the flaw in existing models. “There was a missing piece to the theory,” he said. “And that piece was gene conversion.”

A dual genetic safeguard resulting from hybridization

The emergence of Poecilia formosa as a species stems from an initial hybridization event. This allelic fish arose from the crossbreeding and DNA exchange between two distinct parent species. Because they emerged as hybrids capable of asexual reproduction, mollies have preserved both original genomes intact across all their generations of clones.

This genomic duality offers a decisive advantage during the process of gene conversion. This is a form of genetic repair in which sections of DNA carrying mutations can be corrected using copies of the healthy gene as a template. With two closely related sets of genes, the cells of the Amazon molly have a constant supply of replacement templates available to repair damaged DNA. This process occurs at a significantly higher rate in these fish than in humans.

The precision of this defense mechanism particularly surprised researchers. During his interview with the BBC, Edward Ricemeyer detailed the location of these repairs: “The types of mutations you’d expect to be the worst—the most dangerous, the most harmful—are the exact locations in the genome where we see gene conversion occurring most frequently.”

Asymmetric mutation rates and new perspectives

By scrutinizing the genomes of the two parent species of this hybrid, the scientific team made an unexpected discovery regarding the rate of genetic changes. They observed that the two genomes inherited by the mollies were mutating at completely different rates, with one deteriorating much more rapidly than the other.

This observation initially raised doubts within the scientific community. “It was shocking because it goes against everything scientists thought they understood about mutation rates,” Edward Ricemeyer said in the university’s press release. “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.”

The findings from the study of the Amazon molly could extend beyond the field of marine biology. Scientists believe this genetic conversion could occur in other species that reproduce through asexual cloning, offering a way to maintain a healthy genome for much longer than previously thought. "Better understanding the different ways reproduction occurs helps us better understand ourselves," concluded the study’s lead author. "How we got here, and where we might be headed."

Source: iflscience.com

These all-female fish should have gone extinct millennia ago, but genetics had other plans