Nature
PUBLISHED
A small fish swimming in the waters of Mexico and Texas has caused a big issue for scientists. The Amazon molly is a species that has persisted for over 100,000 years. While that might not sound that unusual, the mollies have done it without the need for male DNA, creating a shoal of entirely female clones.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.The Amazon molly (Poecilia formosa) has survived for so long through a biological process known as gynogenesis. To breed, the female mollies use sperm from a closely related but distinct species to trigger the production of their eggs. Then they jettison the male DNA, producing offspring that are clones of their mothers.
In theory this should work for a while, but without new DNA from sexual reproduction, the idea – known as Muller's ratchet – is that harmful mutations will build up in the genome and eventually cause the species to go extinct. However, because this hasn't happened, researchers are looking into what makes mollies so successful and how they can avoid the dangerous DNA mutations in all of these clones.
“If a genome is supposed to decay and it doesn’t, why?” Wes Warren, a principal investigator in the Bond Life Sciences Center at the University of Missouri, said in a statement. “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.”
Models predict that the molly should have gone extinct after only 10,000 years, not still be thriving more than 100,000 years later.
P. formosa arose as a species through a hybridization event. To learn more about what was going on with their genes, the team looked closely at the genomes of both species that formed the hybrid and the mollies themselves.
The team found that the expected mutations had indeed built up in the genomes of these fish – so how were they managing to survive for so long?
"The theory has been missing a piece," Edward Ricemeyer, who is first author of the study, told BBC Future. "And this piece was gene conversion."
Gene conversion is a form of genetic repair where sections of DNA with mutations can be “fixed” by using copies of the gene as a template. This process seems to happen a lot more frequently in the mollies compared to the same process in humans.
Because the mollies arose from the shared DNA of two species in the form of a hybrid, capable of reproducing without sexual reproduction, they maintained the two genomes they always had through the clones. This gives gene conversion a chance to work on the damaged DNA in cells, since they have two sets of closely related genes to provide the templates.
The team also found that the two genomes of the parent species that gave rise to the mollies were mutating at different rates, with one much faster than the other.
"This was shocking because it goes against everything scientists thought we understood about mutation rates," Ricemeyer said in the statement. "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 kinds of mutations that you expect to be the worst, the most dangerous, the most deleterious, those are the exact places in the genome where we see gene conversion happening the most often," Ricemeyer told the BBC.
The team think that this gene conversion could be happening in other asexual clonally reproducing species and could provide a way to maintain a healthy genome for much longer than previously thought.
"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 is published in Nature.
