Fish wards off harmful mutations with DNA repair trick
The Amazon molly is an evolutionary enigma: an all-female fish that reproduces through cloning. Because it does not mix its DNA with that of a partner, Darwinian logic holds that harmful mutations should accumulate over time, ultimately leading to the extinction of the species.
Yet the Molly (Poecilia formosa) stubbornly refused to follow this scenario. This seems to be due to the fact the fish‘The genome of humanity is repeatedly rewrittenthe researchers report on March 11 in Nature. It uses a copy-and-paste DNA repair mechanism called gene conversion, in which small portions of genetic code are copied from one chromosome to another. These genetic transfers can erase unfavorable mutations and sometimes spread beneficial mutations, allowing natural selection to continue to function.
“It works against this accumulation of mutations,” says Wes Warren, a comparative genomicist at the University of Missouri in Columbia.
The results provide the first evidence that gene conversion can counteract the accumulation of mutations in an asexual vertebrate, that is, one whose offspring are essentially genetic copies, or clones, of the parent.
“If these patterns hold largely true, it suggests that we may need to treat DNA repair and gene conversion as part of the toolbox that shapes the evolutionary fate of clonal species,” says Waldir Miron Berbel-Filho, an evolutionary biologist at the University of West Florida in Pensacola, who was not involved in the research but wrote a paper. accompanying comment in the same number of Nature.
The Amazon molly has thrived for more than 100,000 years in warm rivers and lagoons along the Mexico-Texas border. Although they still rely on males of closely related species to trigger egg development, females propagate clonally with little sign of genetic degradation.
Warren and his colleagues published a version of the fish genome for the first time in 2018. But, due to the limitations of DNA sequencing technologies available at the time, they couldn’t say exactly how the genome controlled its mutations.
The team needed newer sequencing tools that could cleanly separate the fish’s paired chromosomes, each from a different ancestral species that mated long ago to produce the first of this unusual hybrid lineage. Such tools make it possible to study mutations in a much finer manner and to identify the hidden processes that shape the genome over time.
Once this technology became available, Warren and his colleagues reexamined it. Now, researchers could clearly see telltale patterns of genetic conversion occurring across the genome, echoing results others have observed in simpler organisms, including rotifers, water fleas, mites and flatworms.
By generating new pockets of genetic variation, the process “gives natural selection something to act on,” says computational biologist Edward Ricemeyer of the Ludwig Maximilian University of Munich.
However, genetic conversion does more than eliminate harmful mutations and correct beneficial variants. The chromosome changes also appear to iron out genetic mismatches that can give rise to hybrid incompatibility, a situation in which genes inherited from two different parental species don’t work well together.
Mechanism is not a perfect substitute for sex. The molly always accumulates mutations more quickly than a sexually reproducing species would. And the process cannot generate the flood of new gene combinations produced by sexual reproduction.
Still, the findings suggest that the fate of asexual species may not be as bleak as previously thought – and could prompt scientists to refine their thinking about the problem. evolutionary logic behind sexnotes Anne-Marie Dion-Côté, an evolutionary geneticist at the University of Moncton in New Brunswick, Canada, who did not participate in the study.
“It really forces us to think outside the box and out of the textbook,” she says.
