The warm waters of Mexico and Texas are home to a small fish that has produced nothing but daughters for over 100,000 years. Essentially, the offspring are the exact genetic copy of their mother, with no father involved. The fish in focus is the Amazon molly.
Evolutionary theories predict that such asexual, clonal organisms should rapidly go extinct due to the accumulation of harmful mutations. Conventional sexual reproduction helps advantageous traits spread and eliminates harmful ones within a species. Since there is no intermixture of DNA with that of their mates, damaging mutations in mollies cannot be purged effectively.
Various models and simulations estimate that the extinction time is fewer than 10,000 years for species without genetic mixing. And yet, Amazon mollies (Poecilia formosa) have survived well beyond those estimations.
“Asexual reproduction alone isn't supposed to work over the long term, but the fish we studied have been reproducing exclusively asexually for over 100,000 years,” says the study co-author, Edward Ricemeyer, in an email to Refractor.
The Amazon molly arose from a single hybrid mating between two sexual species, a female Poecilia mexicana and a male Poecilia latipinna. Although the Amazon molly still needs to interact with the other closely related species’ sperms to trigger embryo development, the male’s DNA does not merge with the egg’s DNA. Therefore, the sperm contributes no genetic material. This has still not led to functional mutational decay. The study, published in Nature last month, “explains how these fish can avoid their predicted fate of extinction,” he adds.
To understand how they defy these grim prognoses, Ricemeyer and his colleagues sequenced full chromosome-level genomes of the Amazon molly and its two parent species. This allowed researchers to track every DNA change that happened since the fish became asexual.
The team found no evidence of accumulation of harmful mutations, even though the Amazon molly has undergone mutations faster than its sexual progenitors. This is because “they use gene conversion to fix problems in their genomes that are normally fixed by sexual reproduction's shuffling of genes,” Ricemeyer told us.
The gene conversion tracts were widespread in the molly’s genomes. 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.
Gene conversion is like a genetic copy-and-paste mechanism to repair DNA. If a harmful mutation occurs on a chromosome, the fish often rewrites it with a healthy version from its other chromosome.
“Gene conversion happens randomly and can make things worse or better,” Ricemeyer explains. “But it generates different versions of the genome so that natural selection has more options to choose from.”
