Gene Conversion Helps Amazon Molly Persist Asexually for ~100,000 Years, Study Finds

The Amazon molly, an all-female fish species living in the rivers of Mexico and southern Texas, has persisted for around 100,000 years without males. It reproduces via gynogenesis, using sperm from males of related species only to trigger egg development while discarding the male DNA and producing only daughters that are clones of the mother.

A new study used whole-genome sequencing to compare the DNA of Amazon mollies across generations.

The research found that sections of the species' DNA have been repeatedly overwritten by gene conversion occurring more frequently than in most other animals. Edward Ricemeyer, computational biologist at the Ludwig Maximilian University of Munich in Germany and co-author of the study, said gene conversion is doing something similar for the molly's genome to what sex does for sexual species by limiting the accumulation of harmful mutations.

He said gene conversion occurs more often in parts of the genome where the most deleterious mutations are expected.

"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 said. The Amazon molly arose from a hybridization event around 100,000 years ago between a female Atlantic molly and a male sailfin molly.

It carries genetic material from both ancestral species, providing high genetic variation from the outset.

Male sailfin mollies can provide the Amazon molly with sperm but cannot pass their genes to the offspring. Gene conversion is a form of genetic repair that uses one copy of a gene as a template to repair the other and occurs in many organisms including humans. "The theory has been missing a piece.

And this piece was gene conversion," Ricemeyer said. "We thought sexual reproduction was the only proper way to keep a genome healthy… But now we found out that no, there's another way too. " In sexual species, harmful mutations can be shuffled out of the gene pool via recombination, but in clonal species they accumulate according to Muller's ratchet.

Bdelloid rotifers have existed for tens of millions of years without males. Chiara Boschetti, a leading rotifer expert and zoology lecturer at the University of Plymouth in the UK, said bdelloid rotifers acquire DNA from unrelated organisms via horizontal gene transfer. They can survive being dried, cooked, frozen for 24,000 years in Siberian permafrost, and spaceflight.

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