Amazon Molly's Survival: Gene Conversion Keeps an All-Female Clone Species Viable for 100,000 Years

A team led by Wes Warren at Bond Life Sciences Center, University of Missouri, sequenced the genomes of Amazon molly (Poecilia formosa) and its two parental species. They found gene conversion—a DNA-repair process that replaces mutated segments with homologous copies. This repair occurs more frequently in molly than in human genomes, providing a mechanism that counters harmful mutation buildup.

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Amazon mollies reproduce by gynogenesis: females use sperm from a related species to trigger egg development, then discard the male DNA, producing clonal offspring. Evolutionary theory predicts that asexual lineages accumulate deleterious mutations (Muller's ratchet) and should become extinct within a few thousand generations. Models estimated P. formosa would vanish after ~10,000 years, yet it persists in Mexico and Texas for over 100,000 years.

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Sequencing confirmed both parental genomes have accrued mutations, but gene conversion preferentially repairs the most deleterious sites. The two genomes also mutate at different speeds, with one evolving far faster—a pattern that surprised investigators. This dual-genome setup supplies abundant templates for conversion, enabling continuous correction of damaging changes.

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Wes Warren noted the molly “has the best of both worlds – genetic health from sexual reproduction while not needing a male’s DNA.” Edward Ricemeyer emphasized the unexpected mutation-rate disparity and the central role of gene conversion in preserving genome integrity, describing it as a missing piece in the evolutionary puzzle.

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During review, several reviewers doubted the reported mutation-rate differences, stating they “didn’t believe us at first” and demanding more evidence. The authors’ response satisfied the reviewers, leading to publication in *Nature*.

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The contrast between model-based extinction timelines (?10,000 years) and the observed longevity (>100,000 years) reveals a gap in evolutionary theory. Gene conversion offers a plausible explanation, yet the triggers of its heightened activity in mollies remain unknown.

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The findings suggest gene conversion could extend the viability of other asexual organisms and affect long-term genome stability. Future work will test for similar repair mechanisms in additional clonal species and examine conditions that favor such processes.

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• “If a genome is supposed to decay and it doesn’t, why?” — Wes Warren, principal investigator, Bond Life Sciences Center, University of Missouri

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• “This was shocking because it goes against everything scientists thought we understood about mutation rates,” — Edward Ricemeyer, first author

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• “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,” — Edward Ricemeyer

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• “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.” — Edward Ricemeyer

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