The Amazon molly, a small freshwater fish found in Mexico and southern Texas, has puzzled scientists for decades. Unlike nearly all vertebrates, it reproduces asexually through a process called gynogenesis — using sperm from males of related species only to trigger egg development, while discarding their DNA. This means every offspring is a clone, raising concerns about long-term survival due to the risk of harmful mutations piling up over generations, a phenomenon known as Muller's ratchet.

Yet the Amazon molly has thrived for approximately 100,000 years, defying evolutionary expectations. A new study reveals that the species relies on a genetic mechanism called gene conversion — a 'copy-and-paste' repair process — to maintain genome health. By using one gene copy as a template to fix another, the molly reduces the accumulation of dangerous mutations, effectively substituting for the genetic reshuffling usually provided by sexual reproduction.

The fish originated from a hybrid cross between an Atlantic molly and a sailfin molly around 100,000 years ago, giving it unusually high genetic variation from the start. This diversity supports frequent and targeted gene conversion, especially in the most vulnerable parts of the genome. While other asexual species like bdelloid rotifers have also persisted for millions of years, the Amazon molly offers a rare vertebrate example of how life can bypass the need for sex. The findings may also inform research on mutation-related diseases like cancer.