The genome of the asexual fish, the Amazon molly, reveals remarkable good health, reports a study published online this week in Nature Ecology & Evolution. This finding is unexpected, as asexual reproduction is assumed to cause genomes to decay.

Theory predicts several disadvantages of asexual reproduction, such as the accumulation of harmful mutations which lead to genomic decay and eventual extinction. Another challenge is that asexual organisms lack the genetic diversity generated through sexual reproduction, which is important for adapting to new environmental conditions. Organisms that reproduce asexually are thus often considered a biological paradox.

Asexual vertebrates are extremely rare. There are only about 50 naturally occurring fish, amphibian, and reptile species that reproduce asexually. The Amazon molly, Poecilia formosa, was the first all-female vertebrate species described back in 1932. Its common name refers to the Amazons, the all-women warriors of Greek mythology.

The Amazon molly is a hybrid of two distantly related sexual species, and its evolutionary and ecological successes are remarkable - the molly has a long existence and has colonized diverse habitats over a wide geographical range.

Manfred Schartl, and colleagues sequenced the Amazon molly genome and, contrary to expectations, found few harmful mutations, little genetic decay, and a high degree of genetic diversity. The molly genome also reveals a remarkable level of variability in genes relating to immunity. The authors argue that the combination of genetic diversity and broad immune defenses might have allowed the Amazon molly to escape the common fate of asexual organisms - being an easy target for pathogens.