This Fish Species Survived 100,000 Years Without Males

All-female species have been long thought to be evolutionary dead ends. So how has one remarkable fish survived for 100,000 years without males? The answer is revealing new insights into how nature keeps genomes healthy.

In the rivers of Mexico and southern Texas swims a fish that shouldn't exist. In the warm, slow-moving waters, she drifts among her all-female shoal, her silver scales brushing against males of closely related species. It's here that she selects a mate. But in an unusual evolutionary twist, his genes play no part in her offspring. This is a biological heist known as gynogenesis, in which she uses the male's sperm only to trigger egg development, but quickly discards his DNA. She produces only daughters, each a clone of herself.

This fish is the Amazon molly, named after the all-female warrior tribe in Greek mythology, and it has been puzzling scientists for nearly a century. Evolutionary theory suggests that asexual species should quickly die out, as without sex harmful mutations build up in their genomes over time. But this female-only species has persisted for around 100,000 years. By conventional thinking, it should have been a fleeting blip in the tree of life. Yet, this small, unassuming creature endures.

To understand why the Amazon molly's survival without sex is so remarkable, it helps to know: why does sex exist at all?

"Sexual reproduction is a pretty weird and complicated way to reproduce, right?" says Edward Ricemeyer, computational biologist at the Ludwig Maximilian University of Munich in Germany, and co-author of the new study on the Amazon molly. 

Sex is costly, Ricemeyer explains. Individuals must find and compete for a mate, and each parent contributes only half their DNA. Reproduction is often unequal, with females of many species investing far more energy than males in producing, birthing or incubating, and raising offspring.

Asexual reproduction, by contrast, sounds like a much better deal. No need to find (and deal with) a mate, and you can pass on 100% of your genes. Yet across the tree of life, sex – the mixing and recombination of genes from different individuals – is truly dominant.

"If you look at the overall picture, it's 99.9% sex," says Dave Speijer, an evolutionary biologist at the University of Amsterdam, in the Netherlands, specialising in the origins of sexual reproduction.

- Author: Florence Craig, BBC

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