How an all-female fish species evades extinction

The fish Amazon molly exhibits a unique genetic variability.

A rare all-female fish species — native to the border region of Texas and Mexico— continues to thrive, defying existing theories of evolution which predict that asexually reproducing animals cannot survive in the long run, scientists say.

Species that produce asexually are rare among vertebrates. The Amazon molly (Poecilia formosa) — a small fish species— does not produce any male offspring.

The females reproduce asexually through gynogenesis, making their daughters identical clones of themselves. This type of reproduction also means that they need sperm to trigger the cloning process. So the Amazon molly mates with closely related Molly fish to obtain this sperm. The sperm cells even penetrate the egg cell; however, none of the male’s DNA is incorporated into Molly’s eggs. Rather, the egg completely destroys the male genes.

“According to established theories, this species should no longer exist. It should have long become extinct during the course of evolution,” said Manfred Schartl, from the University of Wurzburg in Germany.