How an all-female fish species evades extinction

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 can not 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 the 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.

Researchers explored how the Amazon molly has managed to survive in spite of this. They sequenced the genome of the fish species and compared it with that of related species.
There are two main reasons that argue against asexually reproducing species surviving in the long run.

"Harmful changes occur in any genome at some point. In creatures whose offspring are pure clones, these defects would accumulate over generations until there are no more healthy individuals," Schartl said.
Species that reproduce sexually can easily eliminate such defects when the number of chromosomes is reduced by half during formation of egg and sperm cells to be recombined subsequently during fertilization from half of the maternal and paternal chromosomes, respectively.
There is another argument against the long survival of a species whose offspring are all clones of their mothers.
"These species are usually not capable of adapting to environmental changes as quickly as their sexually producing counterparts," Schartl said.

"So within a few generations, they should be on the losing side of evolution which calls for the "survival of the fittest," he said.
To answer the question why this theory does not apply to the Amazon molly, the scientists studied their genome as well as that of two related fish species that reproduce sexually.
"We found little evidence of genetic degeneration in the Amazon molly, but rather a unique genetic variability and clear signs of an ongoing evolutionary process," Schartl said.
Especially the genes relevant for the immune system exhibit a high level of genetic variability in the genome of P formosa.

The study, published in the journal Nature Ecology & Evolution, concluded that this variability combined with a broad immune response essentially contributes to the fact that the Amazon molly does not share the fate of many other species that reproduce asexually, namely to fall victim to pathogens.