No sex for all-female fish species

The Amazon molly does not produce any male offspring. The females reproduce asexually through gynogenesis. Nevertheless they need sperm to trigger the cloning process.

Species that produce asexually are rare among vertebrates, making the Amazon molly (Poecilia formosa) the big exception. The small fish species, who is native to the border region of Texas and Mexico, 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," Manfred Schartl explains. The biochemist holds the Chair of Physiological Chemistry at the Biocenter of the University of Würzburg. Schartl with an international team of researchers explored how the Amazon molly has managed to survive in spite of this. For this purpose, the researchers sequenced the genome of the fish species and compared it with that of related species. The results of their research are published in the current issue of the journal Nature Ecology & Evolution.

Contradictory to established theories

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 explains. 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 says. So within a few generations, they should be on the losing side of evolution which calls for the "survival of the fittest."

Unique genetic variability

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. The main insight: "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," Manfred Schartl says and he continues to explain that especially the genes relevant for the immune system exhibit a high level of genetic variability in the genome of P. formosa . From this the authors of the study conclude 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.

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