New York, Feb 13 (IANS): Bucking the prediction of imminent extinction, a small fish species which does not produce any male offspring, continues to thrive and researchers have now found why it does not share the fate of many other species that reproduce asexually.

This fish species, the Amazon molly (scientific name Poecilia formosa) reproduces 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 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.

To explore how the Amazon molly - native to the border region of Texas and Mexico -- has managed to survive in spite of this, the researchers sequenced the genome of the fish species and compared it with that of related species.

The results, published in the journal Nature Ecology & Evolution, showed no major genetic damages in the Amazon molly.

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 explained.

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 fertilisation 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".

But after studying their genome as well as that of two related fish species that reproduce sexually, the scientists found little evidence of genetic degeneration in the Amazon molly.

Instead, they found a unique genetic variability and clear signs of an ongoing evolutionary process.

Especially the genes relevant for the immune system exhibit a high level of genetic variability in the genome of P. formosa, Schartl said.

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, the study said.