Amazon molly defies evolution

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Scientists have shown that a fish species that has been alive for up to 100,000 years without sex and producing only female young is defying the power of evolution by avoiding extinction.

The study on the Amazon molly Poecilia formosa by Laurence Loewe and Dunja Lamatsch at Edinburgh University used a complex mathematical computer model to show that based on the theory of the evolution of sex this species should have gone extinct at some point in the last 70,000 years.

The Amazon Molly is a small female-only freshwater fish (3-7 cm) found in a limited range from south-east Texas through to Mexico.

The females mate with males of other related species to trigger the reproductive process but the DNA from the male is not incorporated into the offspring.

The females become sexually mature at 1-6 months of age and, providing ~host males are present, they can produce 60-100 fry every 30-40 days.

However an evolutionary theory known as Muller s ratchet states that in a limited population such as this, any deleterious or harmful genetic mutations would be carried forward to the next generation, meaning that eventually this population should have gone extinct.

The authors of the paper think that the mollies must be using special genetic survival tricks to help the species stay alive.

These tricks may include taking occasional paternal undamaged genetic information from the sperm of sexually reproducing related species, slowly segregating out harmful genes, and even the possibility that this is a species that has a very slow rate of mutation.

Dr Loewe is quoted on the BBC news page saying "What we have shown now is that this fish really has something special going on and that some special tricks exist to help this fish survive. Maybe there is still occasional sex with strangers that keeps the species alive. Future research may give us some answers."

"I think one of the interesting things is that we are learning more about how other species might use these tricks as well. It might have a more general importance he said.

Other ancient creatures that reproduce asexually include Darwinula stevensoni, a small non-marine ostracod that has existed for 20 million years and tiny Bdelloid rotifers which have been alive for around 40 million years.

The common name of Amazon molly alludes to the Amazon warriors, a female-run society in Greek mythology. The species does not occur in the Amazon basin.

For more information see Loewe,L and Lamatsch,D: ~Quantifying the threat of extinction from Muller's ratchet in the diploid Amazon molly (Poecilia formosa) BMC Evolutionary Biology 2008, 8:88