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Muller's ratchet quantified in the Amazon molly

The Amazon molly has long been suspected of being threatened with extinction by Muller's ratchet. Now evolution@home has found that indeed there is a genomic decay paradox in this fish. Thus the search for processes that keep this fish alive is important.

Muller's ratchet quantified in the Amazon molly

Amazon Molly fish are all female (Picture credit: Dunja K Lamatsch)

Amazon Molly fish, which are all female, interact sexually with males of other species in order to trigger their reproduction process. The males' sperm activates development of an embryo, but usually none of the male DNA is passed on to the offspring. Only the mother's genes are inherited. On those rare occasions where male DNA enters the egg, it is stably inherited in separate chromosomes, but the normal mechanisms of recombination are still missing. Hence, from an evolutionary point of view, the Amazon molly should be like any other non-recombining, asexual organism.

When creatures reproduce asexually Muller's ratchet should slowly degrade the genome, because its genes accumulate slightly harmful DNA changes over many generations. Such species will eventually have problems reproducing, and often fall victim to extinction.

Researchers believe Amazon mollies, which live in rivers in south-east Texas and north-east Mexico have originated around 70,000 years ago when a hybridization event between two sexual sister species produced the genome of what we now know as the Amazon molly.

This time seemed too long for Manfred Schartl of the University of Würzburg, Germany. Based on general theoretical work he suspected that the Amazon molly could not survive for so long without some additional help. In 1995 he and his group also identified a possible way out of this dilemma. As reported by Nature and New Scientist, the rare entry of paternal DNA from the sexual sister species into the egg of the Amazon molly could repair damaged genes.

However, the application of the theoretical work on Muller's ratchet to the Amazon molly was not rigorous, because the corresponding theoretical study was concerned with general patterns on a smaller scale and did not test what would have happened in the case of the Amazon molly. For example one might have speculated that the same simulations could have shown that the Amazon molly would survive Muller's ratchet for more than 100,000 years, if realistic population sizes would have been used. Testing realistic population sizes for the Amazon molly was not feasible in 1995, due to the extraordinary computational complexity of this task. Thus, there was the possibility that no genomic decay paradox existed in this species and that we would not have expected it to go extinct.

To test this possibility, evolution@home used its Simulator005 to explore models of Amazon molly evolution that are realistic for this fish. To do this, Dr. Dunja Lamatsch (at the time in Manfred Schartl's group at the University of Würzburg) helped to compile all the data that was needed to build a realistic model of Amazon molly evolution under the assumption that no external DNA from sexual sister species contributes to survival. 

Evolution@home has now calculated the time to extinction for the fish, based on modelling genetic changes over many thousands of generation. The results have recently been published in "BMC Evolutionary Biology" and are freely available for download (PDF).

The results basically confirm that the Amazon molly should indeed have gone to extinction within the time of its existence, so a genomic decay paradox exists for this species when using a simple model of Muller's ratchet with realistic values. This means that some escape mechanisms exist that are not captured in this simple model and the search for a better, more realistic model is important.

As far as one can tell, the Amazon mollies are employing some genetic survival tricks. One such trick may be indeed the occasional use of DNA from the males that trigger development, in order to refresh their gene pool and stay alive. But there are many other factors that could contribute to survival, including advantageous DNA changes, which have been ignored so far.

More research is needed to explain how exactly the Amazon molly survives. Such research is important, since it may help us to understand some basic aspects of evolution that probably play a role in the survival of other asexual species and could even be important for sexual species.

Dr. Lamatsch, now at the Institute for Limnology at the Austrian Academy of Sciences, agrees: "We need more detailed measurements of how the Amazon molly evolves."

Evolution@home plans to eventually return modelling efforts to the Amazon molly to test various potential explanations for how it manages to swim against the evolutionary tide that drives other asexual species so quickly to extinction.

 

More information

The original scientific report that is freely available:

  • Loewe L & Lamatsch D (2008) "Quantifying the threat of extinction from Muller's ratchet in the diploid Amazon molly (Poecilia formosa)" BMC Evolutionary Biology 8:88   PDF | PubMed | DOI | Journal | ISI Web   

 

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