Quantifying the threat of extinction from Muller's ratchet in the diploid Amazon molly (Poecilia formosa).

Laurence Loewe,Dunja K Lamatsch

doi:10.1186/1471-2148-8-88

Abstract

BackgroundThe Amazon molly (Poecilia formosa) is a small unisexual fish that has been suspected of being threatened by extinction from the stochastic accumulation of slightly deleterious mutations that is caused by Muller's ratchet in non-recombining populations. However, no detailed quantification of the extent of this threat is available.ResultsHere we quantify genomic decay in this fish by using a simple model of Muller's ratchet with the most realistic parameter combinations available employing the evolution@home global computing system. We also describe simple extensions of the standard model of Muller's ratchet that allow us to deal with selfing diploids, triploids and mitotic recombination. We show that Muller's ratchet creates a threat of extinction for the Amazon molly for many biologically realistic parameter combinations. In most cases, extinction is expected to occur within a time frame that is less than previous estimates of the age of the species, leading to a genomic decay paradox.ConclusionHow then does the Amazon molly survive? Several biological processes could individually or in combination solve this genomic decay paradox, including paternal leakage of undamaged DNA from sexual sister species, compensatory mutations and many others. More research is needed to quantify the contribution of these potential solutions towards the survival of the Amazon molly and other (ancient) asexual species.

Highlights

The Amazon molly (Poecilia formosa) is a small unisexual fish that has been suspected of being threatened by extinction from the stochastic accumulation of slightly deleterious mutations that is caused by Muller's ratchet in non-recombining populations
We quantified the rate of mutation accumulation due to Muller's ratchet using the best available analytical approximations [9,27,28] and globally distributed individualbased simulations run by Simulator005r6 of evolution@home [7,29,30,31] assuming our estimates of the most realistic parameter combinations
The results show that values for Usdm, the slightly deleterious mutation rate, that are above Usdm ≈ 0.1, lead to the extinction of the Amazon molly within the estimated Tage = 81,000 years of its existence, even if lower and upper limits for Tage are considered

Summary

Introduction

The Amazon molly (Poecilia formosa) is a small unisexual fish that has been suspected of being threatened by extinction from the stochastic accumulation of slightly deleterious mutations that is caused by Muller's ratchet in non-recombining populations. Ancient asexuals are the rare exceptions to this rule and it is of considerable biological interest to know what mechanisms allow them to survive for so long They must have found a way to overcome the longterm fitness-degrading consequences of genetic processes like Muller's ratchet and/or ecological processes like RedQueen dynamics [1,2,3,4,5,6]. Muller's ratchet describes the long-term accumulation of slightly deleterious mutations in asexual populations and has been suggested as a key mechanism for the extinction of asexual species on the long term [3,7,8] Most of these predictions remain at a stage of verbal argument [1], making it very difficult to rule out that Muller's ratchet may not have had (page number not for citation purposes). We employ a simple null hypothesis [see [7]] for testing the threat of extinction from Muller's ratchet in the unisexual fish Poecilia formosa, the Amazon molly

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Results

Discussion

Conclusion