The speed of Muller's ratchet with background selection, and the degeneration of Y chromosomes

Abstract

The rate of accumulation of deleterious mutations by Muller's ratchet is investigated in large asexual haploid populations, for a range of parameters with potential biological relevance. The rate of this process is studied by considering a very simple model in which mutations can have two types of effect : either strongly deleterious or mildly deleterious. It is shown that the rate of accumulation of mildly deleterious mutations can be greatly increased by the presence of strongly deleterious mutations, and that this can be predicted from the associated reduction in effective population size (the background selection effect). We also examine the rate of the ratchet when there are two classes of mutation of similar but unequal effects on fitness. The accuracy of analytical approximations for the rate of this process is analysed. Its possible role in causing the degeneration of Y and neo-Y chromosomes is discussed in the light of our present knowledge of deleterious mutation rates and selection coefficients.

Muller's Ratchet and the Degeneration of Y Chromosomes: A Simulation Study

“…With the parameter combinations that I tested, the effect can be substantial, accelerating the ratchet by a factor of up to 10 when there are twice as many BS loci as focal loci. Results of a similar order of magnitude have been reported in a previous theoretical study of the effect of background selection on the rate of Muller's ratchet in an asexual haploid population (Gordo and Charlesworth 2001).…”

“…What is the effect of background selection on the speed of Muller's ratchet? It has been demonstrated previously that background selection can accelerate the ratchet in asexual haploid populations (Gordo and Charlesworth 2001), and I confirm this effect for the case of sexual diploid populations. 4.…”

The population genetics of mutations: good, bad and indifferent

“…recombination, back mutations or advantageous mutations). Indeed Charlesworth et al (1993b), has explored related scenarios with low levels of recombination under the label 'mutation accumulation' and has also used the classical theory of MR without recombination to quantify processes that can degenerate Y-chromosomes (Charlesworth 1978;Charlesworth & Charlesworth 2000;Gordo & Charlesworth 2001). If selection coefficients are further reduced with all else being equal, mutations will accumulate faster, eventually switching from the slow regime to the 'fast regime', where mutation accumulation is the norm rather than the rare exception.…”

“…U, s, r e , see table 1) are such that the expected critical number of optimal genotypes falls below one and any move of the population towards equilibrium will also bring the best existing genotype closer to extinction or push it over the edge. While the resulting mutation accumulation is still a random process, it is now deterministically unavoidable and hence it has also been called 'quasi-deterministic' (Gordo & Charlesworth 2001). This regime was first described explicitly in the context of the classical theory of MR (Gessler 1995) and subsequently in settings that also allow for advantageous mutations (Desai & Fisher 2007;Rouzine et al 2008).…”

“…To do so, it is important to integrate the linkage theories that deal with different mutational effects (table 2), as these usually span many orders of magnitude in real organisms. Charlesworth has contributed to such integration by considering the effects of BGS in combination with MR (Gordo & Charlesworth 2001) and investigating a wide range of selection coefficients in simulation models with back mutation and recombination (McVean & Charlesworth 2000;Kaiser & Charlesworth 2009). In principle, general multi-locus models provide a way of integrating all the aspects of evolution discussed above (Kirkpatrick et al 2002), but these need large numbers of parameters to be specified.…”