The advantage of sex in evolving yeast populations

Abstract

Sex is a general feature of the life cycle of eukaryotes. It is not universal, however, as many organisms seem to lack sex entirely. The widespread occurrence of sex is puzzling, both because meiotic recombination can disrupt co-adapted combinations of genes, and because it halves the potential rate of reproduction in organisms with strongly differentiated male and female gametes. Most attempts to explain the maintenance of sexuality invoke differences between parents and sexual offspring. These differences may be advantageous in novel or changing environments if new gene combinations are favoured from time to time. Sex would then serve to concentrate beneficial mutations that have arisen independently into the same line of descent. But in a stable environment sex might serve to concentrate deleterious mutations, so that they will be more effectively purged from the population by selection. We have studied the effect of sex on mean fitness in experimental populations of the budding yeast Saccharomyces cerevisiae. Our results show that sex increases mean fitness in an environment to which the populations were well adapted, but not in an environment to which new adaptation occurred, supporting the hypothesis that the advantage of sexuality lay in the removal of deleterious mutations.

A comparison of sexual and asexual replication strategies in a simplified model based on the yeast life cycle

“…If the mutation rate in yeast is not too low, then sex can become the advantageous strategy. This explanation is consistent with experimental work on yeast suggesting that sexual replication in yeast provides a mechanism for removing deleterious mutations from the yeast genome [16].…”

“…As we have mentioned previously, this view is not necessarily contradictory to the results of [16], which argue that sex is a way of removing deleterious mutations. In our opinion, at intermediate mutation rates, and when the cost for sex is sufficiently low, there is an advantage for sex that holds in static environments and in an infinite population.…”

A de novo originated gene depresses budding yeast mating pathway and is repressed by the protein encoded by its antisense strand

“…Yeasts can reproduce both sexually and asexually (facultative sex); selective forces might have favored either vegetative fitness or mating ability under different conditions and a negative correlation between these two traits might exist [28]. It is one of nature's wonders to recruit a new component Mdf1p into the mating pathway to make yeast better able to balance the gain and cost of these two physiological phenomena.…”

“…In fact, in contrast to the superiority of the M + A − strain in rich medium, Mdf1p is unable to promote growth in nonfermentative medium (Supplementary information, Figure S6B), in which sexual reproduction is advantageous [28]. Hence, the non-mating haploid M + A − strain may not be very good at coping with harsh nutri-…”

Mutualism and asexual reproduction influence recognition genes in a fungal symbiont

“…This costly reproductive strategy could be maintained because recombination acts to provide advantageous genotypes necessary for adaptation to changing environments or because recombination acts to eliminate deleterious mutations (e.g., Zeyl and Bell 1997;Taylor et al 1999;Neiman et al 2010). Both of these hypotheses are consistent with the fact that the absence of sexual reproduction decreases the overall fitness of an organism and could ultimately lead to extinction (Butlin 2006;Paland and Lynch 2006;Howe and Denver 2008).…”