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

Abstract

This paper develops simplified mathematical models describing the mutation-selection balance for the asexual and sexual replication pathways in Saccharomyces cerevisiae, or Baker's yeast. The simplified models are based on the single-fitness-peak approximation in quasispecies theory. We assume diploid genomes consisting of two chromosomes, and we assume that each chromosome is functional if and only if its base sequence is identical to some master sequence. The growth and replication of the yeast cells is modeled as a first-order process, with first-order growth rate constants that are determined by whether a given genome consists of zero, one, or two functional chromosomes. In the asexual pathway, we assume that a given diploid cell divides into two diploids. For the sake of generality, our model allows for recombination. In the sexual pathway, we assume that a given diploid cell divides into two diploids, each of which then divide into two haploids. The resulting four haploids enter a haploid pool, where they grow and replicate until they meet another haploid with which to fuse. In the sexual pathway, we consider two mating strategies: (1) A selective strategy, where only haploids with functional chromosomes can fuse with one another; (2) A random strategy, where haploids randomly fuse with one another. When the cost for sex is low, we find that the selective mating strategy leads to the highest mean fitness of the population, when compared to all of the other strategies. We also show that, at low to intermediate replication fidelities, sexual replication with random mating has a higher mean fitness than asexual replication, as long as the cost for sex is low. If the fitness penalty for having a defective chromosome is sufficiently high and the cost for sex sufficiently low, then at low to intermediate mutation rates the random mating strategy has a mean fitness that is a factor of √ 2 larger than the asexual mean fitness. This is consistent with previous work suggesting that sexual replication is advantageous at high population densities, low replication rates, and intermediate replication fidelities. The results of this paper also suggest that S. cerevisiae switches from asexual to sexual replication when stressed, because stressful growth conditions provide an opportunity for the yeast to clear out deleterious mutations from their genomes. That being said, our model does not contradict theories for the evolution of sex that argue that sex evolved because it allows a population to more easily adapt to changing conditions.

Standing Genetic Variation Drives Repeatable Experimental Evolution in Outcrossing Populations of Saccharomyces cerevisiae

“…That our selection regime requires both sex and asexual growth creates an additionally complex environment; it is probable that selection favors different alleles in these different phases, perhaps in an antagonistic manner. Clear tradeoffs have been demonstrated between the choice to outcross versus grow vegetatively (Zeyl et al 2005), and each strategy has adaptive advantages in particular environments (Grimberg and Zeyl 2005;Tannenbaum 2008). Ultimately, we present a case of populations adapting to a novel environment, and are not preoccupied with the particular traits under direct selection.…”

Protein disorder reduced in Saccharomyces cerevisiae to survive heat shock

“…Sexual reproduction also appeared crucial for the survival of yeast cultured under heat stress 56 , 57 . Seven of the ten molecular functions to be significantly overrepresented in the heat stress-duplicated chrIII ( Table S3 ) by a standard GO-term enrichment analysis 51 are involved in reproduction.…”