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

TL;DR

This study uses simplified mathematical models to compare sexual and asexual yeast reproduction, showing sexual strategies can increase population fitness under certain conditions, especially when the cost of sex is low.

Contribution

It introduces a mathematical framework for analyzing mutation-selection balance in yeast, highlighting conditions favoring sexual over asexual reproduction.

Findings

Sexual reproduction with low cost yields higher mean fitness.

Sexual reproduction outperforms asexual at low to intermediate fidelities.

Yeast switches to sexual reproduction under stress to remove deleterious mutations.

Abstract

This paper develops simplified mathematical models describing the mutation-selection balance for the asexual and sexual replication pathways in {\it Saccharomyces cerevisiae}. 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. 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. 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. 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 {\it 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.