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This paper develops simplified mathematical models describing themutation-selection balance for the asexual and sexual replication pathways in{\it Saccharomyces cerevisiae}. We assume diploid genomes consisting of twochromosomes, and we assume that each chromosome is functional if and only ifits base sequence is identical to some master sequence. The growth andreplication of the yeast cells is modeled as a first-order process, withfirst-order growth rate constants that are determined by whether a given genomeconsists of zero, one, or two functional chromosomes. In the asexual pathway,we assume that a given diploid cell divides into two diploids. In the sexualpathway, we assume that a given diploid cell divides into two diploids, each ofwhich then divide into two haploids. The resulting four haploids enter ahaploid pool, where they grow and replicate until they meet another haploidwith which to fuse. When the cost for sex is low, we find that the selectivemating strategy leads to the highest mean fitness of the population, whencompared to all of the other strategies. We also show that, at low tointermediate replication fidelities, sexual replication with random mating hasa higher mean fitness than asexual replication, as long as the cost for sex islow. This is consistent with previous work suggesting that sexual replicationis advantageous at high population densities, low replication rates, andintermediate replication fidelities. The results of this paper also suggestthat {\it S. cerevisiae} switches from asexual to sexual replication whenstressed, because stressful growth conditions provide an opportunity for theyeast to clear out deleterious mutations from their genomes.

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