Origin of sex for error repair II. Rarity and extreme environments

In a previous paper we studied the simultaneous, and at times conflicting, needs of coping with DNA damage, efficient cell replication, and the avoidance of cell mortality. These selective factors operated on sexual and asexual haploid and diploid populations that were reproductively isolated from one another. We concluded, in part, that a sexual type of cell could not expand from extreme rarity in populations dominated by asexual haploid and diploid cells. In the present paper we show that it is relatively easy for a rare sexual mutant to expand in a population dominated by asexual haploid cells if some matings occur between sexual and asexual cell types. We also study the persistence of sex in high mortality, high damage environments, in which neither the asexual diploid nor haploid can survive. The diploid cannot survive because its lower birth rate cannot overcome mortality and the haploid cannot survive because its birth rate cannot overcome gene damage. Sex can persist in these punishing environments by tuning the parameters of the sexual cycle, and the fusion and splitting rates, into a specified region, thereby reaping both benefits of damage repair and efficient replication.