Origin of sex for error repair. I: Sex, diploidy, and haploidy (Q1347239)

Genetic damage is a fundamental problem for living systems. Recombination in a diploid cell can repair a damaged gene, so long as one of the copies is undamaged. During sex, outcrossing generates the diploid state by temporarily fusing two haploid cells (as in the case of meiosis) or by bringing DNA into the cell from outside (as in the case of bacterial transformation). But why should cells alternate between the haploid and diploid states in the first place? Why not just remain diploid, if damage repair is the only problem? Diploid cells are unlikely to become damaged at the same site in both copies and one can imagine that some sort of efficient mitotic recombination in diploid cells might be possible. Consequently, the function of DNA repair, while providing explanations for diploidy and recombination, does not by itself explain the need for recombination plus outcrossing -- that is, sex. There are economic issues to consider when investigating the selective advantage of haploidy and diploidy, and these issues are included in the models studied here. Most basically, diploid cells require twice the genetic resources and nutrients to replicate. In addition, there may be size differences and other intrinsic differences between diploid and haploid cells. Masking of recessive, or nearly recessive, mutations is probably another important advantage of the outcrossing aspects of sex in many organisms with a diploid stage. However, we do not include this factor in the models studied here, although we believe that both kinds of genetic error, deleterious mutations and genetic damage, played a role in the evolution of diploid sexual life cycles. The point of the present paper is to study selection resulting from the need to jointly repair genetic damage and replicate DNA. We leave to future work the problem of considering deleterious mutation and genomic damage together in explicit mathematical models. The work reported here concerns competition between three types of life cycle: sexual, haploid, and diploid. In nature, the sexual process is represented by diverse methods and styles, from mixed infection in viruses to transformation and conjugation in bacteria to syngamy and meiosis in eukaryotes. We have tried to extract the essentials from these myriad representations and incorporate them into a simple model, which allows us to understand if the sexual process might have arisen in response to the simultaneous need for repairing genetic damage and efficient cell replication.