Evolution of Sexual Reproduction: Importance of DNA Repair, Complementation, and Variation

Abstract

We have proposed that sexual reproduction arose in evolution as a DNA repair process which allowed damage in one chromosome to be repaired by the information in another homologous chromosome, and has retained this advantage throughout evolution. Since this process required that two chromosomes be present in a common cytoplasm, additional ways were evolved to take advantage of the redundant information available. The diploid stage of the cycle was probably transient in early organisms, but began to take on a more significant role as genome size increased, since it provided protection against the expression of deleterious mutations. As the diploid stage of the sexual cycle became the predominant stage, genome information content expanded beyond the range of haploid organisms. The shift to diploidy is essentially irreversible, and likewise helps prevent the abandonment of sexual reproduction. A consequence of the DNA exchange reactions that constitute recombinational repair is the reassortment of parental genetic information among progeny. As is widely appreciated, this variation is advantageous in itself under certain conditions. However we argue that a complete evolutionary explanation for the maintenance of sexual reproduction should also include complementation and DNA repair.