Molecular symbionts and the evolution of sex

The main focus of this article is on the evolutionary origin of sex rather than its maintenance in extant organisms. Sexual outbreeding involves a complex set of biological phenomena, and it is useful to consider the probable order in which the components of this process evolved. I propose that conjugation between cells was the initial stage in the evolution of sex. It has been shown that molecular symbionts (such as transposons and plasmids) derive a major selective advantage from conjugation and sexual outbreeding. This strongly suggests that the origin of conjugation between cells may best be understood as a symbiont-encoded adaptive function. In other words, conjugation between haploid cells evolved initially as a means of propagating molecular symbionts. Once efficient mechanisms of conjugation were in place, however, the subsequent elaboration of the other aspect of sex, particularly genetic recombination, were facilitated. Although the molecular symbiont theory is directly relevant only to the origin of sex, it also has implications for understanding the subsequent stages in the evolution of sex.