4 STEPS TO 2 SEXES

Four steps through which parasitic intracellular symbionts could bring about the evolution of two sexes are considered. In the first step, a primitive host population has biparental cytoplasmic inheritance and lacks gametic differentiation: parasitic cytoplasmic elements readily invade and spread by vertical transmission through such host populatio ns, even if they have major deleterious effects on their hosts. The se cond step leads to the establishment of a nuclear mutant in the host ( locus A) that prevents inheritance of the cytoplasm in gametes in whic h it occurs. This mutant comes to equilibrium at an intermediate frequ ency, because a double dose of symbionts is more deleterious than a si ngle dose, and zygotes lacking cytoplasm from both gametes are inviabl e. The third step involves the spread of a mutant at another nuclear l ocus (B), causing self-incompatibility of gametes in which it occurs. If this is closely linked to locus A, the mutant may become establishe d by preventing the deleterious gamete unions. The mutant at locus B m ust, however, start both with an appreciable frequency and be in gamet ic disequilibrium with locus A. In the fourth step a second mutation c ausing self-incompatibility occurs at locus B. This allele spreads by becoming associated with the other allele at locus A, eventually leavi ng the population with two gamete types, or sexes, one predominantly t ransmitting the cytoplasm, and the other eliminating it. It is argued that this is a feasible mechanism for the origin of two sexes.