Factors affecting binary sex evolution with respect to avoidance of vertical transmission of deleterious intracellular parasites

In sexual reproductive systems, the number of sexes is generally binary, viz. male and female. Several theoretical studies have shown that the evolution of this system is possibly related to cytoplasmic DNA, including deleterious cytoplasmic symbionts. When organisms are infected by a symbiont that is transmitted vertically to offspring via gametes, the exclusion or degeneration of the latter may evolve as a characteristic of those organisms. If this necessarily results in the elimination of organelle DNA in gametes, a reciprocal preference between individuals, one transmitting organelles and the other not, may be favored. In this theoretical study, factors affecting such an evolutionary process, in which the symbiont is considered as a parasite infecting vertically, horizontally and naturally, are considered. In addition, host individuals are assumed to recover from the infection to some degree. According to the analysis, a binary sex system can evolve only when uninfected and infected host individuals co-exist in a single host population. This condition can be satisfied only if natural infection occurs. Although recovery from infection has both positive and negative effects on binary sex evolution, the latter is promoted only when natural infection exists. Accordingly, if natural infection does not exist, the evolution of binary sex system is unlikely with respect to deleterious cytoplasmic symbionts, in absent of heterozogotic advantage in vertical transmission.