Facultative sexual reproduction under frequency-dependent selection on a single locus.

The evolution of a facultative sexual strategy that simultaneously produced sexual and asexual individuals was studied theoretically, under negative frequency-dependence of fitness. The organism was considered to be diploid, characterized by two loci concerning fitness and determining sexual strategy, between which a certain degree of linkage existed. The locus concerning fitness was assumed to involve two alleles, resulting in three genotypes, the relative fitness of an individual being defined by a decreasing function of frequency of its own genotype on this locus in the population. The sexual reproductive strategy was considered to be determined by three alleles; asexual, obligate sexual and facultative sexual. Simulations under various linkages between loci and level of frequency dependence of fitness showed that a facultative sexual strategy was generally able to invade and increase in the population. In particular, when the level of frequency dependence was high to some degree, the facultative strain producing many sexual individuals tended to exclusively occupy the population. Namely, the frequency-dependent selection resulted in a predominance of obligate sexual strategy over asexual strategy, simultaneously causing a subordination of the former to the facultative sexual strategy. This indicated that the evolution of sex should be considered carefully with respect to the possibility of invasion of facultative sex.