Résumé
Lloyd’s (1975)
and
Charlesworth & Charlesworth’s (1978)
phenotypic selection models for the maintenance of androdioecy predict that males (female-sterile individuals) must have an advantage in fertility (
K
) of at least two in order to invade a hermaphroditic population, and that their equilibrium frequency (
x
eq
=(
K
− 2)/2(
K −
1)) is always less than 0.5. In this paper, we develop a model in which male fertility is frequency-dependent, a situation not investigated in the previous models, to explore the conditions under which a high frequency of males (i.e. more than 50%) could be maintained at equilibrium. We demonstrate that a gametophytic self-incompatibility (GSI) locus linked to a nuclear sex determination locus can favour rare alleles through male function, by causing frequency-dependent selection. Thus, the spread of a female-sterility allele in a hermaphroditic population may be induced. In contrast with the previous models, our model can explain male frequencies greater than 50% in a functionally androdioecious species, as long as there is (i) dominance of female-sterility at the sex locus, and (ii) a few alleles at the self-incompatibility locus, even if the advantage in fertility of male phenotype is lower than two.