The evolution and maintenance of gynodioecy in sexually and vegetatively reproducing plants

Abstract

The evolution and maintenance of gynodioecy is explored theoretically at the ramet level in models that allow for differences between females and hermaphrodites in vegetative components of fitness as well as in sexual components. For nuclear control of male sterility, it is shown that females may be maintained by selection in an otherwise hermaphrodite population through advantages over hermaphrodites in vegetative fitness components, even if there is no sex difference in maternal fitness components. The advantage required depends on the importance of vegetative reproduction. If females produce more daughter ramets than ovule offspring, the advantage in vegetative fitness components required to maintain females is numerically smaller than the well-known requirement of a two-fold advantage in maternal fitness components. Three further findings in the nuclear models are that (a) with some parameter combinations, females attain frequencies of higher than 0·5, in which case their equilibrium frequency decreases rather than increases with increasing female advantage in seed fecundity; (b) the advantage in adult survivability required to maintain a given equilibrium frequency of females becomes smaller with increasing reliance on vegetative reproduction; and (c) the rate of approach to equilibrium is inversely proportional to the level of vegetative reproduction in the population as a whole. If male sterility is determined solely by cytoplasmic factors, females should be maintained if they have any advantage in vegetative reproduction over hermaphrodites, given that maternal fitness components are equal in the two sexes. For nuclear-cytoplasmic control, it is shown that the premise that a joint polymorphism for cytoplasmic male sterility and nuclear fertility restoration should not be maintained in the absence of differences in fitness between sex genotypes of the same sex phenotype (Charlesworth and Ganders, 1979) may be generalised to situations where sex differences in vegetative reproduction occur. The significance of vegetative reproduction in population genetic studies in general is briefly discussed.