DELPH, L.F.: Environmental influences on the evolution of gender and sexual dimorphism

Flowering plants that are dimorphic in gender typically display significant differences between the sexes in a variety of traits. This sexual dimorphism is thought to have evolved via disruptive selection on homologous characters as a result of differences in expenditure on reproduction or mating success. I will present two examples of how the environment can influence sexual dimorphism and thereby affect breeding-system evolution. It has been hypothesized that harsh environments favor the maintenance of females in gynodioecious populations (those containing hermaphrodites and females) because seed production by hermaphrodites is more plastic than that of females. Hence, hermaphrodites lower their seed production in harsh sites more so than females; this alters the relative seed fitness of the two morphs and favors an increase in females. Such plasticity in seed production should stabilize gynodioecy. I will review studies testing this hypothesis to document its value in explaining sex-ratio variation. The second example concerns populations with stable, high frequencies of females. In such populations the hermaphrodites will be achieving most of their fitness via male function, and a fitness premium is placed on pollen production. Hence, when resources are limiting hermaphrodites in such populations should allocate resources towards growth, as this will enhance pollen production in the following year. In contrast, females should invest in seeds rather than growth. I will present evidence that such sex-specific strategies have evolved and are likely to drive the population closer to dioecy.