EVOLUTIONARY DYNAMICS OF SEX-RATIO AND GENDER DIMORPHISM IN SILENE LATIFOLIA .1. ENVIRONMENTAL-EFFECTS

In 10 sibships of the dioecious Silene latifolia, we studied effects of maternal family on sex ratio and effects of maternal family, plant density, and watering regime on sexual dimorphism. Plant sex ratio was female biased; the significant heterogeneity across families was consistent with a hypothesis of genetic variation for progeny sex ratio, expressed before seed germination. Floral sex ratios were male biased and heterogenous across families; males had many more flowers than females. All traits (biomass, flower number, height, longest branch length, branch number, and flower weight) showed sexual dimorphism. We documented genetic variation in the degree of sexual dimorphism in plant height and in the dimorphic response of flower weight to watering regime. Family mean genetic correlations between females and males were significantly positive far all traits except flower weight. These correlations varied significantly across growing environments for plant height and branch number, suggesting that only in certain environments will such positive correlations act to genetically constrain further dimorphism. The pattern of genetic correlation among traits was very similar in females and males and was not environmentally variable. Under the best growing conditions, however, females exhibited stronger correlations among traits and so were genetically more constrained than males. Female plants grown under water stress showed many negative genetic correlations, a response typical of resource limitation.