| Within-population sex ratio variation along an environmental gradient---a pattern labeled spatial segregation of the sexes---is an extreme form of sex ratio bias commonly found in dioecious plants. Spatial segregation of male and female individuals is surprising in an immobile organism because limited dispersal of gametes should reduce sexual reproductive success, and presumably fitness, of individuals. Little is known about the ecological and evolutionary mechanisms responsible for this pattern.; In California populations of the clonal, perennial grass Distichlis spicata, I examined sex ratio variation along micro-elevation gradients correlated with soil fertility and tidal inundation. First, using RAPD markers, I demonstrated that this species exhibits spatial segregation of male and female genets, rather than simply segregation of ramets or flowering potential, and that sex is genetically determined. Second, although gender-specific juvenile mortality has rarely been examined in plants, I used RAPD markers to identify the sex of seedlings and thereby determine whether differential seed germination and seedling mortality are proximate mechanisms for spatial segregation of the sexes in this species. In majority-female habitats, gender-specific bias in seedling survivorship and seed germination do occur, but only variation in seedling survivorship helps explain the observed sex ratio bias. Third, I examined whether specialization of the sexes to different microhabitats occurs in D. spicata, a scenario not previously shown for a dioecious plant species. I determined that microhabitats differentially affect flowering in the sexes. Moreover, female seedlings have a greater suppressive effect on neighbors than do male seedlings, a finding that supports speculation that females competitively exclude males from high-quality habitats. Fourth, by examining pollen movement and the effects of pollen load, nutrient availability, and male ramet densities on seed set, I found that spatial segregation of the sexes in this species limits the mating success of males and females. These data are the first of their kind, and they give the most comprehensive picture of the causal mechanisms for, and consequences of, spatial segregation of the sexes in any plant species. Data of this kind are essential for evaluating hypotheses concerning the evolution of sexual segregation and dioecy. |
