Geographic variation in primary sex allocation per flower within and among 12 species or Pedicularis (d): proportional male investment increases with elevation

Premise of the study: The study of geographic variation in ecologically important traits within and among taxa is a first step toward understanding the environmental factors that contribute to population differentiation and species divergence. This study examines variation in mean sex allocation per flower (androecium mass/gynoecium mass) among 49 wild populations representing 12 Pedicularis species across an elevation gradient on the eastern Tibetan Plateau. Methods: We used population means to evaluate sources of variation in per-flower sex allocation within and across species. In particular, we evaluate the relative influence of intrinsic (i.e., plant size, estimated as aboveground stem biomass) vs. extrinsic factors affecting mean sex allocation among populations. Key results: Mean sex allocation per flower (the relative investment in male floral organs) is negatively correlated with mean plant size; populations of large plants produce relatively female-biased flowers. This relationship between mean plant size and mean sex allocation is not statistically significant, however, when the effect of elevation is controlled statistically. Among populations within and across species, mean sex allocation increases with elevation. This relationship persists even when the effect of mean plant size is controlled statistically. Factors associated with increasing elevation appear to favor genotypes and/ or taxa with male-biased flowers. Conclusion: Extrinsic environmental conditions may be more important than intrinsic resource status in determining patterns of geographic variation in mean sex allocation among populations or species of Pedicularis. We cannot conclude whether the effect of elevation on mean sex allocation is the result of environmentally induced plasticity, genetically based adaptation, or species sorting, but it is only partly mediated by mean plant size. Key words: elevation; geographic variation; Orobanchaceae; Pedicularis; population differentiation; sex allocation. doi: 10.3732/ajb.0900301