Tactics for male reproductive success in plants: contrasting insights of sex allocation theory and pollen presentation theory.

The basic tenet of sex allocation theory is that an organism's reproductive success, through either male or female function, can be represented as a sex-specific, monotonic, increasing function of the organism's investment of resources in that function. The shapes of these curves determine what patterns of resource allocation can be evolutionarily stable. Although SA theory has stimulated creative thinking about plant sexual tactics, quantifying the shapes of male and female gain curves has proven so difficult that other approaches must be considered. I contrast sex allocation theory to a different, emerging viewpoint, pollen presentation theory (PPT), which attempts to address variation in reproductive success by measuring and modeling the quantitative fates of pollen grains. Models suggest that RS through male function depends heavily on the packaging and gradual dispensing of pollen to pollinators, even with the amount of investment held constant. Many plants do deploy pollen gradually, through morphological and "behavioral" mechanisms that range from obvious to subtle. They may thereby influence many aspects of the evolution of sexual modes in plants, including transitions between dioecy and cosexuality. After reviewing the main implications of the models, I discuss recent work aimed at testing some key assumptions and predictions by functional and comparative studies in the genus Penstemon. Species of Penstemon conform to PPT predictions that bee-adapted flowers will restrict per-visit pollen availability more than hummingbird-adapted flowers.