The evolution and function of stylar polymorphisms in flowering plants

Abstract

We recognize four major classes of stylar polymorphisms in flowering plants: The heterostylous conditions distyly and tristyly, stigma-height dimorphism, and enantiostyly. These polymorphisms differ in the relative positions of sexual organs and in the number of floral morphs that occur within populations. In heterostyly, stigma and anther heights are reciprocally positioned in the two or three floral morphs; in stigma-height dimorphism the two morphs vary in style length but not anther height; whereas in enantiostyly, flowers differ in whether the style is deflected to the left- or right-side of the flower. We distinguish two forms of enantiostyly depending on whether both style orientations occur on the same plant (monomorphic enantiostyly) or on different plants (dimorphic enantiostyly). Stylar polymorphisms have originated independently in numerous animal-pollinated flowering plant families. Both heterostyly and enantiostyly involve distinct floral syndromes suggesting functional convergence in which the position of the pollinator is important for pollen dispersal and male reproductive success. The function of stigma-height dimorphism remains enigmatic although the occurrence of populations with 1 : 1 style-morph ratios suggest that, like heterostyly and dimorphic enantiostyly, they are maintained by disassortative mating. We interpret these sexual polymorphisms as floral designs that increase the precision of cross-pollination and reduce lost mating opportunities associated with self-interference, especially geitonogamy. A single adaptive explanation based on frequency-dependent male mating proficiency can explain the evolution and maintenance of the four stylar polymorphisms in plants. (C) 2000 Annals of Botany Company.