Pollen heteromorphism, defined here as the production within all flowers of a plant of several pollen morphs differing in aperture number, is common in angiosperms. We have focused on the evolution of pollen heteromorphism in the genus Viola, in which about 1/3 of the species are pollen- heteromorphic. We have studied the distribution of pollen heteromorphism in the genus Viola using a molecular phylogeny based on ITS sequence data. We show that pollen heteromorphism has evolved independently at least six times in Viola. A com- parative analysis shows that the occurrence of pollen heteromorphism is correlated with sporo- phytic polyploidy in all sections of the genus apart from section Melanium. This section differs from all other sections on several aspects such as flower morphology, absence of cleistogamous flowers, and a high proportion of heteromorphic species. We discuss the possible adaptiveness of pollen hetero- morphism in this section.
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