Pollen aperture polymorphism in the Angiosperms: importance, possible causes and consequences

Summary To investigate the evolutionary mechanisms that produced the great diversity of pollen morphology, we concentrated on pollen aperture polymorphism. Possible selective forces and evolutionary mechanisms have been investigated previously. In this paper, we aim to test the predictions of a theoretical model and to evaluate the influence of ploidy on pollen aperture polymorphism. A survey of 186 species showed that pollen aperture number variation occurs mainly as different types in all plants (heteromorphism) whereas genetic polymorphism is related to heterostyly. Sporophytic ploidy increases the mean aperture number by shifting the mode to higher-aperturate pollen. However, the different types found within a plant do not correspond to reduced and unreduced gametophytes. The evolutionary consequences of these results are discussed.