Pollen and anther development malfunction in distylous flowers of Palicourea padifolia (Rubiaceae)

Abstract Background: Heterostyly is a genetic polymorphism in which flowers differ between individual plants of a species in heights at which stamens and style are reciprocally positioned. In these species, sexual selection theory predicts that different allocation patterns affect the functioning of polymorphism, enabling the evolutionary transition between heterostyly and dioecy. Questions: Because heterostyly can transit into dioecy, does anther/pollen development differ between floral morphs (LS and SS) of P. padifolia? Is pollen/anther development malfunction associated with one morph more than the other? Study species: Palicourea padifolia (Rubiaceae), a distylous plant. Methods: Tiny floral buds to flowers at anthesis were collected, processed for microphotography, and examined to describe pollen developmental pathways in LS and SS flowers. In addition, we used the TUNEL test to detect programmed cell death. Results: Stages of normal pollen development are fully described and illustrated in LS and SS flowers. Abnormalities due to tapetal degeneration were observed at various developmental stages; at later stages, SS flowers showed more abnormalities than LS flowers. The TUNEL test showed that degeneration was by programmed cell death. Conclusions: Along with previous results of asymmetrical fecundity and pollen transfer of morphs in P. padifolia, our study of pollen development indicates that the SS morph was more prone to lose the male function, though male sterility is far from being complete, which it might be an initial step towards functional dioecy.