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

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.

Functional dioecy in Gleditsia amorphoides (Fabaceae)

The genus Gleditsia (Fabaceae) comprises woody shrubs and trees that grow in temperate and subtropical regions around the world. This genus is characterised by sexual polymorphism and functionally unisexual flowers. Gleditsia amorphoides is the southernmost species of the genus, and is widely used as a source of timber and derived products for industrial applications (galactomannans are extracted from its seeds and saponins are derived from its fruits). The species is endemic to the Chaquenean Forest of South America. It is described as morphologically androdioecious, with male and perfect flowers appearing on different plants. In the current study, we characterised floral morphology, experimentally tested the breeding system and analysed flower visitors. Results indicated that G. amorphoides staminate flowers produce viable pollen grains and that perfect flowers have a functional gynoecium and empty anthers, where pollen abortion occurs early in floral development. The species relies on outcrossing, which depends mainly on pollen carried by insect pollinators, to produce seeds and fruits. We conclude that G. amorphoides is functionally dioecious, with staminate and pistillate floral morphs.