Diversity in anthers and stigmas in the buzz-pollinated genus Senna (Leguminosae, Cassiinae)

The large genus Senna ( Cassiinae, Leguminosae) is an outstanding example of floral structural specialization associated with buzz pollination. This specialization is expressed especially in the androecium ( with a high diversity of anther elaborations) and gynoecium ( with diversity in stigma shape). The floral structure of 69 species from all major clades of Senna was studied, focusing on heteranthery, anther dehiscence, pore position, extension of the lateral furrow of the thecae, cell wall thickening in the anther tip, and stigma diversity ( especially position, form, size, and structure of the orifice and presence of a chamber). Filament union is reported for the first time in the genus; it involves the seven adaxial androecial organs, a pattern unique in legumes. Our investigations identified novel morphological characteristics that are congruent with the clades supported by the molecular phylogeny. Anthers of abaxial stamens with the least differentiated dehiscence pattern, i. e., two separate pores and separate thecae, are found in most major clades ( I, III - V, VII). Anthers with apically confluent thecae, forming a shared chamber, and/ or with a single pore by confluence of two pores, represent specialized patterns ( clades II, IV, VI, VII). Diverse anther tips may reflect different strategies of pollen dispersal; anther pore position may influence pollen flow directions. Anther tip elongation in the abaxial stamens and constriction between the thecae and the anther tip may influence the speed and/ or amount of the released pollen.