More of almost the same: novel pollen descriptions uncover subtle morphological variation in Mimosa L. (Leguminosae, Mimoseae)

Pollen is a source of information on the reproduction, ecology, evolution, and systematics of plants. Mimosa, one of the largest genera in Leguminosae, has pollen organized in tetrads or polyads, with variation in the arrangement, ornamentation, and size of grains. Despite such morphological diversity, pollen has been described for less than 25% of the more than 600 species in the genus. Here, we increase the current knowledge about Mimosa pollen disparity by extending taxon sampling to 31%. Samples were studied under light and scanning electron microscopy. Novel pollen descriptions and data gathered from the literature were used to infer a pollen morphospace and calculate Mimosa pollen disparity. We find tetrads to be the main dispersion unit, but some taxa also present 8, 12 or 16-grained polyads. Dispersal unities vary in the disposition of pollen grains and size, while grain ornamentation, number of pores and type of aperture were less variable. Analyses revealed an increase in morphological disparity after novel descriptions and an uneven distribution of such variation across major Mimosa clades. Our results expanded the taxonomic breadth of Mimosa pollen diversity, filling gaps and suggesting that although the general picture of pollen morphology in Mimosa is well-defined, the boundaries of its variation remain to be explored.