Pollen evolution in the Stryphnodendron clade (Leguminosae, Mimoseae)

Abstract

Pollen morphology is a valuable tool for understanding plant evolution and taxonomy. Although particularly conserved in the legume family (Leguminosae), pollen may show wide variation in some subfamilies and tribes. However, legume pollen diversity may not be consistently considered in the taxonomy of some groups. Here we investigated pollen evolution of the Stryphnodendron clade (Leguminosae, Mimoseae) and how it relates to generic delimitations within the clade. We analyzed six morphological traits sourced from herbarium specimens and the literature for 46 taxa. Discrete traits comprised number of grains in the dispersal unit, outline, and pollen ornamentation, while continuous traits encompassed exine thickness, as well as longer and shorter diameters of dispersal units. All characters were mapped over a phylogenetic hypothesis for the clade using stochastic character mapping and maximum likelihood ancestral state estimation. Our results reveal that some traits, such as number of grains per unit and outline, are generally more conserved than others, such as pollen ornamentation and continuous traits, which are more labile. Pollen ornamentation differentiates two lineages within the Stryphnodendron clade and also the genera Stryphnodendron, Gwilymia and Microlobius. In addition, the recently described genus Gwilymia shows a unique pattern of exine thickness. In conclusion, although number of grains per unit and outline are conserved traits within the Stryphnodendron clade, ornamentation and continuous traits support recognition of distinct lineages both above and at the genus level. These findings reinforce the importance of incorporating pollen morphology in assessments of generic delimitation within the legume family.