Pollen grain evolution in Zornia evidences the homoplastic nature of a stenopalynous genus of Leguminosae

Background and aims – Zornia is the only pantropical genus in the Adesmia clade (Dalbergieae), subdivided into two clades by previous phylogenetic studies. Zornia remains the only genus of the Adesmia clade to be palynologically understudied. We present a comprehensive palynological study for this genus, testing the systematic relevance of pollen morphology in a phylogenetic context. Materials and Methods – Standard acetolysis was performed on all pollen grains of the Zornia species, alongside advanced microscopic techniques (LM, SEM, and TEM). Additionally, a principal components analysis was performed to elucidate patterns of variation in quantitative data among species. Using the most recent phylogenetic framework existing for the taxa, we scored and coded 13 micromorphological characters to test for secondary homologies. Key results – A comprehensive pollen characterisation enabled a complete description of Zornia as stenopalynous, with some differences in pollen grain size and ultrastructure of the operculum, margo, and sexine. The thickness of the pollen nexine (> 0.5 μm) is considered a synapomorphy for the genus Zornia , as shown by our character reconstruction analysis. Conclusions – The presence of colpate apertures is a unifying pollen character of the genus Zornia . In addition, the thickness of the pollen nexine (> 0.5 μm) was recovered as a synapomorphy for the genus, while Zornia Clade A was supported by two homoplasies correlated with ultrasculpture of the apocolpium (psilate-perforate) and sexine thickness (< 0.5 μm) and Zornia Clade B by the exoaperture width (5–10 μm). The stenopalynous nature of Zornia pollen grains is corroborated here by the large number of homoplasies recovered.