Morphological variation and evolutionary trends in pollen of Bacopa (Gratioleae-Plantaginaceae)

Bacopa (Gratioleae-Plantaginaceae) is a plant genus with great morphological variability and little previous information on its palynology. Thus, the aim of the present study is to describe palynological characters of potential taxonomic significance for the delimitation of Bacopa species and to discuss their systematic implications. In addition, the evolution and systematic utility of palynological characters in the genus were also analyzed. The pollen grains were acetolyzed, observed and analyzed under light and scanning electron microscopy, measured and photographed. Analyses based on qualitative and quantitative data were performed based on descriptive and multivariate statistics. Also, the ancestral reconstruction of palynological characters was carried out by stochastic character mapping, based on previous phylogenies. The pollen morphology of 22 taxa was described for the first time. The pollen morphology of Bacopa contributes to taxonomic studies of the genus because it allows the formation of groups that coincide with previously proposed sections. The compound opening (tricolporate) was considered as an ancestral character state, as well as the tectate exine, small size and prolate-spheroidal shape. Bacopa is confirmed as an eurypalynous genus, the number and type of apertures allowed us to differentiate three pollen types that coincide with the subclades determined by molecular phylogeny.