Pollen morphology reveals higher specialisation degree in Schnella section Caulotretus (Cercidoideae: Leguminosae): importance for pollination and taxonomic significance

Schnella is a neotropical genus of legumes in the subfamily Cercidoideae with approximately 50 species, divided into the sections Schnella and Caulotretus. The genus has a complex taxonomic history, and its species were previously classified in Bauhinia or Phanera, until molecular phylogenetic studies supported its recognition as a distinct genus. Nonetheless, the absence of a fully resolved phylogeny has been an important barrier to testing and improving its infrageneric relationships. In this study, we present pollen data to provide valuable insights into the taxonomy of Schnella sect. Caulotretus and its differentiation from Schnella sect. Schnella. Pollen samples were obtained from floral buds of herbarium specimens and then acetolyzed, measured, and described under light, scanning, and transmission electron microscopy. Pollen grains in species of Schnella sect. Caulotretus are isopolar, medium-large to very large, subprolate to prolate, 3-colporate, and with a variable endoaperture. Costa and margo are recorded for most of the species. Exine ornamentation varied between slightly to strongly fossulate-perforate to perforate. Most species show a heterogenous ornamentation pattern between the mesocolpus and apocolpus, and this condition is reported for the first time in the genus Schnella. Pollen morphology within Schnella sect. Caulotretus demonstrates a greater degree of variability than previously documented. Notably, size and exine ornamentation play a pivotal role in species differentiation. Additionally, some species exhibit highly distinctive pollen morphology that deviates significantly from the established morphological patterns within this section. Our findings contribute essential data for the palynological characterization of Schnella sect. Caulotretus and explore the importance of pollen morphology in the delimitation and taxonomy of this section.