Exploring the Palynology of Coleus Lour. (Lamiaceae) from an Evolutionary Perspective

Coleus Lour. is known for its medicinal properties. However, this genus faces taxonomic challenges due to the aggregation of species formerly classified under Plectranthus L'Hér., resulting in numerous synonyms. To support the current taxonomic circumscription, this research focused on the pollen evaluation of Coleus species from both palynotaxonomic and evolutionary perspectives. Seventeen species were examined, including three for which novel data are presented in this study: Coleus amboinicus Lour., C. barbatus (Andrews) Benth. ex G.Don, and C. neochilus (Schltr.) Codd. The pollen grains were acetolyzed and examined using scanning electron microscopy. A similarity dendrogram was constructed based on pollen morphometric data. The phylogeny was developed using Bayesian inference with the aim of mapping the evolution of pollen size and morphology. The similarity dendrogram revealed six distinct pollen groups, differing in ornamentation (rugulate, reticulate, and bireticulate, with perforations and/or granules) and shape (prolate-spheroidal, prolate, subprolate, and oblate). Ancestral state reconstruction indicates that the most recent common ancestor of Coleus likely had subprolate pollen approximately 35 µm in size (polar axis), with an initial rapid diversification in grain size followed by a period of stabilization. Despite the current scarcity of pollen data, Coleus demonstrates significant potential for further palynological studies.