Pollen Diversity in Bromeliaceae (Poales): A Morphological and Micromorphological Review

Abstract Bromeliaceae exhibits significant pollen diversity. Within the family, pollen grains are dispersed either as monads or tetrads and may be monosulcate, bi-, tri-, tetra-, or pantoporate, or even inaperturate. Ornamentation includes reticulate, foveolate, microreticulate, perforate, or psilate patterns. Additional variations are observed in the margins of the apertures and in the exine deposition on the aperture membrane. In general, pollen grains are relatively well-documented in Bromeliaceae. However, there is a lack of systematization and organization to provide a comprehensive and integrated understanding of its pollen structure. This study aims to conduct a general review of pollen morphology in Bromeliaceae, analyze character diversity based on the current phylogeny of the family, and propose the ancestral states of pollen traits. The study sampled 127 species from 47 genera, supplemented by literature data of 220 species (from 14 additional genera). Together, this dataset encompasses over 70% of the genera in the family and includes species from all subfamilies. Ancestral character states were inferred through a maximum parsimony approach. The identified ancestral states include monads as the dispersal unit, heteropolarity, bilateral symmetry, medium size, a basic elliptical shape, monosulcate pollen, a simple sulcus type, margin type III (Puya type), and reticulate ornamentation. Bromelioideae is eurypalynous, with a predominance of biporate pollen grains in Core Bromelioideae. Tillandsioideae is considered stenopalynous, characterized by monosulcate pollen grains. However, the subfamily shows considerable variation in sulcus types. Pitcairnioideae is also stenopalynous and monosulcate but exhibits significant variation in the sulcus and ornamentation. The remaining subfamilies display less variation and are stenopalynous. This study provides a comprehensive overview of pollen diversity in Bromeliaceae and highlights its evolutionary patterns, offering a basis for further research on character evolution and systematics of the family.