Sculptural elements on the ectexine surface of Poaceae pollen from Neotropical forests: patterns and implications for taxonomic and evolutionary studies in this family

Poaceae have been historically recognized by having homogeneous pollen grains. Most of the available studies have focused on species from open areas, and studies of tropical forest representatives of the family are scarce. Some studies have suggested pollination assisted by insects, contrasting with the traditional anemophily known for this family. We analysed pollen surface of 121 specimens belonging to 87 species, eight tribes and four subfamilies of Poaceae from Neotropical forests, based on scanning electron microscopy method. None of the analysed samples had ectexine without ornamentation. Four different patterns were observed, three mixed and one of them simple: microechinate-areolate (all tribes except Streptochaeteae; 71 species); microechinate-perfurate (12 species); microechinate-rugulate (exclusive to Olyreae, the tribe that includes the herbaceous bamboos and is confined to the understory in the Neotropical region; two species) and microechinate (Bambuseae and Streptochaeteae, two species). The present data broaden the knowledge on the palynology of Poaceae from tropical forests, because 14 genera and 75 species have the pollen surface characterized for the first time in this article. Patterns of ectexine variation, especially in subfamilies Anomochlooideae and Bambusoideae, include differences between and within genera, with application to taxonomic and evolutionary studies, and probably to other fields such as ecology, palaeobotany and reproductive biology in the family.