Staminal features in Barnadesioideae (Asteraceae): description, evolution and function

Abstract

Abstract Morphological features of the heads (capitula) of Asteraceae have been used extensively in classification of the family at different levels of the taxonomic hierarchy. Among the various characters, features of stamens have been employed to determine relationships from specific to tribal levels, including size, shape, colour, cell size and shape of the thecae, downwards extensions from the thecae (spurs or tails), the apex and base of the connective between the thecae, the antheropodium that joins the connective to the filament and the filament itself. We investigate variation in these staminal features in 88 species of ten genera of Asteraceae subfamily Barnadesioideae, the sister group of the rest of the family. A new morphological categorization of antheropodia and anther bases is presented, features that show the widest ranges of variation in the subfamily. Other characters, such as apices of the connectives and fusion of filaments, are less variable. Six staminal characters are optimized on a molecular phylogenetic tree for Barnadesioideae. Putative plesiomorphies for the ancestor of the subfamily are hypothesized to be entire apices of the connectives, antheropodia shorter than the basal appendages and filaments free and glabrous. The ancestral conditions for pollen sac extension and for attachment of anthers on the corolla tube are equivocal. Hypotheses are offered on the possible adaptive function of staminal features, such that antheropodia seem significant in structural support of the thecal tube and that spurs and tails may function in protecting the basal portion of the thecae from insect probing.