The evolution of anther morphology in orchids: Incumbent anthers, superposed pollinia, and the vandoid complex

Abstract

The anther is the source of several of the principal characters traditionally used for classification in Orchidaceae, especially in Epidendroideae. In particular, anther and pollinium orientation are important, but little-studied characters; it is not known whether various instances of these states are achieved in the same way. We examined variation in anther and pollinium orientation in Epidendroideae and Vanilloideae using anatomical sections of columns at successive ontogenetic stages and related the observed changes to the final anther morphology to reexamine our homology hypotheses for these characters. Anther bending in vanilloids is achieved primarily by massive expansion of the connective. In nonvandoid epidendroids it is the result of column elongation and tipping of the mature anther, while in vandoids it is due to a redirection of growth in very early ontogenetic stages. Superposed pollinia result from a reorientation of developing thecae; those is Sobralia result from incurving of the thecae, while vandoids have thecae that are directed outward (latrorse). Contrasting ontogenetic and phylogenetic patterns suggest heterochronic changes in these characters, especially in vandoids. These observations allow us to further refine character states that are crucial to our understanding of orchid relationships.