Variation and evolutionary transformation of some characters of the pollinarium and pistil in Epidendroideae (Orchidaceae)

Abstract

We studied the variation in the pollinarium and pistil of Epidendroideae and reconstructed the ancestral states of the characters (pollinia number, pollinium orientation, pollinium with suture, and pollinium texture). The pollinarium is complete (formed by pollinium, caudicle, stipe, and viscidium) in Vandeae, Epidendreae, and Cymbidieae, but the caudicle is absent in some Aeridinae and the viscidium in Laeliinae and Pleurothallidinae. Neottieae, Arethuseae, Sobralieae, Epidendreae, and Xerorchideae included some genera having sessile pollinia. The more frequent state in the family is to have two pollinia, followed by four, eight, and six pollinia. The pistil is unilocular, although it seems to have experienced reversals several times within Epidendroideae because intermediate states were observed (e.g. Vanda and Angraecum). In these cases, a prolongation of the placental tissue is developed that in Huntleya and Peristeria make contact but do not fuse. Most members of the subfamily have pistil composed of three carpels divided into six emerging valves, but only three are fertile. In Cattleya and Sophronitis the sterile valves are much reduced and the pistil seems to have only three valves. We have generated useful and valuable information to understand the evolution of the reproductive organs in Epidendroideae. Probably, these transformations in the pollinarium and pistil have co-evolved in tandem with pollinators to make the pollination more efficient. Our results suggest that the common ancestor of Epidendroideae had a complete pollinarium, formed probably of four juxtaposed granular pollinia without suture, bearing caudicle, tegular stipe and viscidium, but several early transformations occurred during the Epidendroideae diversification.