The pollination mechanism in the 'Pelexia alliance' (Orchidaceae: Spiranthinae)

The biological function of the wedge–like, dorsally adhesive viscidium of the neotropical orchid genera Cyclopogon, Pelexia and Sarcoglottis (grouped in the so-called 'Pelexia alliance') is elucidated by the study of the pollination biology of three species occurring in Sao Paulo State, southeastern Brazil. Cyclopogon congestus is pollinated by the bee, Pseudoaugochloropsis graminea (Halictidae), Pelexia oestrifera by workers of Bombus (Fervidobombus) atratus (Apidae) and Sarcoglottis fasciculata by males and females of Euglossa cordata (Apidae: Euglossini). These three species offer nectar as a reward and are self-compatible, though they need pollinators to set fruits. In spite of the difference in flower sizes and in their pollinators' taxonomic groups, the pollination mechanism is essentially the same for these species. The pollinarium adheres to the ventral surface of the bee labrum. The viscidium needs to be dorsally pressed in order to liberate a glue which fixes the pollinarium to a bee. Pollination is achieved by the interaction of the orchid column and the mouthparts of the bees. Fivation to the ventral surface of the labrum is advantageous for the orchid, since it is a difficult place for the bees to clean. Another advantage is that, since the labrum is articulated, when the bees fold and close their mouthparts, the pollinarium remains protected under the bee's head, thus reducing the risks of pollen loss. Since the wedge-like, dorsally adhesive viscidiurn is a characteristic feature of Cyclopogon, Pelexia and Sarcoglottis , it is suggested that some kind of phylogenetic constraint may exist, impeding the occurrence of pollinators other than bees in these orchid genera. All other flower-visiting animals lack the labrum-like structure needed to fix the pollinarium.