A phylogenetic study of pollinator conservatism among sexually deceptive orchids

Orchids of the genus Chiloglottis are pollinated through the sexual deception of male wasps mainly from the genus Neozeleboria (Tiphiidae: Thynninae). The orchids mimic both the appearance and sex pheromones of wingless female thynnines but provide no reward to the deceived males. Despite the asymmetry of this interaction, strong pollinator specificity is typical. Such plant-pollinator interactions would seem to be relatively flexible in the plant's adaptive response to variation in the local pollinator resource. However, we present DNA sequence data on both orchids and wasps that demonstrate a pattern of pollinator conservatism operating at a range of taxonomic levels. Sequence data from the wasps indicate 15 of 16 Chiloglottis pollinators are closely related members of one clade of Thynninae. A pattern of congruence between orchid and wasp phylogenies is also demonstrated below the generic level, such that related orchids tend to use related thynnine wasps as specific pollinators. Comparative physiological data on the wasp responses to the floral scents of two Chiloglottis species and one outgroup, Arthrochilus, indicate similar attractive volatile chemicals are used by related orchid taxa. By extension, we infer a similarity of sex pheromone signals among related thynnines. Thus, the conservative pattern of pollinator change in sexually deceptive orchids may reflect phylogenetic patterns in the sex pheromones of their pollinators.