Pollinator sharing by the sexually deceptive Green-comb Spider Orchids, Caladenia phaeoclavia, C. parva and C. villosissima (Orchidaceae: Caladeniinae): taxonomic considerations

Abstract

Sexually deceptive orchids are pollinated when male insects perform mating behaviours on the female-mimicking labellum. Such orchids are characterised by extreme pollinator specificity, having only one, or occasionally a few, closely related insect species as pollen vectors. Extreme pollinator specificity may confer complete prezygotic reproductive isolation, thereby avoiding gene flow across species boundaries in the absence of postzygotic barriers. Pollinator specificity is underpinned by precise mimicry of pollen vector sex pheromones. Rapid speciation in sexually deceptive orchid lineages is driven by small changes in allomone chemistry leading to switching of pollinators usually within the same or closely related insect lineages. Such rapid speciation may result in complexes of morphologically and genetically similar cryptic species that present taxonomic challenges. Species boundaries in sexually deceptive orchids are defined by differences in allomone chemistry and are detectable in nature by their specific pollinators, irrespective of minimal morphological or genetic divergence. Conversely, pollinator specificity in sexually deceptive orchids can help to identify examples of taxonomic over-splitting where complexes of morphologically variable orchid species are shown to share the same pollinator species. In this paper three sexually deceptive morphospecies in the Greencomb Spider Orchid complex, Caladenia parva G.W. Carr, C. phaeoclavia D.L. Jones and C. villosissima G.W. Carr, are shown to share the same thynnid wasp pollen vector, Lophocheilus anilitatus (Smith). The three orchids are closely similar morphologically, are likely to be monophyletic and are considered to be the same biological species. However, phylogenetic analysis is needed to confirm their monophyly, and possible polyploidy in C. villosissima may confer postzygotic reproductive isolation allowing it to share pollinators with C. phaeoclavia and C. parva without hybridizing. It is concluded that pollinator specificity has an important role in identifying species boundaries in complexes of cryptic species, but phylogenetic analysis and possible postzygotic isolating mechanisms should be investigated where pollinator sharing is identified among morphospecies before concluding they are monospecific.