Pollination and chemical ecology of Pterostylis (Orchidaceae)

Abstract

Pollination by sexual deception has evolved multiple times within the Orchidaceae, with these orchids known to exploit a wide variety of male insects from the Hymenoptera, Diptera and Coleoptera. Fungus gnats (Diptera; Sciaroidea) have been predicted to be the pollinators of many species of sexually deceptive orchid, yet unlike orchids that sexually attract bees and wasps (Hymenoptera), the pollination and floral chemistry of fungus gnat-pollinated orchids has received relatively little attention. One such example is Pterostylis (Orchidaceae), a diverse terrestrial orchid genus from Australasia. In Chapter 1, I investigate the pollination of Pterostylis cycnocephala and confirm, for the first time in Pterostylis, pollination by sexual deception of male black fungus gnats (Sciaridae). Pollinator baiting and floral dissection experiments revealed that floral odour is responsible for long-range pollinator attraction, but appropriate morphology, in the form of an intricate labellum appendage, is required to elicit the copulatory behaviour needed for pollination. In Chapter 2, I test the long-standing hypothesis that sexual deception is taxonomically widespread in Pterostylis, using pollinator baiting as a tool to investigate the pollination mechanisms of a representative subset of species. My results show that pollination by sexual deception of male fungus gnats is likely to be the dominant pollination mechanism in Pterostylis, although food rewarding systems are also likely to be present. In Chapter 3, I uncover the chemical basis for the sexual deception of male Mycomya sp. (Mycetophilidae) by Pterostylis orbiculata flowers. Using preparatory-GC in combination with laboratory and field bioassays, I show that (6Z,9Z)-1,6,9-tricosatriene, an unusual polyunsaturated hydrocarbon (polyene), is crucial for male fungus gnat attraction and therefore pollination. In Chapter 4, I build on the findings from Chapter 3 by conducting (1) a genus-wide survey of floral volatiles to investigate patterns of polyene presence and abundance across the genus, and (2) a detailed survey of P. orbiculata and five closely related species in the section Urochilus to investigate if closely related species produce different polyenes. Polyenes were present primarily in the labellum extracts of Pterostylis species confirmed or predicted to sexually attract male Mycetophilidae, but not other fungus gnat families. In addition, each of the species in the section Urochilus either produced different polyenes or showed evidence of chemotypic variation associated with polyenes. Overall, this thesis represents the first extensive investigation of the pollination and chemical ecology of Pterostylis orchids, providing new insights into the evolution of pollination by sexual deception of male fungus gnats.