How widespread is pollination by sexual deception of fungus gnats in Pterostylis (Orchidaceae)?

Abstract

Pollination by sexual deception has evolved multiple times in the Orchidaceae, with most known cases involving male Hymenoptera as pollinators. The diverse Australasian orchid genus Pterostylis, characterized by elaborate trap flowers, contains some species pollinated by sexual deception of fungus gnats (Diptera). However, there is considerable variation in floral morphology, suggesting that additional pollination strategies or pollen vectors may be involved. Here, we test the hypothesis that sexual deception of male Diptera is taxonomically widespread by investigating the pollination systems across a representative subset spanning nine out of 10 sections and 18 Pterostylis species. We confirm four new cases of pollination by sexual deception of male fungus gnats (families Mycetophilidae, Keroplatidae, and Sciaridae) and accrued evidence for three further cases. Each of these orchids was pollinated by a single species of fungus gnat, with two species exploiting the same pollinator. Unexpectedly, we observed insect feeding behaviour on two species pollinated by sciarid gnats and phorid flies, respectively, with trace levels of sucrose detected where feeding was observed. Our results show that the sexual deception of male fungus gnats is likely to be the dominant mode of pollination in Pterostylis, although other poorly understood pollination strategies are also present.