New Zealand spider orchid (Corybas trilobus) exploits the reproductive behaviour of fungus gnats (Diptera: Mycetophila) for pollination.

Abstract

Due to a reduce pollinator diversity, plant-pollinator interactions in remote habitats are commonly unspecialised. In New Zealand (NZ), many plants exhibit a generalist pollination syndrome where flowers are open-access and pollinated by a range of insects. Orchids in the genus Corybas seem to be the exception. It is believed they mimic mushrooms and their pollination occur by brood-site deception. This study aims to uncover whether this pollination strategy operates in a form of Corybas trilobus known by the tag-name Corybas Remutaka. We used hand-pollination experiments to determine the breeding system of this orchid. SEM observations and neutral red staining were used to study the floral micro-morphology and location of scent glands, respectively. Pollinators behaviour was observed, and filmed, in the field and ex situ. Olfactory signals were assessed using solid-phase micro-extraction and solvent extractions, and visual cues by photography with a UV sensor. Only hand self- and cross-pollinated flowers formed capsules, while natural fruit-set was low. Male and female individuals of the endemic fungus gnat Mycetophila phyllura (Diptera) were the only pollinators observed. Both sexes can remove and deposit pollinia. None of the typical fungal compounds, or other compounds linked to brood-site deception, were detected in the orchid but a prevalence of alkane, sesquiterpene and fatty aldehydes. Some of these compounds were also detected in a mushroom used by fungus gnats for mating and ovipositioning. Features in the labellum surface and base (e.g. exit holes) and UV reflectance may help directing movement of the gnats within the flower. Corybas Remutaka is a self-compatible, nectarless, pollinator-dependent orchid that relies on a mix of volatiles and micro-morphological features to attract a specific pollinator. The orchid exploits the reproductive instinct of male and female fungus gnats to secure its pollination. This deceptive pollination strategy combines features of rendezvous site pollination and brood-site deception.