The Pollination and Chemical Ecology of Eastern Australian Underground Orchids

Abstract

Orchids are renowned for their diversity of interesting and unusual floral forms and ecological interactions. Amongst the most interesting and unusal orchids are the uniquely Australian Underground Orchids (Rhizanthella spp.). These mycohetereotrophic plants have no leaves and flower under leaf litter, meaning they are very rarely seen and have only been recorded from a handful of locations across Australia. Due to this rarity, there has been relatively little scientific investigation of the genus - particularly the Eastern Australian species - and there remains much to learn about their biology. Chapter 1 presents a timeline of research on Rhizanthella species across Australia, focusing on key findings on the ecology of the genus. Knowledge of the pollination of these species is particularly sparse, with only a few reports of insects carrying pollen in Western Australia and none in Eastern Australian species. I lay out a framework for investigation of the pollination biology of Rhizanthella, considering the general characteristics of several potential pollination strategies. The following chapters address some of the key knowledge gaps to establish where Rhizanthella might fit within this framework, with particular focus on oviposition site mimicry as the prime suspect. Chapter 2 reviews the chemicals known to play a role in orchid pollination broadly, some key synthetic methods for making these compounds, and outlines future directions for the study of orchid volatile chemistry. This provides a basis for comparison with the floral volatile chemistry of Rhizanthella. Chapter 3 presents the first comprehensive visitation survey of any Rhizanthella species. Several small fly species visited artificially exposed inflorescences, including Chloropideae species as the most frequent visitors, but only Megaselia species (Phorideae) were observed removing pollinia. This appeared to be mediated by a motile labellum acting as a pollinator filter to enable only insects of a particular size to remove pollinia. DNA barcoding revealed that the phorid species carrying pollinia represented only a small fraction of the phorid diversity in the area, further indicating a specialised system. Chapter 4 reports the chemical compounds emitted by and present in the tissue of three geographically separate populations of Rhizanthella. The major compounds emitted in all studied populations were two compounds never previously reported as floral volatiles, and rarely in nature from any source: 1,3-dimethoxy-5-pentylbenzene, and 3-methoxy-5-pentylphenol. These compounds were also present in floral tissue extracts, along with several other compounds, predominantly long chain hydrocarbons and benzenoids. Preliminary trapping bioassays hinted at a potential attractive role of these compounds to phorids in one of the two populations tested, although further work will be required to establish this conclusively. Chapter 5 confirms the presence of small amounts of sugars which may be responsible for the feeding behaviours of chloropid visitors described in Chapter 3. Liquid nectar was never detected, but there were tissue-specific patterns of sugars which indicate a 'trail of breadcrumbs'-like gradient leading pollinators into the position to affect pollination. Chapter 6 discusses the findings of this thesis in the context of what they reveal about the pollination strategy of Rhizanthella, linking back to the framework established in Chapter 2. The results do not strongly support oviposition site mimicry, but nor do they confirm any particular alternative strategy. I outline a possible specialised insect-host mimicry pollination strategy, but note that further evidence will be required to link the clues uncovered by this thesis conclusively. Overall, this thesis presents the first extensive investigation of pollination and chemical ecology of Rhizanthella, and provides a foundation for further pollination biology research in Rhizanthella species. Show more