Pollination ecology of Australian sexually deceptive orchids with contrasting patterns of pollinator exploitation

Abstract

Sexual deception, entailing the pollination of flowers through mimicry of female insects, is one of the most remarkable pollination strategies to have evolved. This thesis explores two Australian sexually deceptive orchid systems with contrasting patterns of pollinator exploitation. The first three chapters focus on the genus Cryptostylis, a system with a unique case of pollinator sharing - five Australian species, four of which are largely sympatric, all deceive the same male ichneumonid pollinator. In Chapter One, mark-recapture experiments were used to investigate the consequences of ichneumonid pollination on pollen movement in C. ovata. A high pollinator revisitation rate indicated some potential for self-pollination. In Chapter Two, reproductive barriers contributing to the absence of hybrids between Cryptostylis species were investigated. Pre-pollination barriers, assessed in field experiments, did not prevent hybridisation. Hand cross-pollinations conducted among the four common Cryptostylis species in a greenhouse all produced fruits, however seed mass and the percentage of formed embryos were reduced in hybrids. Major differences in ploidy and chromosome number likely explain this post-pollination fitness reduction. Two Cryptostylis species were found to be self-incompatible, marking the first case of self-incompatibility in the Diurideae. The unique reproductive biology of Australian Cryptostylis, encompassing pollinator sharing, self-incompatibility, and post-pollination reproductive isolation driven by large ploidy differences, may indicate that its mode of diversification may differ greatly to those in other sexually deceptive genera. Chapter Three presents the first phylogeny to encompass both Australian and Asiatic Cryptostylis. An Australian origin of Cryptostylis is supported, with a likely single dispersal event to Asia. Ploidy variation and geographic barriers appear to have played a role in diversification across Cryptostylis. In Chapter Four, the potential presence of pollination ecotypes in Drakaea livida was tested for. Patterns of chemical diversity and pollinator availability across the distribution of the species are investigated. Pollinator choice trials revealed the presence of three discrete ecotypes each attracting its own pollinator species. Patterns of pollinator availability did not correlate with ecotype distribution. Each ecotype possessed a significantly different floral volatile composition. Using Partial Least Squares Discriminant Analysis (PLS-DA), the presence-absence of a subset of taxonomically informative compounds could be used to accurately predict the ecotype of a flower. Different classes of electrophysiologically active compounds were present in different ecotypes. These marked differences in chemical composition between the ecotypes suggest either a long time since their divergence and may hint at a scenario of convergent evolution of floral morphology. In Chapter Five, the ecotype geographic ranges and methods of identifying the ecotypes were investigated. Species distribution modelling predicted each ecotype to have a different core range. Two ecotypes were widespread, while one had a limited distribution within extensively cleared agricultural land, raising conservation concerns. PLS-DA correctly identified the ecotype of a flower when labella extracts were made from pollinated flowers, thereby providing a non-destructive identification technique. The pollinator specificity, morphology, floral chemistry, and ranges of the ecotypes supported them as Evolutionary Significant Units. In conclusion, the ecological and evolutionary consequences of pollination by sexual deception may vary extensively between plant taxa in accordance with their different patterns of pollinator exploitation. The taxonomy, species richness of the pollinator group, and the plant species to pollinator species ratio all influence the evolution and diversification of sexually deceptive orchids. Show more