Chemical ecology and pollination biology of the Australian cycad macrozamia communis
Creators: name: Wallenius, Thomas Christopher
Publisher: name: The Australian National University
Year: 2014
Subjects: subject: Cycads Pollination Australia
subject: Plant chemical ecology
subject: Pollination by insects
subject: Insect pollinators

description: Cycad pollination typically involves an obligate interaction, in which a specialised insect herbivore feeds, oviposits and pollinates during a short period of cone pollen-shed and receptivity. Two key traits of cycad reproductive biology, cone thermogenesis and emission of volatile organic compounds, are thought to mediate dispersal and colonisation behaviours of the insect pollinators. However, the variation of these two cone traits and the physiological responses of pollinators to them have received little attention. For my PhD thesis, I investigated cone thermogenesis and volatile emission in the Australian cycad Macrozamia communis, and the behavioural and antennal responses of the weevil pollinator Tranes lyterioides. The cone traits and physiological responses of the pollinators were investigated in the context of the two main hypotheses of cycad pollination, brood-site deception and volatile-mediated push-pull pollination. Initial research focused on the interaction between thermogenesis and emission levels of four main volatiles in pollen-shedding male and receptive female cones. A significant positive correlation between daily thermogenic events and volatile emission levels was found in both male and female cones, and coincided with the timing of weevil pollination behaviours, suggesting a role of the two cone traits in pollination. Behavioural responses of T. lyterioides weevils to varying concentrations of the four main volatiles, other volatiles and cone odour were explored using dichotomous-choice bioassays, with results indicating that high concentrations of volatiles may promote dispersal of the weevils from cones. Antennal responses of T. lyterioides weevils to varying concentrations of the four major volatiles and their blends were investigated by means of electroantennography. The results showed determining that male and female weevils exhibit high olfactory sensitivity to these volatile blends. While similarities in the levels of thermogenesis and volatile odour bouquets existed between male and female cones, consistent with brood-site deception, the role of variation in the levels of specific volatiles and pollinator responses to them requires further investigation before this hypothesis is confirmed. The consistent and strong repellence of weevil pollinators by all four main cone volatiles supports the "push" aspect of the "push-pull' pollination hypothesis. However, weevils were not attracted to lower concentrations of volatiles and thus no "pull" was found to be exerted. Thus, the host location of T. lyterioides could not be distinguished from that of any other phytophagous insects. It was concluded that further study and refinement of existing hypotheses is still required to elucidate the intricate coordination between cone traits and behavioural and sensory adaptations of the pollinators before we fully understand the pollination system of M. communis.