Macrozamia cycad cone traits mediate thrips and weevil pollinator behavior
Terry I, Moore C, Walter G, Hull C, Roemer R
Department of Biology, University or Utah, 257 South 1400 East Street, Salt Lake City, UT 84112, USA.
Correspondence: terry@biology.utah.edu
Because cycads are the basal clade of extant seed plants, and insects vector pollen between male and female cones, studies of cycad pollinators could reveal much about early pollination mutualisms. The cycad genus, Macrozamia, an Australian endemic, is unusual in having some species pollinated only by thrips in the genus Cycadothrips, whereas all other known cycad pollinators world-wide are beetles. Comparisons of several Macrozamia species pollinated only by Tranes sp. weevil with those pollinated by Cycadothrips sp. only reveal differences in their cone volatile components. Tranes-pollinated species release monoterpenes, primarily linalool and smaller amounts of beta-myrcene, whereas Cycadothrips ¨Cpollinated cycads lack linalool and release several monoterpenes dominated by very high levels of beta-myrcene. Diel periodicity of volatile emissions coincides with cone thermogenesis and insect movement between cones: daytime activity for Cycadothrips-pollinated species and evening activity for Tranes-pollinated ones. Results of electroantennograms (EAGs) and gas- chromatography-electroantennographic detection (GC-EADs) show that insects respond physiologically to specific components within their host cone¡¯s volatile mix. In particular, Cycadothrips chadwicki respond to beta-myrcene and two beta-ocimene isomers, (E)- beta-ocimene and (allo)-beta-ocimene. Tranes sp. from M. machinii respond to both linalool and beta-myrcene. Olfactometer test results indicate that Cycadothrips are attracted by beta-ocimene isomers but are repelled by beta-myrcene. Weevils responded positively to linalool. The timing of specific component emissions may explain insect movements between cones. These tests suggest a possible plant species isolating mechanism that operates through the specific pollinator by affecting insect behavior.
