Push–Pull Pollination

Odour emission, thermogenesis and pollination in cycad cones
Odour emission, thermogenesis and pollination in cycad cones

Cycads know a thing or two about pollination – that’s one reason they’ve been around since the Jurassic period. They are found across much of the subtropical and tropical parts of the world and many can survive in harsh conditions, from semidesert climates to wet rain forest conditions.

All cycads are dioecious, and insect pollinators have to visit both male and female plants for pollination to occur. Effective pollination requires pollinators to move from male to female cones or to move back and forth between the male and female cones. The known pollinators of almost all cycads are insect herbivores whose larvae feed on male cone tissues (including pollen) of the host cycad, so that the male cone serves as the larval brood site.

Floral odour and heat are characteristic of the reproductive structures of some cycads. Ontogenetic (developmental) changes in floral odour and heat production have also been shown to influence pollinator behaviour. The cues affecting insect pollinator behaviour, such as aggregation, attraction, repellence, mating and oviposition, may differ between male and female plants of the host species as well as at different times of the day or stages of cone development. Until recently it was assumed that volatiles and heat production in cycads function solely to attract pollinators to inflorescences, but Terry et al. (2004) observed that insect pollinators actually left male cones of several Australian cycads (Macrozamia spp.) during periods of peak volatile emission, which also coincided with peaks in cone temperature as a result of thermogenesis – the plants appear to get rid of potential pollinators they don’t want, a ‘push–pull’ pollination strategy.

A recent paper in Annals of Botany examines patterns of cone odour emissions and heating in the African cycad Encephalartos villosus and demonstrates that these are different from those observed in Macrozamia cycads and are not consistent with the push–pull pattern as periods of peak odour emission do not coincide with mass exodus of insects from male cones.