Cone chemistry, thermogenesis and specialised erotylid beetle pollination of the West African cycad Encephalartos barteri (Zamiaceae) in Benin.

Abstract

Increasing evidence shows that most cycad species rely on cone thermogenesis and volatile emissions to facilitate pollination by specialised insects that utilise male cones as brood sites. However, African cycads remain poorly studied outside South Africa, leaving major biogeographic gaps in our understanding of pollination systems across the continent. Encephalartos barteri is the only cycad native to West Africa and is currently listed as Vulnerable. We investigated cone thermogenesis, volatile emissions and insect visitors of E. barteri across seven sites in Benin in West Africa. Cone volatiles were collected using dynamic headspace sampling and analysed by gas chromatography-mass spectrometry (GC-MS). Pollination potential was assessed using enclosure and fluorescent dye experiments. Volatile profiles of both male and female cones were dominated by the benzenoid p-methyl-anisole, while the alkene (3E)-1,3-octadiene also occurred in high relative amounts but was largely restricted to male cones. Male cones were thermogenic, reaching temperatures averaging 4.4 °C above ambient. Pollinator surveys revealed a single undescribed erotylid beetle (Coleoptera: Erotylidae) across all sites. In behavioural experiments, dye-marked erotylid beetles deposited fluorescent dye on the micropyles of receptive ovules, and the enclosure of pollen-laden beetles on female cones resulted in seed set, confirming their potential role as pollinators. Our results demonstrate that E. barteri possesses a specialised beetle-mediated pollination system associated with cone thermogenesis and distinctive volatile emissions. This study provides the first evidence of insect pollination in a West African Encephalartos and highlights the importance of conserving specialised pollinator mutualisms in threatened cycads.

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