Evidence for pollination ecotypes in the African cycad Encephalartos ghellinckii (Zamiaceae)

Abstract

Abstract Geographical variation in pollinator assemblages can result in divergent selective pressures and consequently drive the evolution of phenotypically distinct pollination ecotypes. Intraspecific divergence in pollination systems is well known among angiosperms, but is poorly studied among gymnosperms, often considered a relictual group not undergoing recent radiations. We investigated whether geographical divergence in the chemistry of volatiles emitted by cones of the cycad Encephalartos ghellinckii can be linked with functions for attracting different pollinators. We tested the pollination effectiveness of different insects in terms of deposition of fluorescent dye on micropyles and investigated whether daily fluctuations in cone temperature are correlated with insect pollinator activity. We also performed field bioassays to test the responses of insects to key volatiles and laboratory y-maze experiments to determine whether insects discriminate among cones of the two forms. Experiments confirmed that the beetles Metacucujus goodei and Erotylidae sp. nov. are the most effective pollinators of the mountain and lowland forms, respectively. A peak in cone temperatures in the afternoon was associated with extensive movement of insects among cones of mountain plants, but only limited movement of insects among cones of lowland plants. In field bioassays, Metacucujus goodei beetles were attracted mainly to (3E)-1,3-octadiene, a dominant component of the scent of mountain plants, whereas Erotylidae sp. nov. responded to camphene, a dominant component of the scent of lowland plants. When offered a choice, beetles preferred cones from the local population over those from a different population. The results suggest that E. ghellinckii consists of two pollination ecotypes with modifications in scent chemistry being correlated with an apparent shift between different pollinators. The relationship between thermogenesis and pollinator activities varies between ecotypes, with mountain plants showing a push–pull type of system and the lowland plants a system in which insects depart from cones with declining quality. This study highlights the role of insect pollinators in driving the evolution of cycads and the importance of thermogenesis and volatile emissions for mediating some beetle pollination systems.