Brood-site mutualisms represent extreme levels of reciprocal specialization between plants and insect pollinators, raising questions about whether these mutualisms are mediated by volatile signals and whether these signals and insect responses to them covary geographically in a manner expected from coevolution. Cycads are an ancient plant lineage in which almost all extant species are pollinated through brood-site mutualisms with insects. We investigated whether volatile emissions and insect olfactory responses are matched across the distribution range of the African cycad Encephalartos villosus. This cycad species is pollinated by the same beetle species across its distribution, but cone volatile emissions are dominated by alkenes in northern populations, and by monoterpenes and a pyrazine compound in southern populations. In reciprocal choice experiments, insects chose the scent of cones from the local region over that of cones from the other region. Antennae of beetles from northern populations responded mainly to alkenes, while those of beetles from southern populations responded mainly to pyrazine. In bioassay experiments, beetles were most strongly attracted to alkenes in northern populations and to the pyrazine compound in southern populations. Geographical matching of cone volatiles and pollinator olfactory preference is consistent with coevolution in this specialized mutualism.

false2016-03-16T23:02:09

Variation in traits across species distribution ranges is often indicative of diversifying evolution that can lead to speciation. Of particular interest is whether traits vary clinally or abruptly because the latter pattern can be indicative of incipient speciation. Understanding of intraspecific variation in chemical traits is still in its infancy because studies of population variation have tended to focus on morphology or neutral genetic markers. To address these issues, the composition of cone volatile odours was examined in ten populations of the South African cycad Encephalartos villosus across its range in the Eastern Cape and KwaZulu Natal using headspace sampling and analysis by gas chromatography‐mass spectrometry. Because volatiles play a key role in attracting pollinators to cones of Encephalartos cycads and may thus reflect local adaptation to pollinators, pollinator assemblages were also investigated in the ten populations of E. villosus. Volatile compounds from populations in the north of the distribution range were dominated by unsaturated hydrocarbons, whereas, in the southern populations, nitrogen‐containing compound and terpenoids were the major compounds. A shift between southern and northern populations appeared to occur at the Umtamvuna River, where populations had odour profiles with components of both the northern and southern populations. However, one population in the north (Vernon Crookes Nature Reserve) had a quantitatively similar odour profile to the populations in the extreme south of the range. These results reveal strong interpopulation variation in the cone scent of E. villosus, including variation in the relative emission of dominant compounds that may play key functional role in this pollination system. However, pollinator assemblages did not differ across the different populations, which suggest that these patterns were produced by co‐evolution or drift, rather than by pollinator shifts. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106, 514–527.

There are pronounced developmental changes in volatile emissions and heating in E. villosus cones, as well as strong daily changes in thermogenesis. Daily patterns of volatile emissions and pollinator abundance in E. villosus are different from those observed in some Macrozamia cycads and not consistent with the push-pull pattern as periods of peak odour emission do not coincide with mass exodus of insects from male cones.

No abstract

No abstract

No abstract

No abstract