Geographical variation in cone volatile composition among populations of the African cycad Encephalartos villosus

Abstract

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.

Understanding intraspecific variation of floral scent in light of evolutionary ecology

“…Among studies that investigated geographical variation in floral scents, a sub-sample also provided some information on pollinator assemblage, making it possible to investigate whether scent and pollinator variation show some consistency. This has been done at different taxonomic scales, by comparing several populations within a given plant species (Dötterl et al, 2005(Dötterl et al, , 2007Mant et al, 2005;Svensson et al, 2005;Jhumur et al, 2008;Schlumberger and Raguso, 2008;Vereecken andSchiestl, 2008, 2009;Ibanez et al, 2010;Soler et al, 2011;Parachnowitsch et al, 2012;Suinyuy et al, 2012;Gross et al, 2016), different geographical ecotypes or subspecies (Chess et al, 2008;Schlumberger and Raguso, 2008;Suchet et al, 2011;Breitkopf et al, 2013;Doubleday et al, 2013) or closely related allopatric species (Svensson et al, 2006). Among the studies included in the present review, we found that variation in floral scents only rarely matches (only in five studies) with variation in pollinator identities.…”

“…The magnitude of such variation is therefore difficult to rank among studies, as it ranges from strikingly different chemotypes (e.g. Suinyuy et al, 2012) through blends that vary only through the occurrence of one or a few minor compounds (e.g. Dormont et al, 2010) to cases in which all investigated individuals produce exactly the same compounds (e.g.…”

Evolutionary dynamics of a cycad obligate pollination mutualism – Pattern and process in extant Macrozamia cycads and their specialist thrips pollinators

“…The central role that coning characteristics play in pollination suggests that these thrips species should be tightly associated with specific cycads or, perhaps, with groups of cycads that share coning characteristics, as occurs in other cycad systems (Suinyuy et al, 2012(Suinyuy et al, , 2013. One Macrozamia pollinator, C. chadwicki, pollinates 10 Macrozamia species (Forster et al, 1994;Jones et al, 2001;Mound, 1991;Terry et al, 2008) that are distributed mostly independently of one another in different environments across a broad area of eastern Australia (Fig.…”

Improving our chemistry: challenges and opportunities in the interdisciplinary study of floral volatiles

“…51 Both of these examples illustrate chemical aspects of the geographic mosaic of coevolution, as the pollinators in each case are different populations of the same moth and weevil species, respectively, and both represent nursery pollination systems that vary geographically in degree of mutual dependence. More difficult yet is to understand the extent to which the presence or absence of different VOC blends are the result of selection acting semi-independently on multiple genetic loci or other genetic processes.…”