Different Geographical Distributions of Two Chemotypes of Barbarea vulgaris that Differ in Resistance to Insects and a Pathogen
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The interactions of plants with herbivores and pathogens have been suggested to drive the evolution of resistances in plants and in some cases new lineages and taxa. However, such divergence may require reproductive isolation, e.g., in allopatry. In the crucifer Barbarea vulgaris, some plants are resistant to the flea beetle Phyllotreta nemorum, due to production of specific saponins, whereas others are susceptible. Resistant and susceptible plants additionally differ in resistance to the pathogen Albugo candida, content of glucosinolates, and leaf pubescence, and they are genetically strongly divergent and partially reproductively incompatible. This suggests that at some point they were separated for a considerable length of time. Previously, the insect susceptible P-type had been described only from Denmark, Sweden, and Estonia, whereas the resistant G-type is widely distributed in Western Europe. Here, we tested whether the two plant types have divergent geographical distributions and maintain their distinct trait associations throughout their range. The insect-susceptible type was found in Russia, the Baltics, and parts of Fennoscandia, but not in Central Europe. In contrast, the insect resistant type was found from Finland and westwards. Their different trait associations were consistent within the two ranges. We therefore suggest that the two plant types diverged in allopatry at some time in the past, and evolved different resistances in response to local antagonists. The two plant types probably maintain their distinctness due to a hybridization barrier. Thus, the present distributions of the two types may be shaped by both historical processes and current differential biotic selection.
KeywordsAdaptation Allopatry Evolutionary divergence Plant-antagonist interaction Resistance
All contributors of plant material for this and previous studies are sincerely thanked. We thank the anonymous reviewers that contributed constructive comments to an earlier version of the manuscript. Vinnie Deichmann helped with DNA analyses, Karen R Munk with metabolite analyses. This work was carried out as part of Stina Christensen and Christine Heimes PhD projects, funded by the Faculty of Science, University of Copenhagen, and by a grant from The Danish Council for Independent Research, Technology and Production Sciences to Thure Hauser (no 09–065899).
T.P.H., C.H. and S.C. designed the research. C.H. and S.C. tested flea beetle resistance, S.C. tested Albugo resistance. C.H., V.K. C.E.O. and S.C. analyzed saponins, C.H., N.A. and S.C. analyzed glucosinolates, S.C. analyzed genetics, and T.P.H. and C.H. scored leaf pubescence. The manuscript was written by the authors based on a draft prepared by S.C.
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