Ecotypic differentiation of two sympatric chemotypes of Barbarea vulgaris (Brassicaceae) with different biotic resistances

Abstract

Evolution of ecotypic differentiation and maintenance of divergence in sympatry is more likely if populations occupy different habitats. Two genetically divergent chemotypes of Barbarea vulgaris (Brassicaceae) with contrasting insect and disease resistances (G- and P-type) occupy different geographical ranges in Eurasia but co-occur in Denmark. Here, they grow mostly in separate populations, posing the question why they have not merged. In a 2-year reciprocal transplant experiment, we tested whether the two plant types prefer different habitats and are adapted to these, possibly influenced by selection by herbivores. Vegetation and soil analyses indicated that the G-type occurs in slightly drier, less productive and more alkaline sites than the P-type. Plants planted at sites of their own chemotype had a higher biomass in the first year than plants of the non-resident chemotype, but were more damaged by beetle herbivory. This suggests that herbivores have adapted genetically or plastically to the resident chemotype. G-plants were less damaged by molluscs than P-plants, which may be caused by their content of saponins or glucosinolates. Levels of glucosinolates in siliques were higher in G-plants, but varied strongly among sites. Survival, reproduction and biomass in the second year did not differ between plants growing at sites of their own chemotype and plants of the other chemotype. Thus, the two chemotypes of B. vulgaris are adapted to different habitats to some extent, which could contribute to maintain their differentiation in sympatry. Herbivory by some insects, however, may counteract this.