Genetic and geographic structure of an insect resistant and a susceptible type of Barbarea vulgaris in western Europe

Abstract Interactions between herbivores and plants are believed to have been important
drivers of biodiversity. However, to drive an initial resistance divergence into different
evolutionary lineages and taxa, these interactions have probably been embedded in other
processes of divergence, like allopatric isolation. The cruciferous plant Barbarea vulgaris
ssp. arcuata occurs in Denmark in two types: one (G) is resistant to most genotypes of the
flea beetle Phyllotreta nemorum, the other (P) is susceptible. The two types additionally
differ in hairiness and glucosinolates, they are genetically strongly divergent, and reproduction
between them is reduced. To determine whether the two plant types and their
resistance polymorphisms are also present outside Denmark, and to understand how they
have evolved, we analysed 33 European populations of B. vulgaris for resistance, hairiness,
glucosinolates, and microsatellite markers. Most populations had traits indicative of the G
type, including the already characterized Danish G populations. In contrast, only two
populations outside Denmark were of the P type; one from northern Sweden and one from
Estonia. Genetically, the G populations formed two genetic clusters that were strongly
divergent from a genetic cluster containing P populations. A fourth genetic cluster, which
contained only a single population and no Danish plants, belonged morphologically to the
subspecies ssp. vulgaris. The divergence found in Denmark between a resistant G and a
susceptible P type is thus part of a larger divergence in Europe. Judging from the trait
correlations, genetic divergence, and partial reproductive incompatibility, the plant types
must have been isolated from each other for quite some time. The two P populations
outside Denmark came from the north and east, suggesting a more eastern distribution. If
so, resistant and susceptible types could have diverged during the ice age and later met in
Scandinavia. However, more samples from Eastern Europe are needed to clarify this.