Genetic diversity and similarity in the Barbarea vulgaris complex (Brassicaceae)

Two subspecies of Barbarea vulgaris are taxonomically recognized as ssp. vulgaris and ssp. arcuata. In addition, two types of Barbarea vulgaris ssp. arcuata occurs in Denmark. The G-type is resistant to an herbivorous flea beetle (Phyllotreta nemorum) whereas the P-type is susceptible. A previous study suggested that the P-type evolved by a loss of resistance from a resistant progenitor. We analyzed the genetic relatedness among eight Barbarea taxa: B. vulgaris spp. vulgaris, B. vulgaris ssp. arcuata G- and P-types, hybrids between the types, B. verna, B. intermedia, B. stricta, B. orthoceras and B. australis, using AFLP and SSR markers. A clear distinction between the G- and P-types was revealed. Both were distinct from B. vulgaris ssp. vulgaris, the G-type less so than the P-type. Barbarea verna and B. intermedia formed unambiguous clusters, whereas the remaining taxa produced less discrete groupings. Possible evolutionary scenarios for flea-beetle resistance and susceptibility are discussed, including lineage sorting from a polymorphic ancestral population, and de novo loss of resistance in the P-type of B. vulgaris ssp. arcuata.