Temporal and host-related variation in frequencies of genes that enable Phyllotreta nemorum to utilize a novel host plant, Barbarea vulgaris

The flea beetle, Phyllotreta nemorum L. (Coleoptera: Chrysomelidae), is an intermediate specialist feeding on a small number of plants within the family Brassicaceae. The most commonly used host plant is Sinapis arvensis L., whereas the species is found more rarely on Cardaria draba (L.) Desv., Barbarea vulgaris R.Br., and cultivated radish (Raphanus sativus L.). The interaction between flea beetles and Barbarea vulgaris ssp. arcuata (Opiz.) Simkovics seems to offer a good opportunity for experimental studies of coevolution. The plant is polymorphic, as it contains one type (the P-type) that is susceptible to all flea beetle genotypes, and another type (the G-type) that is resistant to some genotypes. At the same time, the flea beetle is also polymorphic, as some genotypes can utilize the G-type whereas others cannot. The ability to utilize the G-type of B. vulgaris ssp. arcuata is controlled by major dominant genes (R-genes). The present investigation measured the frequencies of flea beetles with R-genes in populations living on different host plants in 2 years (1999 and 2003). Frequencies of beetles with R-genes were high in populations living on the G-type of B. vulgaris ssp. arcuata in both years. Frequencies of beetles with R-genes were lower in populations living on other host plants, and declining frequencies were observed in five out of six populations living on S. arvensis. Selection in favour of R-genes in populations living on B. vulgaris is the most likely mechanism to account for the observed differences in the relative abundance of R-genes in flea beetle populations utilizing different host plants. A geographic mosaic with differential levels of interactions between flea beetles and their host plants was demonstrated.