Evolutionary variations on a theme: host plant specialization in five geographical populations of the leaf beetle Chrysomela lapponica

The ancestral host plants of Chrysomela lapponica are Salicaceae rich in salicylic glycosides (SGs), which serve as precursors for larval chemical defensive secretions. Nevertheless, some populations have shifted to plants poor in SGs or even lacking these compounds. To study whether this shift is accompanied by adaptations to novel SG-poor host plants, we reared C. lapponica larvae from five geographical populations on host plants with high (Salix myrsinifolia) or low (S. caprea) SG content. Individuals from two populations (Finland and Kola region in Russia) associated in nature with SG-rich S. myrsinifolia showed higher survival and shorter developmental time on native host species than on foreign SG-poor S. caprea, thus demonstrating local adaptations to their ancestral SG-rich host plant. Individuals from a Belarus population associated in nature with SG-poor S. caprea showed higher survival on this species than on foreign SG-rich S. myrsinifolia, thus demonstrating local adaptation to the novel SG-poor host. On the other hand, individuals from two other populations associated in nature with SG-poor plants (Baikal and Ural region) performed equally well on both SG-rich S. myrsinifolia and SG-poor S. caprea in our rearing experiments, thus showing no local adaptation to a specific SG-host type, but rather a wide feeding niche including several Salicaceae species of different SG-type. Our results suggest that diet breadth of C. lapponica is a local phenomenon, and that adaptation strategies to novel host plants may differ between populations of a single leaf beetle species.