Host use evolution in Chrysochus milkweed beetles: evidence from behaviour, population genetics and phylogeny

In two sister species of leaf beetles with overlapping host associations, C hrysochus auratus and C. cobaltinus, we established diet breadth and food p reference of local populations for evaluation together with genetic differe ntiation between populations. While C. auratus turned out to be monophagous on the same plant wherever we collected the beetles, the studied populatio ns of C. cobaltinus fed on three different plant species in the field. Plan t preference and ranking of the potential host plants significantly differe d between these populations. The amount of genetic differentiation between populations was measured by a polymerase chain reaction-restriction fragmen t length polymorphism (PCR-RFLP) assay of a 1300 bp mitochondrial DNA (mtDN A) sequence. In addition, the dominant genotypes of all populations were se quenced. No genetic differentiation between the populations of C. auratus c ould be detected in the RFLP assay and sequence divergence was low (= 0.3%) . In C. cobaltinus, on the other hand, genetic differentiation between popu lations was high, revealing a lack of gene flow over a much smaller scale a nd a maximum of 1.3% sequence divergence. C. cobaltinus thereby has the pre requisites for host race formation on different plants from the original ho st spectrum. Our sequence-based phylogeny estimate allows us to reconstruct historical diet evolution in Chrysochus Starting from an original associat ion with Asclepiadaceae, the common ancestor of C. auratus and C. cobaltinu s included Apocynaceae in its diet. The strict specialization on Apocynum a nd the loss of acceptance of Asclepiadaceae observed in C. auratus could ha ve resulted from a process similar to that displayed by C. cobaltinus popul ations.