Phylogenetic analyses of the leaf beetle genus Galerucella: evidence for host switching at speciation?

It is still the subject of lively debate whether sympatric speciation is a general mode of speciation as opposed to allopatric speciation. In herbivorous insects, host switching, i.e. colonization of, and adaptation to, a new host by a herbivore, has been proposed as one of the driving mechanisms of sympatric speciation. Evidence for host switching as a speciation driving mechanism can be inferred from phylogenies of herbivores and host plants: if the host plant phylogeny is randomly distributed over the herbivore phylogeny, this indicates host switching. The Chrysomelid beetle genus Galerucella is a good taxon to study for evidence of host switching, because several closely related Galerucella species form sympatric species complexes associated with various unrelated plant species. Here we present the phylogenetic relationships of 10 species in the genus Galerucella, based on the mitochondrial gene fragments of the NADH-2 (410 bp) and CO-I (659 bp) genes, and analyzed with Bayesian, Maximum Likelihood and Maximum Parsimony methods. The resulting molecular phylogenetic tree proved to be largely congruent with morphologically based taxonomy. The host-associated taxa of the Galerucellanymphaeae species complex are not defined as distinct gene pools under the phylogenetic species concept (PSC), however, the species complex as a whole is. Two results indicate the contribution of host switching to the speciation of Galerucella: (1) the host-associated taxa of the G. nymphaeae species complex have diverged very recently and (2) constrained ML analyses showed that host use constraints led to a significantly different Galerucella tree compared to unconstrained analyses. This evidence for host switching, together with the observation that several sister taxa using unrelated host plants live in sympatry, suggests that sympatric speciation by host race formation can be an important mode of speciation in this genus.