Phylogenesis of Insect/Plant Interactions: Have Phyllobrotica Leaf Beetles (Chrysomelidae) and the Lamiales Diversified in Parallel?

Abstract

The relative importance of conservative versus locally adapted traits for species interactions is an increasingly common theme in evolutionary ecology. Obligate interactions such as those between parasites and hosts often exhibit such strong phylogenetic conservatism that current associations may reflect diversification in parallel. Parallel phylogenesis, documented for animal parasites, has been doubted for insect/plant interactions, but phylogenetic studies of highly specific insect/plant associations are very few. A comparison of phylogeny estimates for the strictly monophagous Phyllobrotica leaf beetles and their lamialean hostplants shows nearly complete concordance, strongly supporting the hypothesis of parallel diversification. The cladogram concordance is significant or nearly so (consensus index values equalling or exceeding the critical value) under randomization distributions based on Adams (though not Nelson) consensus trees. The one clear exception shows unusual natural history, suggesting an isolated host transfer. Insect distributions and plant fossil ages are consistent with a mid-Tertiary age for both clades, further disfavoring the alternative hypothesis of entirely subsequent evolution. The dependence of both larval and adult beetles on the hostplants, larval endophagy, and possible dependence of beetles on toxic host compounds for defense against predators are suggested to underlie the evolutionary persistence of this interaction. Current host use in these beetles appears to reflect primarily the phylogeny of the interaction, strengthening the thesis that history can play a major role in structuring insect/plant relationships.