Phylogeny and history of host-plant association in the leaf beetle genus Trirhabda LeConte (Coleoptera: Chrysomelidae: Galerucinae)

The leaf beetle genus Trirhabda (Chrysomelidae: Galerucinae) contains 26 described species from America north of Mexico feeding on 14 host plant genera from 4 tribes of the family Asteraceae and one genus of the family Hydrophyllaceae. Although species of the genus have been used in many studies for biological control of weeds, and as model organisms for testing behavioral, physiological, and ecological questions, a study of species relationships was lacking. In this doctoral dissertation, the phylogeny of the genus was reconstructed from molecular data (∼1500 nucleotides), containing partial sequences of mitochondrial 12S rRNA and COI genes and the entire nuclear ribosomal internal tran-scribed spacer two (ITS2 rRNA), and 27 external morphological characters. A secondary structure models of Trirhabda ITS2 rRNA was developed in order to maximize positional homology of primary sequence alignment of the rRNA. This secondary structure model was predicted using an approach that combined an energy minimization method with comparative sequence analysis. The predicted models of the secondary structures were then used in refining the primary sequence alignment.; The phylogeny was used to trace the evolution of the host plant utilization. Combined as well as partitioned molecular data consistently recovered seven clades within Trirhabda, but relationships among them were not well supported. Although morphological data alone contain a number of homoplastic characters, their addition to the molecular data provided corroboration of the molecular phylogeny. All phylogenetic hypotheses showed that host plant association is phylogenetically conservative. However, Trirhabda experienced one shift to a distantly related family, from the composite family (Asteraceae) to the waterleaf family (Hydrophyllaceae). The genus experienced 12 other host plant shifts: 6 between tribes and 6 between genera within a tribe. The history of host plant association within Trirhabda is partially congruent with phylogenetic relationship among the four tribes of the family Asteraceae. The pattern of the host plant shift is attributed to the similarity in secondary chemistry of the host plants and not to coevolution between the host and the herbivore.