Dissertation Research: Phylogeny, Host Plant Shifts, and Geographical Origin of the genus Trirhabda (Coleoptera: Chysomelidae)

Rutgers University New Brunswick, New Brunswick NJ

Investigators

Abstract

0073200 Kjer and Swigonova The diversity of beetles, grouped under the insect order Coleoptera, far exceeds the diversity of any other animal or plant group. Because more than half of all known beetles are herbivorous, studies of mechanisms of speciation in relation to host plant utilization, and potential coordinated evolution of beetles and their host plants (coevolution), should provide insights to understanding the evident evolutionary success of beetles. Hypotheses of evolutionary relations (phylogeny) provide a foundation for the study of such mechanisms. The leaf beetles (Chrysomelidae) belong to the fourth largest family of beetles, making them among the most successful organisms on the earth. Graduate student Zuzana Swigonova, advised by Dr. Karl Kjer, is studying the chrysomelid genus Trirhabda, in order to determine evolutionary relationships among the species, determine whether closely related species feed on closely related plants, reveal the history of host plant preference and host plant shifts among the species, disclose any pattern between species relatedness and geographic distribution, and identify the area that is the most probable biogeographical origin of the genus. The dissertation focuses on leaf beetles of the genus Trirhabda (Coleoptera: Chrysomelidae) which contains 26 species from America north of Mexico ("northern species") and 8 species from Mexico and Central America ("southern species"). The northern species have been studied in many ecological aspects and much is known about their host plant preference and life cycle. In contrast, there is no information about host plant association or ecology of the southern species. Six out of the eight southern species are found in Mexico. Beetles of the northern species feed on a narrow range of plants in the families Asteraceae and Hydrophyllaceae. Unlike other species from the family Chrysomelidae, they overwinter in the egg stage. Larvae and adults feed on leaves and inflorescence of the host plant. Pupation takes place in the soil below the host plant. The life history of each of the Trirhabda species is strongly influenced by the life history of their host plant. Several northern species of the genus have been used as biological agents in control of weeds in fields undergoing secondary succession. Despite the fact that Trirhabda species have been used in many ecological studies and despite the economic importance of some of the species, there has been no systematic study proposing evolutionary relationships within this genus. Phylogenetic relationships will be inferred from three independent data sets: one morphological and two molecular. Morphological data will include carefully examined and reevaluated characters described in the literature. New characters, including sensillae of the mouth parts, are to be examined using scanning electron microscopy. Molecular data will include DNA sequences of mitochondrial small subunit ribosomal RNA, the first subunit of mitochondrial cytochrome oxidase, and nuclear ribosomal ITS (internal transcribed spacer) sequences.