Molecular Studies of Cospeciation in a Host-Parasite Assemblage

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9527583 HAFNER Despite considerable study, biologists still know very little about relative rates of DNA change in distantly related groups of organisms, such as mammals versus insects. In this research, Mark Hafner and Xuhua Xia will investigate a host-parasite system, from which they will generate comparative data about rates of DNA change. The hosts in this study are pocket gophers, which are small, herbivorous rodents found throughout much of North and Central America. The parasites are chewing lice, which are small, ectoparasitic insects that live in the fur of their hosts. The association between chewing lice and their hosts is considered obligate, because the lice cannot survive off their natural hosts. This intimate and long-term association between pocket gophers and chewing lice presents an unusual opportunity to examine both the pattern and rate of molecular change in the DNA of these organisms. Previous work has shown that each speciation event in the pocket gophers is accompanied by a speciation event in their lice. This means that one can measure the amount of molecular change between the species of lice that live on these gophers. Since time is a constant, such a comparison yields a direct measure of relative rates of DNA change in the gophers versus their lice. Sequences of two mitochondrial genes (cytochrome oxidase I and cytochrome b) will be used to construct independent phylogenies of gophers and lice. These trees will be compared statistically to evaluate overall branching similarity, hence degree of cospeciation in the gopher-louse assemblage. Finally, lengths of homologous branches in the gopher and louse trees will be compared to determine relative rates of DNA change in gophers versus lice.