DISPARATE RATES OF MOLECULAR EVOLUTION IN COSPECIATING HOSTS AND PARASITES

DNA sequences for the gene encoding mitochondrial cytochrome oxidase I in a group of rodents (pocket gophers) and their ectoparasites (chewi ng lice) provide evidence for cospeciation and reveal different rates of molecular evolution in the hosts and their parasites. The overall r ate of nucleotide substitution (both silent and replacement changes) i s approximately three times higher in lice, and the rate of synonymous substitution (based on analysis of fourfold degenerate sites) is appr oximately an order of magnitude greater in lice. The difference in syn onymous substitution rate between lice and gophers correlates with a d ifference of similar magnitude in generation times.