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

The close correspondence often observed between the taxonomy of parasites and their hosts has led to Fahrenholz's rule, which postulates that parasites and their hosts speciate in synchrony. This leads to the prediction that phylogenetic trees of parasites and their hosts should be topologically identical. We report here a test of this prediction which involves the construction of phylogenetic trees for rodents and their ectoparasites using protein electrophoretic data. We find a high degree of concordance in the branching patterns of the trees which suggests that there is a history of cospeciation in this host-parasite assemblage. In several cases where the branching patterns were identical in the host and parasite phylogenies, the branch lengths were also very similar which, given the assumptions of molecular clock theory, strongly suggests that the speciation of these hosts and ectoparasites was roughly contemporaneous and causally related.

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Genetic variation among populations of chewing lice (Geomydoecus actuosi) was examined in relation to chromosomal and electrophoretic variation among populations of their hosts (Thomomys bottae) at a contact zone. Louse demes were characterized by low levels of genetic heterozygosity (H̄ = 0.039) that may result from founder effects during primary infestation of hosts, compounded by seasonal reductions in louse population size. Louse populations sampled from different hosts showed high levels of genetic structuring both within and among host localities. Microgeographic differentiation of louse populations is high (mean F = 0.092) suggesting that properties of this host-parasite system promote differentiation of louse populations living on different individual hosts. Among-population differentiation in lice (F = 0.240) was similar to that measured among host populations (F = 0.236), suggesting a close association between gene flow in pocket gophers and gene flow in their lice.

Although most studies of codivergence rely primarily on topological comparisons of host and parasite phylogenies, temporal assessments are necessary to determine if divergence events in host and parasite trees occurred contemporaneously. A combination of cophylogenetic analyses and comparisons of branch lengths are used in this study to understand the host-parasite association between heteromyid rodents (Rodentia: Heteromyidae) and their sucking lice of the genus Fahrenholzia (Phthiraptera: Anoplura). Cophylogenetic comparisons based on nucleotide substitutions in the mitochondrial COI gene reveal a significant, but not perfect, pattern of cophylogeny between heteromyids and their sucking lice. Regression analyses show a significant functional relationship between the lengths of analogous branches in the host and parasite trees, indicating that divergence events in hosts and parasites were approximately contemporaneous. Thus, the topological similarity observed between heteromyids and their lice is the result of codivergence. These analyses also show that the COI gene in lice is evolving two to three times faster than the same gene in their hosts (similar to the results of studies of other lice and their vertebrate hosts) and that divergence events in lice occurred shortly after host divergence. We recommend that future studies of codivergence include temporal comparisons and, when possible, use the same molecular marker(s) in hosts and parasites to achieve the greatest insight into the history of the host-parasite relationship.

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