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Abstract
Although molecular-based phylogenetic studies of hosts and parasites are increasingly
common in the literature, no study to date has examined two congeneric lineages of
parasites that live in sympatry on the same lineage of hosts. This study examines
phylogenetic relationships among chewing lice (Phthiraptera: Trichodectidae) of the
Geomydoecus coronadoi and Geomydoecus mexicanus species complexes and compares these
to phylogenetic patterns in their hosts (pocket gophers of the rodent family Geomyidae).
Sympatry of congeneric lice provides a natural experiment to test the hypothesis that
closely related lineages of parasites will respond similarly to the same host. Sequence
data from the mitochondrial COI and the nuclear EF-1alpha genes confirm that the two
louse complexes are reciprocally monophyletic and that individual clades within each
species complex parasitize a different species of pocket gopher. Phylogenetic comparisons
reveal that both louse complexes show a significant pattern of cophylogeny with their
hosts. Comparisons of rates of nucleotide substitution at 4-fold degenerate sites
in the COI gene indicate that both groups of lice have significantly higher basal
mutation rates than their hosts. The two groups of lice have similar basal rates of
mutation, but lice of the G. coronadoi complex show significantly elevated rates of
nucleotide substitution at all sites. These rate differences are hypothesized to result
from population-level phenomena, such as effective population size, founder effects,
and drift, that influence rates of nucleotide substitution.