The louse genera Brueelia (Ischnocera) and Myrsidea (Amblycera) are broadly codistributed on songbirds (Passeriformes), but differ in a variety of life history characteristics. We used mitochondrial and nuclear DNA sequences to assess levels of genetic divergence and reconstruct phylogenies of these 2 genera, focusing especially on Catharus thrushes in North America. We then qualitatively compared the phylogenies and levels of divergence within these 2 genera of codistributed parasites. Neither Brueelia nor Myrsidea appears to cospeciate with Catharus thrushes or passerine birds in general. The Myrsidea phylogeny exhibits significant levels of biogeographic structure, whereas the Brueelia phylogeny does not. Myrsidea and Brueelia also differ in their levels of intra-generic genetic divergence, with Myrsidea showing higher levels of genetic divergence and host specificity than Brueelia. Our genetic data support traditional morphology-based taxonomy in several instances in which the same species of Brueelia has been reported on multiple host taxa, e.g., all migrant Catharus spp. carry B. antiqua, with little haplotype divergence. Myrsidea found on each Catharus sp. are in general genetically distinct, except for M. incerta, which parasitizes both Catharus ustulatus and Catharus minimus. The strong biogeographic signal in the Myrsidea phylogeny and higher relative levels of host specificity of Myrsidea spp. suggest that infrequent host-switching, followed by speciation, is shaping the evolutionary history of this group. In contrast, the relatively lower host specificity of Brueelia spp. suggests that host-switching, combined with more frequent ongoing dispersal, has been more important in the evolutionary history of Brueelia.

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