Molecular systematics and population genetics of whale lice (Amphipoda: Cyamidae) living on gray whale islands

Gray whales (Eschrichtius robustus) are 'living islands' to a diverse assemblage of crustacean ectoparasites that includes at least three species of highly specialized whale lice (Cyamus spp.). These lice are obligate parasites that undergo direct development on the gray-whale host, and are dependent on direct physical contact to colonize a new host whale. Given their high degree of morphological specialization and obligate relationship with whales, whale lice might be expected to have a close, long-term evolutionary association with gray whales. Such a relationship can lead to an evolutionary history of the parasite that closely mirrors that of its host and a highly correlated demographic history between the host and its parasites. Here I use a 738 base pair fragment of the mitochondrial DNA (mtDNA) cytochrome c oxidase subunit I gene (COI) gene to: (i) examine the phylogenetic relationships among, and genetic diversity within, gray-whale lice (C. scammoni, C. kessleri, and C. ceti); and (ii) to infer historical demographic patterns within each species of whale louse. Whale lice samples were collected from five different gray-whale hosts. Gray-whale lice exhibited relatively high levels of genetic diversity suggesting large effective population sizes and gene flow among different gray-whale hosts. Each species of louse was phylogenetically distinct and reciprocally monophyletic, indicating congruence between morphological and mtDNA phylogenies. The phylogeny also suggests that collectively these whale lice do not form a monophyletic group, supporting the hypothesis of independent, historical colonizations onto the gray-whale host. All louse species exhibited relatively high levels of genetic diversity. Mismatch distributions for all three gray-whale lice are consistent with long-term, stable historical population sizes and all three lineages coalesce to approximately 800,000 years before present. The polyphyletic relationships of gray-whale lice provide three independent replicates for indirectly examining the demographic history of their host, the gray whale.