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OF PIKAS AND PARASITES: HISTORICAL BIOGEOGRAPHY OF AN ALPINE HOST-PARASITE ASSEMBLAGE
The goal of my dissertation research is to quantify the genetic consequences of Quaternary climatic oscillations for an alpine host-parasite assemblage. I focus on a mammalian alpine specialist (Ochotona princeps; American pika) that is distributed across North America?s Intermountain West, and a suite of its obligate nematode (Cephaluris, Labiostomum, Graphidiella, Murielus) and cestode (Schizorchis) endoparasites. In chapters one and two I reconstruct the population history of O. princeps to understand how climate-driven distributional shifts by the species have structured genetic diversity. Pika populations are partitioned into five non-overlapping mitochondrial lineages associated with different mountain ranges. Pulses of range expansion and contraction associated with past climatic oscillations have maintained lineage cohesion by promoting gene flow among sky island populations. Low-elevation basins may have helped maintain differentiation between lineages by limiting opportunities for genetic admixture, but two nuclear loci revealed genetic patterns consistent with recent gene flow across mitochondrial lineage boundaries. Thus, historical contact between lineages probably occurred, but it had different outcomes for different genetic loci. Northern lineages retain the genetic signature of glacial-age population expansion, likely associated with climate-driven shifts to lower elevations, and all lineages exhibit evidence of recent (post-Pleistocene) population decline. An ecological niche model suggested that continued climate warming could dramatically reduce the distribution of American pikas, potentially eliminating a major mitochondrial lineage. Chapter three builds on the phylogeographic framework established for O. princeps by testing for concordant population histories across several parasites. There is little phylogeographic congruence between the host and its parasites, suggesting independent evolutionary trajectories for the parasites. Indeed, close relationships between parasite populations associated with different host lineages indicate relatively extensive parasite gene flow across host lineage boundaries. Finally, chapter four uses host-parasite comparative phylogeography to elucidate the unresolved origins of O. princeps and its northern sister taxon, O. collaris. Four parasite lineages show that populations associated with O. collaris are phylogenetically nested within populations from O. princeps. This repeated pattern strongly indicates that O. collaris arose from a low-latitude ancestor, and demonstrates the utility of parasites for revealing cryptic host history.
co-evolution; climate change; glaciations; parasite; biogeography; Ochotona princeps; Pleistocene; statistical phylogeography; American pika; cestode; nematode; ecological niche modeling
dissertation or thesis