| Parasites affect nearly every organism on earth, and can be major drivers of host ecology and evolution. Due to the incredible diversity of parasites, and their tight host associations, parasites are ideal for understanding long-term patterns of micro- and macroevolution. Recent studies using molecular techniques have facilitated our understanding of host-parasite coevolution. However, the ecological factors governing coevolution remain poorly understood. In this dissertation, I focus on the influence of transmission ecology on host-parasite coevolution. Using a model system consisting of the wing lice and body lice (Phthiraptera: Ischnocera) of Columbiform birds, I compare louse transmission dynamics to elucidate its role in driving patterns of community ecology and long-term coevolution. In a series of captive and field experiments, I show that wing and body lice do not differ in their rate of horizontal transmission via the direct contact of hosts. However, I demonstrate that wing lice can use an alternate mode of transmission, phoresis, while body lice are not phoretic. Phoretic wing lice "hitchhike" rides on mobile hippoboscid flies (Diptera: Hippoboscidae) to transmit between hosts. Differences in phoresis appear to be consistent with models of species coexistence via a tradeoff in competitive and colonization abilities. Wing lice, the inferior competitor, may be able to coexist with body lice, the superior competitor, through increased transmission to competition-free hosts. Moreover, differences in phoresis are consistent with marked differences in bird-louse coevolutionary history between wing and body lice. Hippoboscid flies are less host specific than the wing lice they transport, and I demonstrate that phoresis facilitates louse dispersal to novel host species. Encountering novel host species is a key factor in eroding host specificity and cophylogenetic congruence between hosts and parasites. Further tests show that morphological and behavioral adaptations for specialization on different microhabitat regions of the host may drive this difference in phoresis between wing and body lice. Finally, in a broad review of phoresis in Philopterid lice, I show that phoresis does indeed promote encounters with novel host species. Additionally, across louse taxa, phoretic lice appear to be less host specific than nonphoretic lice, which is consistent with the patterns observed in our experiments with Columbiform lice. |
