The blacklegged tick, Ixodes scapularis, is the tick vector species responsible for transmitting the Lyme disease pathogen (i.e., Borrelia burgdorferi) in the eastern United States (US). Although this vector species is established throughout the southern US, geographical patterns of Lyme disease incidence indicate that cases tend to be localized in the northeastern and north central US. Several hypotheses have been proposed to explain this gradient in incidence, including regional differences in abiotic conditions, host diversity, tick genetics, and tick questing behavior. This dissertation explores how abiotic conditions and tick genetics may contribute to existing and future patterns of Lyme disease incidence, via their effects on I. scapularis life cycle processes. Using a four-year common garden experiment, I investigated the existing variation in life cycle processes (i.e., emergence and survival) among four widely-dispersed populations located within northern and southern areas of high and low Lyme disease incidence, respectively (Chapter 1). I then explored the potential mechanistic roles of abiotic and genetic explanatory factors underlying the observed among-population variation in emergence and survival. To address the observed among-site variation in emergence, I evaluated the accuracy of temperature-development models in predicting the emergence of ticks across populations, explored empirical evidence for the effects of genetics and plasticity on emergence, and identified potentially important explanatory factors contributing to the accuracy of temperature-development models in predicting emergence timing (Chapter 2). To address the observed among-site variation in survival, I explored empirical evidence for differences in survival among local and transplanted ticks, ticks placed at northern and southern sites, and ticks from northern and southern sites of origin; I also identified potentially important explanatory factors contributing to the observed variation in survival (Chapter 3). In Chapter 1, I found that there was significant among-site variation in emergence and survival, and that these life cycles may be extended at northern sites, relative to southern sites. This chapter also provided the first documentation of bimodal emergence for this species. In Chapter 2, I discovered that temperature-development models significantly under-predicted emergence timing across all life stages and populations used in this study, showed evidence of genetics and plasticity affecting emergence among sites, and identified genetics, plasticity, and key abiotic conditions as potentially important explanatory factors influencing the accuracy of these temperature-development models. In Chapter 3, I demonstrated that southern conditions may be less conducive than northern conditions to tick survival, based on observed larval survival patterns; I also showed that ticks of northern origin may be more robust than ticks of southern origin, based on observed nymphal survival patterns. I supported these trends in survival with the identification of the interaction between genetics and plasticity, abiotic conditions, and diapause as important explanatory factors in best-fitting models of I. scapularis survival. The findings of this dissertation highlight the potentially important contributions that abiotic and genetic factors have on variation in emergence and survival among I. scapularis populations. Understanding how these factors affect the life cycle processes of this may have important implications for predicting disease risk, as populations of this vector species invade new areas and are exposed to new abiotic regimes via climate change.
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