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Abstract
Understanding the determinants and consequences of parasite specialization across hosts is essential to predicting cross-species transmission and pathogen threats to humans and wild systems. My research focuses on milkweed butterflies (Danainae) and their neogregarine protozoan parasites, including Ophryocystis elektroscirrha (OE). In this dissertation, I used field studies, experiments, mechanistic models, and analysis of museum collections to describe the occurrence of infection across host species, identify factors that determine the ability of pathogens to infect different host species, and predict the resulting ecological outcomes of cross-species transmission. First, I investigated host resistance, tolerance, and immunological barriers to infection in two Danaus host species (monarchs and queens) and their respective parasite strains. I found evidence of imperfect parasite specialization to conspecific hosts and differences in host resistance, tolerance, immune investment, and parasite virulence. Next, I explored the ecological dynamics of cross-species infection of OE among monarchs and queens by quantifying parasite exposure at field sites where monarchs and queens overlap, and by developing and analyzing mechanistic compartment models of infection parameterized with empirical observations. I found a six-fold higher percentage of parasite-positive monarchs versus queens across field sites. Model analysis showed that, when the two species co-occur, monarchs decrease the abundance of queens through direct larval competition. Monarchs also increase OE prevalence in queens by shedding more infectious spores and consuming a larger amount of milkweed, increasing the likelihood of environmental parasite exposure by concentrating spore deposition on the remaining milkweed. Lastly, I expanded knowledge of the host range, global distribution, and prevalence of Ophyocystis parasites in milkweed butterflies using thousands of museum specimens. Parasites recovered from five Danaus hosts showed geographic heterogeneity in prevalence and differences in spore size, suggesting both parasite divergence and opportunities for cross-species transmission. Overall, my dissertation advances knowledge of host-parasite ecology by exploring the biological and ecological factors that influence parasite host range and cross-species transmission. Results here underscore the interplay of host, parasite, and environmental factors in determining cross-species infection and host range, and establish milkweed butterflies and their protozoan parasites as a study system with applications to conservation and ecological theory.