Cross-species infection in milkweed butterflies: Host resistance, tolerance, and immune defense to a protozoan parasite

To understand and predict outcomes of pathogen transmission in insects, it is important to identify intrinsic and extrinsic mechanisms driving insect-shared pathogen interactions at the individual and population levels. This research examines causes and consequences of host specialization and cross-species transmission, focusing on a debilitating protozoan Ophryocystis elektroscirrha (OE) that infects wild monarch butterflies (Danaus plexippus). Similar OE-like parasites also infect several other milkweed butterfly species that overlap in geographical range with monarchs. We used transmission experiments on monarchs and the sympatric queens (D. gilippus) to ask the following questions: (1) How do resistance and tolerance to infection differ among milkweed butterfly species exposed to native vs. non-native parasite strains? (2) Does differential investment in immunity account for variation in infection between host species? Results showed that monarchs are more susceptible to infection than queens, and that queens are highly resistant to monarch-origin parasites. Queens were more tolerant to infection than monarchs, experiencing less reductions in lifespan with increasing spore loads. Queens also showed greater immune defense in the form of phenoloxidase activity, and a higher concentration of phagocytic hemocytes. This work is part of a broader effort examining the host range and geographic distribution of OE and OE-like parasites, parasite genetic and phenotypic divergence, and the dynamic consequences of cross-species transmission. Because OE is widespread and prevalent in monarchs and can reduce monarch abundance, identifying the degree to which parasites have specialized across different butterfly species and the fitness consequences of infection are crucial.