Parasites pose major threats to their hosts’ fitness, and hosts can protect themselves against infection through innate immune responses, anti-infection behaviors and microbial defenses. Monarch butterflies are specialist feeders of milkweed host plant species, which vary in their toxicity of secondary chemicals called cardenolides. Monarchs are commonly infected with a protozoan parasite, and infected females preferentially lay their eggs on high-cardenolide species of milkweed, which reduce infection in their offspring.
Results/Conclusions
While previous work suggests that cardenolides directly interfere with parasite infection, current studies also indicate that milkweeds alter the monarch gut microbiome, and that the altered microbiome contributes to parasite resistance. These results indicate that indirect effects in tri-trophic interactions can affect disease resistance by alterations of herbivore microbiomes.