Host defense mechanisms against parasites undergo strong selection, and it is increasingly clear that microbial symbionts can provide such defenses. Monarch butterflies have undergone a long coevolution with their toxic milkweed host plants and detrimental protozoan parasites. Previous work has shown that monarchs obtain resistance to their parasites when feeding on milkweeds with higher concentrations of secondary chemicals called cardenolides. Like all other animals, monarchs host a diversity of microbes in their guts, and we have tested the hypothesis that plant-conferred resistance is the result of an alteration of the monarch gut microbial community to an anti-parasitic state.
Results/Conclusions
We fed larvae on different plant species, dissected the larval midgut and sequenced bacterial 16s rDNA sequences. Results showed that larvae fed on these different plants had different microbial communities. In follow-up experiments, we transferred fecal pellets from caterpillars reared on high-cardenolide plants to caterpillars reared on low-cardenolide plants. This resulted in increased parasite resistance, suggesting that the gut microbiota are modulated by high-cardenolide milkweed and confer protection against parasite infection. Overall, these results suggest that gut microbes alter the resistance phenotypes of monarchs and that milkweeds play an important role in the coevolution between monarchs, parasites and gut microbes.