Host plant species shapes the interconnected microbiomes of monarchs and milkweed

Plants produce defensive chemicals against insect herbivores that may also alter plant and insect associated microbial communities. However, it is unknown if defensive chemicals impact insect and plant microbiomes in a similar way or enhance communities for microbes that can degrade these compounds. Monarch butterflies (Danaus plexippus) feed on milkweed species (Asclepias spp.) that vary in production of toxic cardiac glycosides, which could alter associated microbiomes. We therefore sought to understand if different milkweed species, with varying cardiac glycoside profiles, alter the diversity and structure of monarch, rhizosphere, and phyllosphere microbial communities. It seems each milkweed species may be selecting for different sets of microbial taxa across associated microbiomes. The degree of shared microbial taxa found across monarchs and the rhizosphere implies that milkweed species are exhibiting a top down effect on associated microbial communities. Also, it means that monarchs may acquire their microbiomes through environmental sources or through horizontal transfer from feeding on phyllosphere tissues or encountering soil.