Role of microbial community in insect-plant interactions

Microbes can mediate insect-plant interactions and have been implicated in majorevolutionary transitions to herbivory. However, whether microbes also play a role inmore modest host shifts or expansions in herbivorous insects is less clear. Here weevaluate the potential for gut microbial communities to constrain or facilitate hostplant use in the Melissa blue butterfly (Lycaeides melissa). These butterflies feedexclusively on the leaves and flowers of legumes (Fabaceae) and have recently colonizedalfalfa (Medicago sativa), which was introduced in the western US in the mid 1800s as aforage crop. We conducted a larval rearing experiment where caterpillars from twopopulations were fed plant tissue from two hosts (alfalfa or a native lupine). We used16S rRNA sequencing to assay gut microbial communities of the caterpillars and of theplants they were fed. These data were then used to quantify the relative effects ofsample type (frass versus whole caterpillar), diet (plant species), butterfly populationand development (caterpillar age) on the composition and diversity of gut microbialcommunities, and secondly, to test for a relationship between microbial community andlarval performance. Gut microbial communities varied over time (that is, withcaterpillar age) and were also shaped by diet. Conversely, we found little to no effect ofbutterfly source population on microbial community. Caterpillar age affected bothcommunity composition and diversity, such that gut microbial diversity declined overtime. We only found evidence of an effect of diet on community composition, notdiversity. We found some evidence that gut microbe community composition wasassociated with caterpillar weight, particularly in caterpillars fed a native host (Lupinusargenteus). This suggests gut microbial community could affect performance in L.melissa in a host plant-dependent manner, and thus could in some cases, determinewhether plant species or population can be successfully colonized.