The molecular basis and ecological consequences of a plant-insect interaction

The ability of an herbivorous insect to feed and develop on a given host plant is a complex, multifaceted trait that can be affected by genetic variation within the insect and plant species. Past work on the genetics of host use has focused mostly on either the plant or insect side of a plant-insect interaction, but rarely both. Here we jointly analyze genetic variation in the Melissa blue butterfly (Lycaeides melissa) and a host plant it has recently colonized (alfalfa; Medicago sativa) to quantify the genetic contribution from each species to the outcome of this newly evolved plant-insect interaction. We use a common garden rearing experiment and multilocus genome-wide association mapping to show that caterpillar performance depends on plant and insect genetic variation, with insect genetics being more important early in development and plant genetics contributing more later in development. Moreover, we find evidence of mostly additive genetic effects within and between species, which combine to explain ~30% of the variation in caterpillar performance. Much of the plant genetic effect can be explained by heritable variation in plant phytochemicals, including saponins. Thus, our results provide a mechanistic understanding of variation in a species interaction with possible consequences in terms of host plant occupancy at the landscape level.