Genomic Insights on the Recent Evolution of Novel Host Use in the Melissa Blue Butterfly (Lycaeides melissa)

The factors that shape the evolution of animal diets remain poorly known. For herbivorous insects, the expectation has been that trade-offs exist, such that adaptation to one host plant reduces success on other potential hosts. We investigated the genomic basis of alternative host plant use in Melissa blue butterflies (Lycaeides melissa) by analyzing genetic variation in natural and experimental butterfly populations. We showed that distinct Melissa blue butterfly populations have independently colonized alfalfa since the 1800s when this plant was introduced, and that these populations have adapted to this novel resource. We documented segregating polygenic variation within and among butterfly populations for performance on alfalfa, and showed that different instances of adaptation to alfalfa have occurred via selection on a mixture of the same and different genes. Genetic variants in transposable elements and those in or near genes with metal-ion binding function might be particularly important for host adaptation. We documented very few loci with genetic trade-offs that would inherently constrain diet breadth by preventing the optimization of performance across hosts. Instead most genetic variants that affected performance on one host had little to no effect on the other host.