Co-diversification of a desert-adapted gall inducer clade with its host plant in the west

The mechanisms that give rise to extreme diversity, like that seen in the evolution of herbivore insect lineages, are of great interest to the field of entomology. Research into drivers of speciation is of particular importance in in gall-inducing insects, which use a specialist relationship to plant host in order to create a suitable gall for development. In recent years, there is growing evidence that gall inducers are even more specialized to plant host then we previously believed (e.g. host races), and that host-associated differentiation of gall-inducing populations may be a major cause of speciation in this highly species-rich group. What remains unclear is whether differentiation in gall inducers more commonly occurs following plant host adaptation (co-diversification), or as a result of a novel host-switching, and if either of these processes are a more consistent driver of diversification within gall-inducing radiations. We test for congruence in the evolutionary histories of a gall-inducing tephritid fly lineage (genus Aciurina) and its host plant taxon (Ericameria nauseosa) with paired samples from across the western U.S. using morphology and reduced representation genome sequencing. We present an extensive plant and insect co-phylogeny informed by geography and discuss the diversification patterns and their potential for understanding the ecological conditions suitable for substantial species radiations, like that of herbivorous insects.