Parallel host-plant-associated differences in an extended phenotype between populations of six species of gall-forming insects

Understanding the role of divergent host-use in promoting population differentiation among herbivorous insects provides valuable insight into the origin of diversity in this species-rich group. A strong signature of divergent selection in promoting population differentiation is provided by parallel phenotypic divergence across multiple insect species adapting to the same set of divergent host plants. In this study, we explored whether a key phenotype of gall-forming insects-gall size-exhibits parallel patterns of divergence across a community of gall-forming insect species because of adaptation to different host plant species. A previous study found parallel divergence in gall morphology across two gall-forming insect species adapted to two sister species of oaks (Fagaceae). Here, we expand this question to four additional members of the gall-forming insect community that specialise on the same two host plant species: Quercus geminata (Qg) and Quercus virginiana (Qv). Consistent with the previous result, all four additional gall-forming species tended to induce larger galls on Qg than on Qv, with two of the four species exhibited statistically significant differences and one species showed marginally significant differences. Moreover, we found gall size was similar for two gall-forming species when cross-reared on natural versus planted hosts, ruling out environmental causes of parallel host-associated gall size difference across multiple species. Our study documents parallel divergence in a key phenotype of gall-forming insects associated with different host environments, shedding light on the important role of divergent host use in driving the evolution of gall morphology.