Rapid ecological speciation in gall inducers

Ecological speciation in phytophagous insects is often mediated by adaptation to the host plant (e.g., chemical defenses, phenology) -which can subsequently lead to non-random mating on divergent hosts. However, the strength of ecological selection may vary across insect guilds. For those whose habitat is largely contained within or upon a host, including gall-inducing insects and parasites more broadly, selection can be particularly intense, potentially driving rapid diversification as organisms adapt to host defenses and physiology. Here, we investigate the divergence between Aciurina bigeloviae and A. trixa, two sympatric tephritid flies that induce morphologically distinct galls on different varieties of a single host plant species, Ericameria nauseosa. Host-use assays and hybrid crosses reveal strong "immigrant inviability" (i.e., premating isolation due to poor adaptation to non-native habitats) with purebred flies unable to induce galls on novel hosts and decreased gall induction success in both F₁ and F₂ generations. Together these findings indicate Aciurina species have high genetic specialization and suggest that mismatches in gene networks between fly and plant lead to reduced fitness. Genomic analyses revealed high genetic differentiation (F_ST) and low gene flow, despite an estimated divergence time of only 72-110 kya. Endosymbionts, while potentially important in reproductive isolation, were inconsistently present. Together, our results support the hypothesis that gall inducers experience elevated rates of ecological speciation due to their intimate developmental integration with host plants. This work highlights the role of host plant genotype in driving insect diversification and underscores how rapid reproductive isolation can arise in gall-inducing systems.