Geographic and behavioural isolation promote the differentiation of parapatric host-associated forms in bud-galling midges (Diptera: Cecidomyiidae)

Host-associated differentiation (HAD) is widely recognized as a step towards speciation in phytophagous insects because adaptations to different host plants often lead to reproductive isolation. Various pre-mating and post-mating barriers can reduce gene flow between host-associated populations (HAPs), but establishing their relative contribution and the order in which they took effect during speciation is difficult, because thorough information is needed on past and present distribution ranges, behaviour and life history of the taxa involved. We investigated evidence for HAD in the gall-midge Dasyneuriola sp., which induces bud galls on the parapatric desert shrubs Suaeda fruticosa and Suaeda asphaltica in Israel. Based on mitochondrial sequence and amplified fragment length polymorphism analyses, we show that the populations from the two host plants are at an early stage of separation resulting from geographic and behavioural barriers between them. There was no assortative mating between the HAPs, but asymmetry between them was found in host fidelity by ovipositing females and in larval performance on the host plants. Together with findings on attack levels by natural enemies, these findings suggest that the original host plant in this system was S. asphaltica. We conclude that genetic differentiation in Dasyneuriola sp. was probably triggered by geographic separation, manifested by altitudinal rather than simple geographic distances, and that ecological barriers currently promote further divergence between the HAPs.