Comparative genomics of specialization: the genetic basis of interspecific variation in pine use in Neodiprion sawflies

Pine sawflies in the genus Neodiprion are among the most important defoliating insects in the U.S., with frequent outbreaks that can result in loss of growth and tree mortality in valuable timber species in forest stands, plantations, and ornamental plantings. While most Neodiprion are restricted to hosts in the genus Pinus, there is considerable variation among species in the number and identity of pine hosts used. Here, we take advantage of this well-characterized variation to test the hypothesis that host-plant specialization involves predictable genetic changes; specifically, a widespread loss of chemosensory genes, elevated rates of molecular evolution in intact chemosensory genes, and an overall decrease in gene expression. To this end, we will: (1) sequence and assemble 19 Neodiprion genomes; (2) annotate candidate chemosensory genes (olfactory receptors, gustatory receptors, and odorant-binding proteins); (3) use comparative methods to test for associations between host-use patterns and chemosensory gene evolution; and (4) compare gene expression levels between a pair of sister species differing in host use. Together, these data will increase fundamental knowledge of the genetic mechanisms underlying host specialization in Neodiprion and, by elucidating the extent to which these mechanisms are predictable or idiosyncratic, plant-feeding insects as a whole. This knowledge could facilitate the development of novel chemosensory-based control strategies for pine sawflies and other economically important pests.