Body size as a magic trait in two plant-feeding insect species

When gene flow accompanies speciation, recombination can decouple divergently selected loci and loci conferring reproductive isolation. This barrier to sympatric divergence disappears when assortative mating and disruptive selection involve the same “magic” trait. Although magic traits could be widespread, the relative importance of different types of magic traits to speciation remains unclear. Because body size frequently contributes to host adaptation and assortative mating in plant-feeding insects, we evaluated several magic trait predictions for this trait in a pair of sympatric Neodiprion sawfly species adapted to different pine hosts. A large morphological dataset revealed that sawfly adults from populations and species that use thicker-needled pines are consistently larger than those that use thinner-needled pines. Fitness data from recombinant backcross females revealed that egg size is under divergent selection between the preferred pines. Lastly, mating assays revealed strong ..., Geographic variation in host needle width and adult body size: Because Neodiprion eggs must be completely embedded in pockets that adult females carve within pine needles, egg size and female ovipositor size are likely constrained by needle width (Bendall et al. 2017). Given these constraints, we predicted that geographic variation in adult body size would correlate with geographic variation in host needle width. Although the ideal way to test this prediction would have been to collect host needles and sawfly adults from the same sites, we only had access to pine and sawfly samples that were collected at different times and without this hypothesis in mind. We therefore analyzed geographic variation in host needle width and adult body size separately, with the following expectations: N. lecontei adults and host needles will be bigger than those of N. pinetum and geographic trends in body size would mirror those observed for needle width (i.e., have slopes with the same sign and similar m..., All data files are .csv files and can be opened in R.