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 size-assortative mating within and between species in three
different crosses, with the strongest prezygotic isolation between
populations that have the greatest interspecific size differences.
Together, our data support body size as a magic trait in pine sawflies and
possibly many other plant-feeding insects. Our work also demonstrates how
intraspecific variation in morphology and ecology can cause geographic
variation in the strength of prezygotic isolation.