Hybridization leads to host-use divergence in a polyphagous butterfly sibling species pair.

Climate warming has lead to increased genetic introgression across a narrow hybrid zone separating the eastern and Canadian tiger swallowtails (Papilio glaucus and Papilio canadensis). This situation has led to the formation of an allochronically separated hybrid population with a delayed emerging phenotype or "late flight". Here, we assess how the recombination of the parental genomes that lead to this phenotype may have facilitated another major ecological shift, host-use divergence. We first contrast the ovipositional profiles of the late flight population to that of the parental species P. glaucus and P. canadensis. Subsequently we contrast the larval survival and growth of the late flight, a P. canadensis and a P. glaucus population, and a population from the northern edge of the hybrid zone on five hosts. Our results indicate that the ovipositional preference of this hybrid swarm is identical to that of the introgressing parental species, P. glaucus. Due to the absence of the preferred hosts of P. glaucus (Liriodendron tulipifera L. and Ptelea trifoliata L.) where the late flight occurs, this ovipositional pattern implies a functional specialization onto a secondary host of both parental species, Fraxinus americana L. In contrast, the larval host-use abilities represent a mixture of P. glaucus and P. canadensis, indicating divergence in larval host-use abilities has not taken place. However, high genetic variability (genetic coefficient of variation) is present for growth on F. americana in the late flight hybrid swarm and tradeoffs for larval performance on the preferred hosts of the parental species are evident; indicating a strong potential for future specialization in larval host-use abilities. This current scenario represents an instance where a shift in a major ecological trait, host use, is likely occurring as a byproduct of a shift in an unrelated trait (delayed emergence) leading to partial reproductive isolation.