Climate change induced hybridization leads to a host shift in a polyphagous butterfly

Background/Question/Methods

The high diversity of plant feeding insects is believed to be due to their radiation onto the wide chemical diversity of plants. In particular this divergence is believed to be due to the colonization of novel hosts, making novel niches available for adaptive radiation through specialization to take place. The polyphagous swallowtail butterfly sibling species Papilio glaucus and Papilio canadensis represent an ideal study group for studying such host shifts. They exhibit many ecological similarities, but differ significantly in their host use abilities. In addition, recent increased movement of Papilio glaucus into P. canadensis territory, due to climate warming, has lead to the formation of an allochronically separated hybrid swarm population or “late flight”. This recombination of the P. glaucus and P. canadensis genomes is likely to have produced novel combinations of detoxification enzymes and ovipositional preferences, which may lead to novel host associations. In this study we assess how the formation of this delayed emerging phenotype may have affected the host use traits in this “late flight” population using bioassays for both oviposition and larval host use abilities on plants of variable quality for both parental species. We first contrast the ovipositional profiles of the “late flight” population to P. glaucus and P. canadensis using oviposition assays containing host plants of P. glaucus and/or P. canadensis from seven different plant families. Secondly we contrast survival and growth of the “late flight”, P. glaucus, P. canadensis, and a population from the P. glaucus and P. canadensis hybrid zone in Michigan, U.S.A. on hosts from five host plant families. Finally, we present a quantitative genetic analysis for larval host use abilities of the “late flight” population using estimates of the broad sense heritability and coefficients of genetic variation from larval growth assays on five hosts.

Results/Conclusions

Our results indicate that the ovipositional preference of this hybrid swarm is identical to that of P. glaucus, although the preferred host of P. glaucus is absent where this hybrid swarm occurs. In contrast, the larval host use abilities represent a mixture of P. glaucus and P. canadensis host use abilities and high genetic variability is present for host use on most, but not all, hosts tested. These results indicate that the recombination leading to this phenotype with delayed emergence is also leading to a host shift onto a secondary host of both parental species.