Shifts in genetic and morphological clines of a butterfly hybrid zone may be a response to a 30 year period of climate change

Background/Question/Methods

Clinal variation in species traits that correspond with climatic gradients are predicted to shift as climate change alters this underlying gradient.  However, despite considerable effort, surprisingly few studies have documented such changes. Using historically (1980s) and recently (2010s) collected specimens from a butterfly hybrid zone (Papilio glaucus, Papilio canadensis) we explored whether a number of morphological and genetic traits that are known to differ between these two species have shifted in response to recent warming.  Specifically to test whether the hybrid zone has shifted over the last 30 years we compared the historic and contemporary distribution of a species diagnostic restriction site in Cytochrome Oxidase 1 (COI) and a single nucleotide polymorphism from the coding region of Lactate Dehydrogenase (LDH) that is putatively involved in adaptation to temperature. Using 16 neutral (microsatellite) loci we also investigated the extent of gene flow between species and whether the amount of introgression between species has changed over time.  A number of species diagnostic wing morphology traits, including a melanic band putatively involved in thermoregulation were also measured and compared between time periods to test whether these traits have also shifted over the last 30 years.

Results/Conclusions

Both the morphological and genetic trait clines of the P. glaucus – P. canadensis hybrid zone have shifted over the last 30 years and these changes correspond with documented warming in the region. Based on genetic structure analysis of 16 polymorphic microsatellite markers, we found extensive gene flow between species with little change over the last 30 years, suggesting that these species are hybridizing in the wild. We also found that the species diagnostic maker (COI) has significantly increased in frequency at higher latitudes over the last 30 years, resulting from either a shift in P. glaucus further north or through increased success of this marker to establish in P. canadensis populations.  Similarly, we found many wing traits have also changed during this period, with butterflies becoming smaller and producing less melanism, particularly in the southern portion of the hybrid zone where warming has been the greatest. Taken together, these results suggest that the P. glaucus – P. canadensis butterfly hybrid zone is undergoing phenotypic and genetic changes that may be a response to rapid warming.