Patterns of divergence across the geographic and genomic landscape of a butterfly hybrid zone associated with a climatic gradient

Abstract

21Hybrid zones are a valuable tool for studying the process of speciation and for identifying 22 the genomic regions undergoing divergence and the ecological (extrinsic) and non-ecological 23 (intrinsic) factors involved. Here, we explored the genomic and geographic landscape of 24 divergence in a hybrid zone between Papilio glaucus and Papilio canadensis. Using a genome 25 scan of 28,417 ddRAD SNPs, we identified genomic regions under possible selection and 26 examined their distribution in the context of previously identified candidate genes for ecological 27 adaptations. We showed that differentiation was genome-wide, including multiple candidate 28 genes for ecological adaptations, particularly those involved in seasonal adaptation and host plant 29 detoxification. The Z-chromosome and four autosomes showed a disproportionate amount of 30 differentiation, suggesting genes on these chromosomes play a potential role in reproductive 31 isolation. Cline analyses of significantly differentiated genomic SNPs, and of species diagnostic 32 genetic markers, showed a high degree of geographic coincidence (81%) and concordance (80%) 33 and were associated with the geographic distribution of a climate-mediated developmental 34 threshold (length of the growing season). A relatively large proportion (1.3%) of the outliers for 35 divergent selection were not associated with candidate genes for ecological adaptations and may 36 reflect the presence of previously unrecognized intrinsic barriers between these species. These 37 results suggest that exogenous (climate-mediated) and endogenous (unknown) clines may have 38 become coupled and act together to reinforce reproductive isolation. This approach of assessing 39 divergence across both the genomic and geographic landscape can provide insight about the 40 interplay between the genetic architecture of reproductive isolation and endogenous and 41 exogenous selection. 42 43 peer-reviewed)