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