Signatures of mito-nuclear climate adaptation in a warbler species complex

Mitochondrial (mtDNA) and nuclear (nDNA) genes interact to govern
metabolic pathways of mitochondria. When differentiated populations
interbreed at secondary contact, incompatibilities between mtDNA of one
population and nDNA of the other could result in low fitness of hybrids.
Hermit Warblers (S. occidentalis) and Townsend’s Warblers (Setophaga
townsendi) exhibit distinct mtDNA haplotypes and a few nDNA regions of
high differentiation, whereas coastal S.
townsendi displays a mix of these genetic patterns consistent
with ancient hybridization of S. occidentalis and
inland S. townsendi. Of the few highly-differentiated nDNA
regions between inland S.
townsendi and S. occidentalis, two of these
regions (on chromosome 5 and Z, respectively) are also differentiated
between coastal and inland S. townsendi, similar to the
mtDNA pattern. These two nDNA regions are associated with mitochondrial
fatty acid metabolism. Moreover, these nDNA regions are correlated with
mtDNA ancestries among sites, a pattern consistent with mito-nuclear
co-adaptation. Such mito-nuclear coevolution might be driven by
climate-related selection, because the mito-nuclear ancestry is correlated
with climatic conditions among sampling sites. These results suggest that
cryptic differentiation in this species complex has been shaped by
climate-correlated adaptation associated with mito-nuclear fatty acid
metabolism.