The role of neo-sex chromosome architecture in promoting mitonuclear co-adaptation in Eopsaltria australis

The genome assembly of a female Eastern Yellow Robin Eopsaltria australis from the coastal lineage complements one from the inland lineage that is already publicly available. This species has recently emerged as a highly promising model for testing hypotheses around the importance of mitonuclear interactions in speciation, because its distribution exhibits a pattern of mitochondrial differentiation that is closely associated with different thermal conditions in coastal and inland environments, rather than a north-south geographic cline as most of its nuclear background does. One nuclear region on the Eastern Yellow Robin chromosome, homologous with passerine chromosome 1A, follows the pattern of spatial differentiation seen in the mitogenome and is located on a neo-sex chromosome system at least in the inland lineage, in which the majority of chromosome 1A has fused with the ancestral passerine sex chromosomes. Sex linkage has been postulated to play an outsized role in reproductive isolation and speciation processes, and we formulated specific hypotheses in this study to test whether neo-sex chromosome architecture is associated with increased rates of divergence between lineages, particularly on nuclear-encoded genes with mitochondrial function.