Smeds L, Warmuth V, Bolivar P, Uebbing S, Burri R, Suh A, Nater A, Bureš S, Garamszegi LZ, Hogner S, Moreno J, Qvarnström A, Ružić M, Sæther SA, Sætre GP, Török J, Ellegren H
Nat Commun 6 (-) 7330 [2015-06-04; online 2015-06-04]
The typically repetitive nature of the sex-limited chromosome means that it is often excluded from or poorly covered in genome assemblies, hindering studies of evolutionary and population genomic processes in non-recombining chromosomes. Here, we present a draft assembly of the non-recombining region of the collared flycatcher W chromosome, containing 46 genes without evidence of female-specific functional differentiation. Survival of genes during W chromosome degeneration has been highly non-random and expression data suggest that this can be attributed to selection for maintaining gene dose and ancestral expression levels of essential genes. Re-sequencing of large population samples revealed dramatically reduced levels of within-species diversity and elevated rates of between-species differentiation (lineage sorting), consistent with low effective population size. Concordance between W chromosome and mitochondrial DNA phylogenetic trees demonstrates evolutionary stable matrilineal inheritance of this nuclear-cytonuclear pair of chromosomes. Our results show both commonalities and differences between W chromosome and Y chromosome evolution.