Highly differentiated sex chromosomes are common in many species, and have evolved through processes including cessation of recombination, degeneration of Y and W chromosomes and the evolution of dosage compensation (Naurin et al. 2010, 2011). It is difficult to study these processes in old, already highly heteromorphic, sex chromosomes, such as the X and Y chromosomes in mammals and the Z and W in birds. Studies of sex chromosome evolution have therefore often focused on much younger parts of the sex chromosomes (so called neo-sex chromosomes), resulting from the fusion of autosomal material with the ancestral sex chromosomes.
In birds (ZW) sex chromosomes, and particularly the Z chromosome, have been remarkably stable over millions of years, a property they share with sex chromosomes in mammals. However, we have recently detected a novel neo-sex chromosome in passerine birds. We initially found some evidences for that a large part of chromosome 4a, an autosome in the zebra finch genome, is sex-linked in two species of warblers (family Sylviidae); the common whitethroat (Sylvia communis) and the great reed warbler (Acrocephalus arundinaceus), respectively.
In the common whitethroat, chromosome 4a had highly elevated levels of sex-biased gene expression, and in the great reed warbler some markers on 4a were sex-linked. We are presently using molecular tools to study a large set of loci on chromosome 4a in warblers and closely related passerine lineages. This data make it possible to evaluate when the neo-sex chromosome arose and how large part of chromosome 4a is sex-linked. We also study the evolutionary consequences of sex linkage in birds by comparing sequence data from both the neo-sex chromosome and the much older avian Z- and W-chromosomes.
Dawson DA, Åkesson M, Burke T, Pemberton JM, Slate J, Hansson B (2007) Gene order and recombination rate in homologous chromosome regions of the chicken and a passerine bird. Molecular Biology and Evolution 24, 15371552.
Naurin S, Hansson B, Bensch S, Hasselquist D (2010) Why does dosage compensation differ between XY and ZW taxa? Trends Genet. 26, 1520.
Naurin S, Hansson B, Hasselquist D, Kim Y-H, Bensch S (2011) The sex-biased brain: sexual dimorphism in gene-expression in two species of songbirds. BMC Genomics 12, 37.