Selection sweeps of the W chromosome or mt DNA?
The W chromosomes of birds have very low rates of variation suggesting that the genes on this chromosome been subjected to selective sweeps (Berlin and Ellegren 2004; Ellegren 2013; Smeds et al. 2015). This low rate of variation in the genes on the avian W chromosome led Berlin et al. (2007) to hypothesize that selective sweeps on genes in the W chromosome would also result in selective sweeps on the mt genome, due to strict maternal linkage (perfect co-transmission) of mt DNA and the W chromosome. Following this logic, Berlin et al. (2007) proposed that evidence of selective sweeps of the mt genome would be evidence for positive selection on W genes. However, it could also work the other way: selective sweeps of the mt genome would cause selective sweeps and loss of variation in the W chromosome (Marais 2007; Lane 2008). The fact that the W chromosome of all birds investigated show signs of loss of genetic diversity via selective sweeps is thus, potentially, further support for the idea that the mt DNA barcode pattern is a consequence of selective sweeps. Birds are not the only taxa with ZW sex determination and co-transmission of mt and W chromosomes but to date there is much more sequencing data available for the W chromosome of birds than for any other ZW taxa. A broader survey of genetic diversity of W-linked genes might make it possible to distinguish whether selection on the W-linked genes or mt-linked genes is responsible for observed patterns of low genetic diversity.