The natural reservoir of Borna disease virus (BDV) is unknown. In this paper, we show that mallards (Anas platyrhyncos) and jackdaws (Corvus monedula) can be subclinically infected carriers of this virus. From faecal samples collected at a bird pond, we were able to amplify fragments of the BDV p24 and p40 genes. Following cloning and sequencing, a phylogenetic analysis revealed that these birds carry strains of BDV closely related to but distinct from the reference strains BDV V and He/80. To our knowledge, this is the first confirmed finding of BDV in wild birds.

Sex chromosomes may provide a context for studying the local effects of mutation rate on molecular evolution, since the two types of sex chromosomes are generally exposed to different mutational environments in male and female germ lines. Importantly, recent studies of some vertebrates have provided evidence for a higher mutation rate among males than among females. Thus, in birds, the Z chromosome, which spends two thirds of its time in the male germ line, is exposed to more mutations than the female-specific W chromosome. We show here that levels of nucleotide diversity are drastically higher on the avian Z chromosome than in paralogous sequences on the W chromosome. In fact, no intraspecific polymorphism whatsoever was seen in about 3.4 kb of CHD1W intron sequence from a total of >150 W chromosome copies of seven different bird species. In contrast, the amount of genetic variability in paralogous sequences on the Z chromosome was significant, with an average pairwise nucleotide diversity (d) of 0.0020 between CHD1Z introns and with 37 segregating sites in a total of 3.8 kb of Z sequence. The contrasting levels of genetic variability on the avian sex chromosomes are thus in a direction predicted from a male-biased mutation rate. However, although a low gene number, as well as some other factors, argues against background selection and/or selective sweeps shaping the genetic variability of the avian W chromosome, we cannot completely exclude selection as a contributor to the low levels of variation on the W chromosome.