Molecular evolutionary genomics of birds

Insight into the molecular evolution of birds has been offered by the steady accumulation of avian DNA sequence data, recently culminating in the first draft sequence of an avian genome, that of chicken. By studying avian molecular evolution we can learn about adaptations and phenotypic evolution in birds, and also gain an understanding of the similarities and differences between mammalian and avian genomes. In both these lineages, there is pronounced isochore structure with highly variable GC content. However, while mammalian isochores are decaying, they are maintained in the chicken lineage, which is consistent with a biased gene conversion model where the high and variable recombination rate of birds reinforces heterogeneity in GC. In Galliformes, GC is positively correlated with the rate of nucleotide substitution; the mean neutral mutation rate is 0.12-0.15% at each site per million years but this estimate comes with significant local variation in the rate of mutation. Comparative genomics reveals lower d(N)/d(S) ratios on micro- compared to macrochromosomes, possibly due to population genetic effects or a non-random distribution of genes with respect to chromosome size. A non-random genomic distribution is shown by genes with sex-biased expression, with male-biased genes over-represented and female-biased genes under-represented on the Z chromosome. A strong effect of selection is evident on the non-recombining W chromosome with high d(N)/d(S) ratios and limited polymorphism. Nucleotide diversity in chicken is estimated at 4-5 x 10(-3) which might be seen as surprisingly high given presumed bottlenecks during domestication, but is lower than that recently observed in several natural populations of other species. Several important aspects of the molecular evolutionary process of birds remain to be understood and it can be anticipated that the upcoming genome sequence of a second bird species, the zebra finch, as well as the integration of data on gene expression, shall further advance our knowledge of avian evolution.