Chromosome localization-dependent compositional bias of point mutations in Alu repetitive sequences.

The Alu repetitive sequence family originated from a common ancestor. Its members, apparently free from functional constraints, are interspersed throughout primate genomes. We have found that base substitutions occurring during the evolution of primates caused a decrease in the average G + C content of those members of the family that are located in an A + T-rich region of the genome. The family members that are located in a G + C-rich genomic region have not changed their, already high, G + C content. This suggests that the regional differences in G + C content, which are responsible for chromosomal banding, are caused by an accumulation of mutations that, although selectively neutral in the majority, show different compositional bias in different regions of the vertebrate chromosome.