Heterogeneity of the Transition/Transversion Ratio in Drosophila and Hominidae Genomes

Mutation rate varies between sites in the genome. Part of this variation can be explained by well-recognized short nucleotide contexts, but a large component of this variation remains cryptic. We used data on interspecies divergence and intraspecies polymorphism in Drosophila and Hominidae to analyze variation of the average rate of the 12 possible kinds of single-nucleotide mutations and in the transition/transversion ratio κ at single-nucleotide resolution. Both the average mutation rate and κ vary by a factor of ∼3 between nucleotide sites. The characteristic scale of variation in κ is up to at least ∼30 nucleotides in Drosophila and ∼5 nucleotides in Hominidae. Genome segments with locally elevated mutation rates possess lower values of κ; however, a substantial fraction of variation in κ cannot be directly explained by the local mutation rates.