Estimating the neutral rate of nucleotide substitution using introns

Evolutionary biologists frequently rely on estimates of the neutral rate of evolution when characterizing the selective pressure on protein-coding genes. We introduce a new method to estimate this value based on intron nucleotide substitutions. The new method uses a metascript model that considers alternative splicing forms and an algorithm to pair orthologous introns, which we call Introndeuce. We compare the intron method with a widely used method that uses observed substitutions in synonymous coding nucleotides, by using both methods to estimate the neutral rate for human-dog and mouse-rat comparisons. The estimates of the 2 methods correlate strongly (r(S) = 0.75), but cannot be considered directly equivalent. We also investigate the effect of alignment error and G + C content on the variance in the intron method: in both cases there is an effect, and it is species-pair specific. Although the intron method may be more useful for shorter evolutionary distances, it is less useful at longer distances due to the poor alignment of less-conserved positions.