Rapid evolution of goat and sheep globin genes following gene duplication

Statistical analyses of DNA sequences of globin genes (beta A, beta C, andgamma) from goat and sheep (including new sequence information for thesecond intron of sheep beta A and gamma, kindly provided by A. Davis and A.W. Nienhuis) indicate that the rates of nonsynonymous substitution in thesegenes have been greatly accelerated following the gene duplicationseparating gamma and the ancestor of beta A and beta C and the geneduplication separating beta A and beta C. In both cases the accelerationwas apparently due to relaxation of purifying selection (functionalconstraints) rather than advantageous mutations because accelerationoccurred only in less important parts of the beta globin chain. The ratesof nonsynonymous substitution in these genes are estimated to be about 2.3x 10(-9) per site per year, which is three times higher than that for thedivergence between human beta and mouse beta major globin genes. Ouranalyses further suggest that the rate of synonymous substitution infunctional genes and the rate of substitution in pseudogenes areapproximately equal and are between 2.8 x 10(-9) and 5.0 x 10(-9) and thatthe rate of substitution in introns is about 3.0 x 10(-9). The divergencetime between beta A and beta C and that between gamma and the beta A-beta Cpair are about 12 and 30 million years, respectively. The proportion oftransition mutations is estimated to be 64%, two times higher than expectedunder random mutation but considerably lower than the 96% estimated foranimal mitochondrial DNA.