Pattern of nucleotide substitutions in growth hormone-prolactin gene family: a paradigm for evolution by gene duplication.

The growth hormone-prolactin gene family in mammals is an interesting example of evolution by gene duplication. Divergence among members of duplicated gene families and among species was examined by using reported gene sequences of growth hormone, prolactin and their receptors. Sequence divergence among species was found to show a general tendency in which a generation-time effect is pronounced for synonymous substitutions but not so for nonsynonymous substitutions. Divergence among duplicated genes is characterized by the relatively high rate of nonsynonymous substitutions, i.e., the rate is close to that of synonymous ones. In view of the stage- and tissue-specific expression of duplicated genes, some of the amino acid substitutions among duplicated genes is likely to be caused by positive Darwinian selection.