Variable evolutionary rates in the molecular evolution of mammalian growth hormones

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Abstract
In mammals pituitary growth hormone (GH) shows a slow basal rate of evolution (0.22 ± 0.03 × 10−9 substitutions/amino acid site/year) which appears to have increased by at least 25–50-fold on two occasions, during the evolution of primates (to at least 10.8 ± 1.3 X 10−9 substitutions/amino acid site/year) and artiodactyl ruminants (to at least 5.6 ± 1.3 X 10−9 substitutions/amino acid site/year). That these rate increases are real, and not due to inadvertent comparison of nonorthologous genes, was established by showing that features of the GH gene sequences that are not expressed as mature hormone do not show corresponding changes in evolutionary rate. Thus, analysis of nonsynonymous substitutions in the coding sequence for the mature protein confirmed the rate increases seen in the primate and ruminant GHs, but analysis of nonsynonymous substitutions in the signal peptide sequence, synonymous substitutions in the coding sequence for signal peptide or mature protein, and 5′ and 3′ untranslated sequences showed no statistically significant changes in evolutionary rate. Evidence that the increases in evolutionary rate are probably due to positive selection is provided by the observation that in the cases of both ruminant and primate GHs the periods of rapid evolution were followed by a return to a slow rate similar to the basal rate seen in other mammalian GHs. Differences between the biological properties of GHs have been identified which may relate to these periods of rapid adaptive molecular evolution. On the basis of sequence data currently available (but excluding rodent GHs which show an intermediate rate, the basis of which is not clear) for most (≈90%) of evolutionary time mammalian GHs have been in the slow phase of evolution, with possibly most of the few amino acid substitutions that have occurred being neutral in nature. But most (≈80%) of the amino acid substitutions that have been introduced into GH during the course of mammalian evolution have been accepted during the rapid phases and were adaptive in nature.

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