Variable evolutionary rates in the molecular evolution of mammalian growth hormones

Abstract

In mammals pituitary growth hormone (GH) shows a slow basal rate of evolution (0.22 +/- 0.03 x 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 non-synonymous 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Independent origin of the growth hormone gene family in New World monkeys and Old World monkeys/hominoids

“…The episodic evolutionary pattern is an interesting and unusual feature of GH genes among both mammals and primates (Wallis 1994(Wallis , 1996. However, the driving force of this rapid evolution, whether adaptive evolution or relaxation of purifying selection constraint, is controversial.…”

“…request). Mendoza et al (2004) got a different tree topology based on smaller species' sequences and the deduced amino-acid distance for the following reasons: the pituitary GH gene evolved under different rates in different lineages (Wallis 1994, Liu et al 2001); as our analysis shows, different family members evolved under different selection constraints; and amino-acid sequences generally offer less information than full-length nucleotide sequences. Thus, our phylogenetic tree should be more reliable.…”

“…In mammals generally, the pituitary GH gene evolved very slowly, with the two exceptions of the primates and the artiodactyls. Especially in the higher primates, a burst of rapid evolution occurred, in which about 62-64 residues changed in the mature hormone sequences compared with the proposed ancestor sequence of eutherian GH (Wallis 1994). The period of the rapid evolution has been dated to before the separation of lineages leading to New World monkeys (NWM) and Old World monkeys (OWM)/hominoids, but after the divergence of the tarsier from the higher primates (Liu et al 2001, Wallis et al 2001.…”

Placental Growth Hormone-Related Proteins and Prolactin-Related Proteins

“…Both GH and PRL have undergone rapid evolution in the lineage leading to ruminants [26,45,46]. PRL is duplicated in all well-studied ruminants whereas most species have a single GH-like gene.…”

“…Molecular evolution of GH is only modestly accelerated in muroid rodents whereas the evolution of PRL has been dramatically accelerated [45,46]. GH is represented by a single gene in rodent genomes but PRL has given rise to a large cluster of genes, most of which are expressed in the placenta.…”