Endocrine Abstracts

Pituitary growth hormone (GH) like a number of other protein hormones, shows an episodic pattern of evolution, in which periods of prolonged near-stasis are interrupted by short bursts of rapid change. During mammalian evolution two episodes of rapid change of GH have been identified, one in the Artiodactyla and one in the Primates. The latter underlies the well known species specificity of human GH. Cloning and characterisation of GH genes for a number of mammalian species has helped delineate these periods of rapid evolution more precisely, and has shown that they were 'shorter and sharper' than had previously been established. In artiodactyls the burst occurred on the line leading to ruminants, after divergence of various other cetartiodactyl groups (camelids, pig and cetaceans) but before divergence of any of the ruminant groups. In primates the period of rapid evolution occurred on the line leading to higher primates, after divergence of the prosimian slow loris, but before divergence of new-world monkeys. The burst of rapid change in primates appears to have stopped at about the time of the gene duplications that gave rise to a cluster of GH-like genes in man and New- and Old-world monkeys. Various lines of evidence support the idea that these episodes of rapid GH evolution in mammals were the result of adaptive change, possibly involving acquisition by the hormone of a second function, the importance of which varied ('function switching').