Growth hormone and prolactin in New World monkeys

In most non-primate mammals pituitary growth hormone (GH) is encoded by a single gene. This is true also for most prosimians, but in higher primates the GH locus comprises a cluster of GH-related genes, including the gene encoding pituitary GH at the 5' end, followed by a number of genes (and/or pseudogenes) at least some of which are expressed in the placenta. Such GH gene clusters arose independently in Old World Monkeys/apes and in New World Monkeys (NWM), as a consequence of multiple, tandem gene duplications followed by divergent evolution. GH-related genes have been investigated in a number of NWM species, but only in marmoset (Callithrix jacchus) has the complete structure of the cluster been determined. Here the pituitary-expressed GH gene is followed by 2 GH-like genes, probably expressed in placenta, and five GH-like pseudogenes. In some NWM the GH gene cluster is more complex than this, comprising up to 40 GH-related genes. Sequence comparison and phylogenetic analysis suggests that the GH-like proteins in NWM can be divided into three groups, designated (1) pituitary GH, (2) GH-like proteins Group A (ghlA) and (3) GH-like proteins Group B (ghlB). Most species have at least one representative in each of these groups. Phylogenetic analysis also shows that rates of evolution of GH in primates have been very variable. An episode of rapid GH evolution preceded the divergence of OWM/apes and NWM. Following the independent duplications that gave rise to GH clusters on each of these lineages the rate of evolution of pituitary GH decreased, but the evolution of the other GH-like proteins remained high, apparently reflecting adaptive evolution. The biological function(s) of the GH-like proteins in NWM remain largely unknown. In contrast to the complexity shown by GH-related genes and proteins in higher primates, the related hormone prolactin is encoded by a single gene in all primates and shows only modest variation between NWM species