Duplicate growth hormone genes in sheep and goat.

In most mammals a single gene codes for pituitary growth hormone (GH) and is not associated with closely-related genes. An exception occurs in higher primates. In man a cluster of five genes on chromosome 17 codes for GH-like proteins; one of these (at the 5* end of the cluster) codes for pituitary GH, while the other four code for genes expressed in the placenta, including two genes for placental lactogen, one for a placental lactogen-like protein, and one for a GH variant (Chen et al. 1989). In rhesus monkey, studies on cDNA indicate that there are at least five genes that code for GH-like proteins, one expressed in the pituitary and four in the placenta, although these may not be exactly equivalent to the human GH-like genes (Golos et al. 1993). There is also evidence for multiple GH-like genes in a new world monkey (O C Wallis & M Wallis, unpublished observations). These clusters of GH-like genes have presumably arisen as the consequence of duplications of the single GH gene found in most mammals (and other tetrapods), followed by divergent evolution. It is notable that evolution of GH in primates involved a burst of rapid change (contrasting with general conservation of GH structure seen in most mammals) which appears to have occurred before the gene duplications that gave rise to the gene cluster(s) (Wallis 1994, 1996). Until recently it appeared that duplication of the GH gene in mammals was confined to primates. Placental lactogens are found in some other mammalian groups (ruminants, rodents) but appear to have arisen independently of those found in primates, by duplication of the prolactin gene (Schuler & Kessler 1992, Wallis 1992). However, a number of reports have now appeared indicating that there are duplicate GH genes in at least some caprine ruminants (goats and sheep). These have interesting implications for the evolution of the GH gene in ruminants.