**3. The evolution of GH gene family**

Growth hormone is a classic molecule in the study of the molecular clock, exhibiting a relatively constant rate of evolution in most orders of mammals, except primates and artiodactyls, where a dramatically enhanced rate of evolution (25–50-fold) has been reported. The rapid evolution of primate growth hormone occurred after the divergence of tarsiers (prosimians from Southeast Asia, with four extant species, living in tropical rain forests and included in the prosimian group, although some researchers see them as a link between prosimians and simians) and simians, but before the separation of old world monkeys (OWM) from new world monkeys (NWM). This event of rapid sequence evolution coincided with multiple duplications of the growth hormone gene, suggesting gene duplication as a possible cause of the accelerated sequence evolution. Multiple gene duplications and several gene conversion events both occurred in the evolutionary history of this gene family in OWM/hominoids. GHN genes in both hominoids and OWM are under strong purifying selection, while GHV genes in OWM and hominoids evolved at different evolutionary rates and underwent different selective constraints [47]. A key question is how hormone-receptor preferences have arisen among the duplicates, given that GH and PRL came from the same common gene from which they split 300–400 million years ago, moreover when both receptor genes (GHR and PRLR) show a surprising asynchrony in hormone and receptor gene duplications [48, 49].

Growth hormone exhibits a rare episodic pattern of molecular evolution characterized by sustained burst of rapid changes that are imposed very slow evolution on long periods. For example, there was a remarkable period of rapid change in the evolution of GH in primates or an ancestor and gave rise to the species-descending specificity [50, 51]. This pattern, also seen in placental growth hormones, are a consequence of selection, may reflect changes in the functions of GH additional to its basic growth-inducer effects [50, 51].

The biological specificity of GH is accompanied by significant differences in amino acid sequences, signifying an unusual episodic pattern of molecular evolution

#### **Figure 7.**

*Phylogenetic tree for mammalians GH (modified from ref. [53]). Y-axis represents million years in evolution; 0 is the current moment. Numbers of substitutions are indicated along the branches of the tree. Thick line indicates a period of rapid evolution for GH. There was another period of rapid evolution, which occurred earlier and gave rise to GH in alpaca, deer, and sheep, but this was omitted to simplify the figure, as a number of species were also omitted. Note the differences between rat and mouse, as well as between slow Loris, marmoset, and rhesus and man.*

*Growth Hormone Gene Family and Its Evolution DOI: http://dx.doi.org/10.5772/intechopen.108412*

in which long periods wherein the sequence is markedly conserved (near-stasis) are interrupted by occasional bursts of rapid changes [50–53]. This is likely caused by positive Darwinian selection [52].

The burst of rapid changes on GH in the lineage leading to higher primates is particularly marked with substitution about 35% of all amino acid residues. This burst occurred before the separation of lineages leading to man and OWM (in fact, sequences of GHs of man and rhesus monkey are very similar) (**Figure 7**). However, there are not evidences for duplications of the GH gene in non-primate mammals.

The episode of rapid change seen for GH evolution in primates appears to be specific to the coding sequence for the mature protein hormone; the pattern of evolution seen for other components of the GH gene is different. Thus, when sequences of the signal peptide, 5′ untranslated region (**Figure 2**), or introns are analyzed, the burst of rapid evolution seen for the hormone is not present. The burst of change is specific to the protein-coding component of the gene indicating that its cause relates to the protein, resulting from adaptive change in response to selection, or loss of selective constraints due to loss of function. The episode of rapid acceleration is particularly marked for residues associated with receptor binding. There are evidences for an episode of rapid change in the GH receptor during primate evolution [53].

Of interest is the fact that regulatory sequences involved in the transcription of the gene are also affected by evolutionary changes. For example, a negative regulatory element (NRE3) is conserved in most mammals, including slow loris and marmoset. Two binding sites for the transcription factor Pit-1 are in the corresponding position for other mammalian GH genes [53]. It is notable that the distal and proximal sites in slow loris are much closer than in all other mammalian species, reflecting a deletion of about 14 nucleotides in the slow loris gene, which removes the site of the cyclic ANP response element existing in the GH gene. This region is not deleted in other mammalian GH genes, with the exception of marmoset and rabbit, but its different sequence makes it unlikely its functioning as a CRE [54]. A glucocorticoid response element, present in the first intron of the human GH gene, does not appear to be present in the marmoset or the slow loris gene. However, a number of putative negative thyroid hormone responsive elements exist in the human GH gene appear in the marmoset gene, but not in the slow loris gene. Therefore, these differences between these regulatory elements in human, marmoset, and slow loris GH genes indicate that the regulation of GH in these primate species shows significant differences. Of the many differences between non-primate and human GHs, that at position 169 (His in in pig and other non-primates, Asp at the equivalent position, 171, in man) appears to be most important in determining species specificity [55].

As we have seen, although in summary form, there are many changes in the molecular form of GH throughout the evolution, much more marked the more phylogenetically the species are apart from each other [56].

These changes seem to be related to the acquisition of new properties of GH, which leads the original gene to participate only in the regulation of growth. This would explain why GH has such high multiple actions in the body, rather than simply acting for growth, for which the initial gene seemed to emerge.
