Molecular Evolution of Growth Hormone and Classification of Fishes

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The goal of the project is to construct a molecular phylogenetic tree of the growth hormone and prolactin family on the basis of sequence comparisons and to semi-quantify the phylogenetic distance of animals using ratios of amino acid changes. Although a substantial amount of structural information is now available on growth hormones and prolactins, it is still limited to mammalian and teleostean species. We have isolated growth hormones and prolactins from the species which are key-species in phylogeny and determined the complete amino acid sequences of the followings ; growth hormones from reptiles (crocodile and sea turtle), amphibian (bullfrog). Primitive bony fish (sturgeon), teleosts (Atlantic cod, catfish, flounder), elasmobranch (blue shark) ; prolactin from mammals (sei and sperm whale), reptiles (alligator, crocodile, sea turtle), amphibian (bullfrog), primitive bony fish (lungfish), teleosts (catfish and flounder). In the course of this investigation, we found a novel pituit … More ary protein from teleost species, such as Atlantic cod, flounder, and chum salmon. Elucidation of the complete amino acid sequences revealed that the proteins are distantly and similarly related to grow hormone and prolactin. Consequently, the protein, which is a new member of the growth hormone prolactin family, was named "somatolactin". A molecular phylogenetic tree of growth hormones which was constructed by the method of Fitch and Margoliash consists of two trunks, tetrapods and teleosts. The teleost trunk is much bigger than the tetrapod trunk, indicating the greater conservation of tetrapod growth hormones during molecular evolution. The degree of mutational changes is in good conformity with phylogeny. Therefore, phylogenetic distances of these species could be semi-quantified by amino acid changes per 100 residues. Evolutionary relationship of growth hormone, prolactin and somatolactin was also estimated by sequence comparison. The estimation suggests that these pituitary hormones diverged from a common ancestor by gene duplication. Less