Varying signals of the effects of natural selection during teleost growth hormone gene evolution

The growth hormone (GH) gene of teleost fish exhibits a higher degree of variability compared with other vertebrate groups. However, the different selective constraints at the sequence level are not well understood. In this study, maximum-likelihood (ML) models of codon substitutions were used to investigate Darwinian adaptive evolution of the GH gene in teleost fishes. Complete GH gene sequences of 54 fish species were classified into 4 orders, and the variable nature of GH was examined by determining the dN and dS rate variation and the rates of molecular evolution for each teleost order. The results indicate that although the overall evolution rate for teleost GH is high ((1.15 +/- 0.01) x 10(-9) substitutions/(aa site x y)) compared with the "slow phases" in mammals ((0.21 to 0.28 +/- 0.05) x 10(-9)), the vital structure of this gene has been retained. While the majority of the amino acid changes appear to be due to relaxation of purifying selection, some positively selected sites were detected in regions with no specifically identified role in protein function. The positively selected regions observed in salmoniformes lineage suggests a possible role for positive selection driving functional divergence in paralogous forms of the GH gene after whole-genome duplication in this lineage.