The evolution of somatostatin in vertebrates

Somatostatins (SS) play important roles in the regulation of growth in vertebrates. In the present study, we identified six SS genes in zebrafish and named them SS1, SS2, SS3, SS4, SS5 and SS6. We subsequently found that five SS genes (SS1, SS2, SS3, SS4 and SS5) also existed in stickleback, medaka, Takifugu and Tetraodon. Phylogenetic analysis showed that vertebrate SS genes were grouped into five clades. Using a comparative genomic approach, we further investigated the evolutionary origin of these SS genes in vertebrates, and the results revealed that: (1) SS1, SS2 and SS5 were generated by two rounds of genome duplications (2R) that happened during the early stages of vertebrate evolution; (2) SS4 is an SS1 paralog generated by a third genome duplication (3R) that occurred to most teleost fish; and (3) SS3 and SS6 were produced by tandem duplication of SS1 and SS2 in teleost fish. RT-PCR analysis revealed that all six SS genes were functionally expressed in different zebrafish tissues. These data indicate that both genome-wide duplication and local duplication contribute to the expansion of SS genes in vertebrates.