Somatolactin in the white sturgeon and African lungfish and its evolutionary significance

Somatolactin (SL) is a newly characterized pituitary hormone belonging to the growth hormone-prolactin family. Until now SL has been identified only in teleosts, the most highly derived ray-finned fishes. We report here the cloning of SL cDNAs from two species of bony fish, the white sturgeon (Acipenser transmontanus) and the African lungfish (Protopterus annectens). Overlapping partial cDNA clones corresponding to teleost SLs were amplified by polymerase chain reaction (PCR) from either single-strand or double-strand cDNA from pituitary glands. Excluding the poly(A) tail, the sturgeon SL cDNA is 881 base pairs (bp). This is comparable to 1.0 kb estimated by Northern blot analysis. It contains a 696-bp open reading frame encoding a prehormone of 232 amino acids (aa) with a signal peptide of 24 aa and a mature protein of 208 aa. Excluding the poly(A) tail, the lungfish SL cDNA is 938 bp. This is comparable to 1.1 kb estimated by Northern blot analysis. It contains a 696-bp open reading frame encoding a prehormone of 232 aa with a signal peptide of 26 aa and a mature protein of 206 aa. The deduced aa sequences of sturgeon and lungfish SLs show 76-60% and 65-54% identity with teleost SLs, respectively. These values are significantly higher than the 30% identity with nonteleostean growth hormones and prolactins. Immunostaining of sturgeon pituitary with anti-salmon SL serum demonstrated that the SL cells were localized in the pars intermedia, as in teleosts. The present results demonstrate that the SL gene is present in two divergent lineages, the Actinopterygii (Chondrostei: white sturgeon) and the Sarcopterygii (Dipnoi: African lungfish).