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

Somatolactin (SL) is a newly characterized pituitary hormone belonging to t he growth hormone-prolactin family. Until now SL has been identified only i n teleosts, the most highly derived ray-finned fishes. We report here the c loning of SL cDNAs from two species of bony fish, the white sturgeon (Acipe nser transmontanus) and the African lungfish (Protopterus annectens). Overl apping partial cDNA clones corresponding to teleost SLs were amplified by p olymerase chain reaction (PCR) from either single-strand or double-strand c DNA 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 prehormo ne of 232 amino acids (aa) with a signal peptide of 24 aa and a mature prot ein of 208 aa. Excluding the poly(A)I tail, the lungfish SL cDNA is 938 bp. This is comparable to 1.1 kb estimated by Northern blot analysis. It conta ins a 696-bp open reading frame encoding a prehormone of 232 aa with a sign al peptide of 26 aa and a mature protein of 206 aa. The deduced aa sequence s of sturgeon and lungfish SLs show 76-60% and 65-54% identity with teleost SLs, respectively. These values are significantly higher than the 30% iden tity with nonteleostean growth hormones and prolactins. Immunostaining of s turgeon 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 Acti nopterygii (Chondrostei: white sturgeon) and the Sarcopterygii (Dipnoi: Afr ican lungfish). (C) 1999 Academic Press.