Studies on the GH/SL gene family: Cloning of African lungfish (Protopterusannectens) growth hormone and somatolactin and toad (Bufo marinus) growth hormone

The lungfishes (lobe-finned fish) occupy a unique position in vertebrate ph ylogeny, being regarded as the closest extant relatives to the tetrapods. T he putative pituitary hormone somatolactin (SL) has hitherto been found onl y in teleost fishes, and the presence of this protein in tetrapods or lobe- finned fishes has not been ascertained. It was therefore of interest to det ermine the structure of SL in the African lungfish (Protopterus anncctens), as this information would be useful for designing probes to facilitate the detection of SL genes in amphibians and other tetrapods. The structural re lationships between SL, growth hormone (GH), and prolactin (PRL) strongly s uggest that these proteins evolved from a common ancestor. To obtain a more complete picture of the evolution of these hormones in lungfish, African l ungfish GH has been cloned and sequenced. The cDNA sequence of a toad (Bufo marinus) GH was determined to facilitate maximum parsimony analysis of GH sequences. Cladistic analysis confirmed that lungfish and amphibian GH sequ ences form a clade distinct from the GH sequences of ray-finned fishes. A d istance matrix analysis of SL sequences indicated that lungfish SL had the lowest primary sequence identity with goldfish SL (47%) and the highest wit h flounder SL (66%). The detection of SL in a lungfish indicates that the g ene duplication within the SL/GH/PRL family, which gave rise to SL, must ha ve occurred in a common ancestor of the ray-finned fishes (Actinopterygii) and the lungfishes (Sarcopterygii) and tetrapods. (C) 1999 Academic Press.