MOLECULAR-CLONING OF CDNAS ENCODING 2 GONADOTROPIN-BETA SUBUNITS (GTH-I-BETA AND GTH-II-BETA) FROM THE MASU SALMON, ONCORHYNCHUS-MASOU - RAPID DIVERGENCE OF THE GTH-I-BETA GENE

Two types of cDNA (GTH-I beta and -II beta) encoding the beta subunit of masu salmon (Oncorhynchus masou) gonadotrophin were cloned using th e reverse transcription-polymerase chain reaction for pituitary mRNAs. The nucleotide sequence of GTH-I beta cDNA was 469 bp long, encoding 137 amino acids, and GTH-II beta cDNA was 476 bp long, encoding 142 am ino acids. These two masu salmon beta subunit types showed low homolog ies of 52% (nucleotide sequence) and 33% (amino acid sequence). The ev olutionary interval between masu and chum salmon was estimated to be 5 .65 and 1.43 million years by comparing GTH-I beta and GTH-II beta res pectively. These time values are markedly inconsistent with the evolut ionary time (3.0 million years) estimated from fossil records and an i sozyme study. Southern blot analyses showed that the I beta gene restr iction fragment lengths differed among five teleosts, whereas, with on e exception, the II beta gene showed well conserved patterns. Therefor e, the GTH-I beta gene may have diverged at a faster rate than the GTH -II beta gene.