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M Higa, T Kitahashi, Y Sasaki, H Okada, and H Ando

Two types of genes encode salmon gonadotropin-releasing hormone (sGnRH), which is thought to act on both sexual maturation and reproductive behavior, in salmonids. We characterized the two sGnRH genes (sGnRH-I and -II) and their upstream regions in masu salmon, Oncorhynchus masou, since such information is a prerequisite for molecular approaches to salmon reproduction. The two genes have similar exon-intron structures composed of four exons and three introns. Sequence analyses of the two genes showed that coding regions are highly conserved, but upstream regions are distinctively divergent. In the upstream regions, only the sGnRH-II gene has a large palindromic sequence, which has been proposed to be involved in control of transcription via estrogen receptors. In contrast, the sGnRH-I gene is missing the large palindromic sequence, but has three distinct palindromes in the upstream region. These results may suggest divergent transcription regulatory mechanisms between the two sGnRH genes in masu salmon. The differences in the upstream regions of sGnRH genes in Atlantic salmon (Salmo salar), sockeye salmon (Oncorhynchus nerka) and masu salmon are discussed with respect to the evolution of sGnRH genes in salmonid fish.