INCOMPLETE DEVELOPMENT OF THE BRAIN-PITUITARY-GONADAL AXIS MAY UNDERLIE THE DELAY IN THE INITIATION OF PRECOCIOUS MATURATION IN MALE SOCKEYE-SALMON

Precocious males first appear in the second year in sockeye salmon, wh ereas in masu salmon they appear in the first year. In underyearling m ale masu salmon, precocious maturation is accelerated under short phot operiod with an increase in salmon gonadotropin-releasing hormone (sGn RH) and pituitary gonadotropin subunit (GTH I beta and GTH II beta) co ntents, suggesting that the brain-pituitary-gonadal axis is already co mpleted. In this study, the development of the brain-pituitary-gonadal axis in underyearling male sockeye salmon was investigated by manipul ating photoperiod in order to clarify the relationship between its dev elopment and initiation of precocious maturation. Fish were divided in to short (8L-16D) and long photoperiodic (16L-8D) groups in June and w ere reared for 4 months until October. sGnRH contents in the brain and pituitary, and GTH I beta and GTH II beta contents in the pituitary w ere measured. sGnRH contents in the hypothalamus and the pituitary, an d GTH I beta contents in the pituitary increased under short photoperi od, although gonadal maturation was not induced under either photoperi od. However, pituitary GTH II beta contents did not increase in either photoperiodic group. These results suggest that in contrast to masu s almon, the brain-pituitary-gonadal axis is not completed in underyearl ing male sockeye salmon. This species-specific difference in the devel opment of the brain-pituitary-gonadal axis may underlie differences in the initiation of precocious maturation.