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, whereas in masu salmon they appear in the first year. In underyearling male masu salmon, precocious maturation is accelerated under short photoperiod with an increase in salmon gonadotropin-releasing hormone (sGnRH) and pituitary gonadotropin subunit (GTH I beta and GTH II beta) contents, suggesting that the brain-pituitary-gonadal axis is already completed. In this study, the development of the brain-pituitary-gonadal axis in underyearling male sockeye salmon was investigated by manipulating photoperiod in order to clarify the relationship between its development and initiation of precocious maturation. Fish were divided into short (8L-16D) and long photoperiodic (16L-8D) groups in June and were 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 were measured. sGnRH contents in the hypothalamus and the pituitary, and GTH I beta contents in the pituitary increased under short photoperiod, although gonadal maturation was not induced under either photoperiod. However, pituitary GTH II beta contents did not increase in either photoperiodic group. These results suggest that in contrast to masu salmon, the brain-pituitary-gonadal axis is not completed in underyearling male sockeye salmon. This species-specific difference in the development of the brain-pituitary-gonadal axis may underlie differences in the initiation of precocious maturation.