1Laboratory of Breeding Science, Graduate School of Fisheries Science, Hokkaido University, Minato, Hakodate, 041-8611, 2Nanae Fresh Water Laboratory, Field Science Center for Northern Biosphere, Hokkaido University, Sakuramachi, Nanae, Kameda, 041-1105, Japan
Although tetraploid is important for the mass production of sterile triploids by hybridizing with diploid, its survival capacity is extremely low. In this study, we aimed to identify the cause of such a high mortality of embryos treated with hydrostatic pressure shock (700kg/cm2, 7min duration, 5-7h after fertilization at 10°C) in masu salmon. In treated eggs, cleavage rates were delayed and aberrant cell divisions were observed during the early cleavage stage. Histological observation in the blastula stage revealed frequent occurrence of aberrant blastoderms including anuclear and/or small blastomeres. Ploidy analysis of eyed and hatching embryos revealed successful production of pure tetraploid individuals. However tetraploid survivors exhibited abnormal appearances. In the groups treated at 6.5 and 7h, no embryonic body was observed at hatching stage. Judging from the results of each treated group, the optimum timing for the inhibition of first cleavage was 6h after fertilization, when the eggs were cytologically staged as prometaphase to metaphase of the first cleavage. Aneuploids and mosaics were also detected in treated embryos by flow cytometry. These results suggest that abnormal cleavage and blastomeres mosaicism caused by the treatment at the first cleavage give rise to high mortality.