Cytogenetic effects of hydrostatic pressure treatment to suppress the first cleavage of salmon embryos

Tetraploids in chum and masu salmon were induced by suppressing the first cleavage by hydrostatic pressure of 680 kg/cm². Ploidy level was scored by counting nucleoli using silver-stained preparations. True tetraploids were induced in some chum embryos treated at 13.2 h after fertilization; however, aneuploids or polyploid mosaics were also found. These showed abnormal embryogenesis. Chromosomes isolated from the nucleus or precociously moving to the centrosome were observed in the dividing cells. Chromosome bridges between two centrosomes were also observed, as well as chromosome fragments, rings and gaps. Even if damages induced by mitotic suppression with hydrostatic pressure treatment is minor at the first cleavage, it will be amplified through rapid cell cycles.