Ploidy level determination in genetically manipulated northern pike based on the number of active nucleoli per cell

Silver-staining of cell preparations followed by counting of the mean numbe r of active nucleoli per cell enabled determination of the ploidy level in genetically manipulated northern pike Esox lucius. Haploid pike resulted fr om activation of eggs with sperm irradiated with ultraviolet light, and tri ploid specimens were obtained by applying high-temperature shock. Cells of haploid individuals had one (almost 100% of cells examined) or, sporadicall y, two nucleoli. Depending on the specimen, mean number of nucleoli per nuc leus ranged from 1.0 to 1.06. In diploid fish, the mean number of active nu cleoli per cell ranged from 1.58 to 1.81. In triploids, the mean ranged fro m 2.08 to 2.46. Positive identification of ploidy level in northern pike re quired examination of up to 40 cells per individual, as determined by the r egression analysis. The method required a very small amount of any kind of tissue, enabling determination of ploidy level in juvenile fish without sac rificing them.