BRDU REPLICATION BANDING STUDIES ON THE CHROMOSOMES IN EARLY EMBRYOS OF SALMONID FISHES

The availability of chromosome replication banding for fish karyotype analysis was investigated in early embryos of three salmonid species: rainbow trout (Oncorhynchus mykiss), masu salmon (O. masou), and chum salmon (O. keta), at around 100 to 110 thermal units, after exposure to 5-bromodeoxyuridine (BrdU). The optimum conditions for obtaining reproducible replication bands were BrdU treatment of three to five embryos at the final concentration of 400 to 700 mu g/ml in 10 ml physiological saline, in the presence of 10(-5) M fluorodeoxyuridine and 10(-4) M deoxycytidine, for 2 to 2.5 days at 5 degrees C in the dark. Under these conditions, distinct banding patterns were obtained in the chromosomes of all three species. The karyotype of each species was subsequently determined. In addition, chromosome number variation, observed in one diploid embryo with 61 chromosomes and two haploid ones with 31 chromosomes of rainbow trout, was shown to be due to a Robertsonian centric fission of one biarmed chromosome, although the affected chromosome was not the same in each embryo. The present findings suggest that the BrdU replication banding in the chromosomes of early embryos is a promising technique in fish karyotype analysis, in terms of its technical simplicity and a high reproducibility to induce distinct chromosome bands.