Taxonomy of Finfish (1+2)

6.1.5.4 Nor Banding

Nuclear organizer region on the chromosomes can be detected by Silver staining the metaphase chromosomes. Silver staining of NOR band is relatively a simple method. The mechanism of silver staining is not fully known yet but it is believed that silver stain binds to the non-histone proteins of the transcriptionally active NOR region. The number and location of NOR bands are useful for systematic purpose. In some species it is found that the number of NOR bands is related to ploidy level. For example, in certain species belonging to the Order Acipenseriformes, the increase in ploidy is also accompanied by increase in the number of NOR bands. Acipenser ruthenus (2n=118+4) possesses two pairs of chromosomes. But in the related species A. baceri (2n=250+8) and A. naccani (2n=246+8), four pairs of NOR bands are present, which suggest that these species are tetraploid. However, this is not universal. In some species like Salvelinus NOR bands are found in multiple chromosome location. Interestingly, sex-specific differences in NOR location was observed on the metaphase chromosomes of Mosquito fish, Aplocheilus panchhax.