ABSTRACT
Unlike the chromosomes of homeothermic vertebrates, the chromosomes of poikilothermic vertebrates generally do not show euchromatic, serial banding patterns (G-, R-, Q-bands) after treatment with G-banding methods (using trypsin digestion as described by Seabright 1971, or incubation in hot 2x SSC according to Sumner et al. 1971) or with DNA base-specifi c fl uorochromes (Medrano et al. 1988; Sumner 1990). Thus, banding procedures developed to investigate chromosomes of higher vertebrates (especially birds and mammals) are rarely (e.g., Blaxhall 1983; Gold et al. 1990; Yu et al. 1994) reproducible in fi sh chromosomes. The structural basis of euchromatic or serial banding patterns has been associated with the compartmentalisation of the genome of higher vertebrates into AT-rich and GC-rich isochores, not present in the chromosomes of fi shes, most amphibians and reptiles (Medrano et al. 1988; Schmid and Guttenbach 1988; Schmid et al. 1990; Sumner 1990). In any case comparative chromosome banding studies in lower vertebrates are often limited to the determination of 2n, NF and chromosome morphology by using basic staining techniques to describe the conventional traits (such as Giemsa staining), or the pattern of constitutive heterochromatin by using C-banding
(CBG), or the fl uorescence banding patterns (by using DNA base-specifi c fl uorochromes). The identifi cation of the nucleolus organising regions (NORs) using GC-specifi c
fl uorescent agents (e.g., Mithramycin M-MM, Chromomycin
A3-CMA3) and impregnation with AgNO3 is also a widely used method in the tool-box of fi sh cytogenetics. Sequential chromosome banding consists of the sequential application of different staining and banding methods to the same metaphases, thus substantially increasing the information on the chromosome structure. Therefore, sequential banding can be viewed as a methodological compensation for the inapplicability of euchromatic banding techniques in cytogenetics of higher vertebrates.
