The chromosome complement of the coho salmon (Oncorhynchus kisutch) has been analysed by means of C-banding, silver and fluorochrome staining, and in situ digestion with restriction endonucleases. C-banding shows heterochromatic regions in the centromeres of most chromosomes but not in the telomeric areas. The fifteenth metacentric chromosome pair contains a large block of constitutive heterochromatin, which occupies almost all of one chromosome arm. This region is also the site where the ribosomal cistrons are located and it reacts positively to CMA3/DA fluorochrome staining. The NORs are subject to chromosome polymorphism, which might be explicable in terms of an amplification of ribosomal cistrons. The digestion banding patterns produced by four types of restriction endonucleases on the euchromatic and heterochromatic regions are described. Two kinds of highly repetitive DNAs can be distinguished and the role of restriction endonucleases as a valuable tool in chromosome characterization studies, as well as in the analysis of the structure and organization of fish chromatin, are also discussed.
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The authors thank Dr J. Santos, Dr C. Sentis and Dr J. Fernandez-Piqueras for their technical assistance and their valuable comments. We are grateful to Dr L. Sanchez, Ms A. Vinas and Mr F. Nieto for their important contribution to cell culture. We also thank our colleague Dr J. Trout for his revision of the English text. This work was funded by a CICYT Project No. PB87/0881.
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