CONSERVATIVE NATURE OF THE NUCLEOLUS ORGANIZER REGION IN 3 SPECIES OFMEDITERRANEAN MUGILIDS

We have studied the distribution of the Nucleolus Organizer Regions on the chromosome complements of three species of Mediterranean Mugilids : Liza aurata, Liza ramada and Chelon labrosus, by means of the silver nitrate staining method. Only one representative pair of NOR-chromoso mes was detected in each species, the submetacentric or metacentric nu mber 24, common to all of them. The terminal NOR position on the short arm as well as the lack of variation in size indicate the conservativ e nature of the NORs in the Family Mugilidae. A certain variation of t he nucleoli number (1 to 4) in interphasic nuclei was detected. This m ay be explained by the selectivity of the silver staining for the tran scriptionally active NORs. Associations involving the NOR-chromosomes reveal a slight deleting of argirofilic proteins attached to them.