NOR polymorphism in the Iberian species Chondrostoma lusitanicum (Pisces :Cyprinidae) - re-examination by FISH

Chromosomal polymorphism regarding the number of chromosomal NOR sites in t he cyprinid fish Chondrostoma lusitanicum reported previously (Rodrigues & Collares-Pereira, 1996) was re-examined using fluorescence in situ hybridiz ation (FISH) with a ribosomal DNA (rDNA) probe. All positive CMA3-bands con tained ribosomal DNA documented by either two or four FISH positive signals in the respective karyotypes. This polymorphism suggests the occurrence of structural rearrangements of translocation type in rDNA region from one an cestral NOR-bearing chromosome pair ubiquitous among leuciscine cyprinid fi shes to another pair. The absence of individuals heterozygous for this poly morphism is discussed.