Localization of ribosomal genes in Pleuronectiformes using Ag-, CMA(3)-banding and in situ hybridization

In this paper we present the analysis of nucleolar organizer regions (NORs) in five species of the order Pleuronectiformes (Scophthalmus maximus, Scop hthalmus rhombus, Platichthys flesus, Solea solea and Solea lascaris), a gr oup of fish poorly studied cytogenetically. In spite of the small chromosom e sizes, which characterize the karyotypes of this group, we implement both classical (Ag- and CMA(3)-staining) and molecular (in situ hybridization w ith a major rDNA probe) techniques for NOR location. NORs were localized in a single chromosome pair in all species studied and showed size variation within each species. Evidence of a NOR-site polymorphism was detected in Pl atichthys flesus and Scophthalmus maximus. NOR location was apparently the same within families (S. rhombus vs. S maximus., and S. solea vs. S. lascar is) but clearly differed among families, and supports the phylogenetic rela tionships proposed for the order Pleuronectiformes.