Physical localization of active and inactive rRNA gene Loci in Hordeum marinum spp gussoneanum (4x) by in situ hybridization

Two populations of the diploid and 10 populations of the tetraploid cytotype of Hordeum marinum ssp. gussoneanum were studied for the presence of chromosomal segments harbouring rDNA. Conventional cytological methods established the presence of only one satellited (SAT) chromosome pair in both cytotypes. This was supported by silver staining revealing two NORs and two standard-sized nucleoli. Two additional micronucleoli were observed in a few interphases of two tetraploid populations indicating the presence of an extra chromosome pair with very low nucleolus-forming activity. In situ hybridization with the wheat rDNA probe pTA71 identified intense signals at the nucleolar constrictions of the SAT chromosomes of both cytotypes and weaker signals in a chromosome pair of the tetraploid cytotype, morphologically similar to the SAT chromosomes but without visible nucleolar constrictions. This confirms the presence of rDNA in two chromosome pairs in the tetraploid cytotype. The morphological similarity between these two pairs and the SAT-chromosome pair of the diploid cytotype as well as an identical position of the signals in all three pairs give additional support to an autoploid origin of the tetraploid cytotype. The rDNA at the nucleolar constrictions consisted of two segments of condensed rDNA of different sizes connected by diffuse rDNA. The rDNA of the chromosome pair without nucleolar constrictions was condensed supporting that this conformation is connected with inactivity. The tripartite structure of the rDNA at the nucleolar constrictions corresponds to similar tripartite structures observed after silver staining and Giemsa C-banding.