Physical mapping, expression patterns and interphase organisation of rDNA loci in Portuguese endemic Silene cintrana and Silene rothmaleri

Double target in situ hybridization to root tip metaphase and interphase ce lls of Silene cintrana and Silene rothmaleri was used to allocate the posit ion of 18S-5.8S-25S and 5S rRNA genes. In both species, the 18S-5.8S-25S rD NA probe labelled four sites located on the short arms of two submetacentri c chromosomes. Only one locus for 5S rDNA was mapped adjacent to 18S-5.8S-2 5S genes in a subterminal position on the centromere side: in S. rothmaleri the 5S rDNA locus was adjacent to the small 18S-5.8S-25S locus while in S. cintrana it was near the large one. The NOR activity analysed by Ag-staini ng in metaphase cells revealed proportionality between in situ labelling di mensions and Ag-NORs. In both species all rDNA loci were potentially active , although in S. rothmaleri a tendency for the expression of only one locus was observed. Interphase organisation analysis of rDNA showed some differe nces between both species that were correlated with NOR activity.