DETECTION OF A VARIABLE NUMBER OF RIBOSOMAL DNA LOCI BY FLUORESCENT IN-SITU HYBRIDIZATION IN POPULUS SPECIES

Fluorescent in situ hybridization (FISH) was applied to related Populu s species (2n = 19) in order to detect rDNA loci. An interspecific var iability in the number of hybridization sites was revealed using as pr obe an homologous 25S clone from Populus deltoides. The application of image analysis methods to measure fluorescence intensity of the hybri dization signals has enabled us to characterize major and minor loci i n the 18S-5.8S-25S rDNA. We identified one pair of such rDNA clusters in Populus alba; two pairs, one major and one minor, in both Populus n igra and P. deltoides; and three pairs in Populus balsamifera, (two ma jor and one minor) and Populus euroamericana (one major and two minor) . FISH results are in agreement with those based on RFLP analysis. The pBG13 probe containing 5S sequence from flax detected two separate cl usters corresponding to the two size classes of units that coexist wit hin 5S rDNA of most Populus species.