RDNA CLONING AND RAPID HYBRID IDENTIFICATION IN POPULUS SPP (SALICACEAE)

Ribosomal DNA genes from P. deltoides have been cloned and specific sequences of the 25 S and 18 S rDNA region, labelled by digoxigenin, have been used to determine the rDNA structure of Populus tremula, P. fremontii, P. maximowiczii, P. yunnanensis, P. nigra, P. wislizenii, P. alba. The restriction maps of the coding region appeared to be similar among the examined species and with those of P. deltoides and P. trichocarpa, reported in a previous paper. Inter- and intraspecific variation in rDNA repeat unit length have been revealed after EcoRI digestions. SstI and XbaI restriction sites have been found at different positions in the IGS of some species. The polymorphic fragments generated by SstI digestion allowed the identification of the hybrid origin of some genotypes. The number of rDNA genes in the genome of P. deltoides has been estimated to be about 2 000 copies. Finally, the usefulness of these studies in Populus spp. taxonomy and forestry genetics is discussed.