RIBOSOMAL DNA STUDIES IN POPLARS - POPULUS-DELTOIDES, POPULUS-NIGRA, P-TRICHOCARPA, POPULUS-MAXIMOWICZII, AND POPULUS-ALBA

The nuclear rDNA physical maps of Populus nigra, P. deltoides, P. trichocarpa, P. maximowiczii, and P. alba have been built up for the restriction enzymes EcoRI, EcoRV, BamHI, PstI, and SacI. There is no HindIII site. A large variability appeared between the species in the intergenic spacer mainly for EcoRI and PstI. For several clones of each species two to three major unit types coexist in the genome, while several minor units as well as length variant units or new unit types have been found. The variability between the species is due to different major unit types, while the variability between clones in one species is due to the minor unit types. Every species carries several rDNA unit types either different in size (variable length unit) or variable in restriction sites (variable unit types). Four thousands copies of rDNA units were found in P. nigra, P. deltoides, and P. maximowiczii. The clones belonging to the same species carry the same major unit types but are different in their minor variable length units or minor unit types. The hybrid clones carry the sum of the major unit types of the two parental species. These facts suggested the existence of three rDNA loci. The combination of one enzyme EcoRI and one probe (flax entire rDNA unit) allowed to easily recognize each of the five species. Conversely, we detected the presence of rDNA fragments characteristic to P. deltoides in some clones belonging to P. nigra. We conclude that these clones are likely to have a hybridization in their stands but without observable phenotypic consequences. This technique will be used to verify the purity of each P. nigra candidate clone before exploiting it in a breeding or propagation program.