Abstract: This study examined the physical localizations of the 25S rDNA on the mitotic metaphase chromosomes of seven species in the five sections of Populus, using fluorescence in situ hybridization (FISH) . Four 25 rDNA loci were detected on the chromosomes of five wild diploid species (2n = 2x = 38) , P. tomentosa , P. nigra var. thevestina , P. pseudo-simonii, P. lasiocarpa and P. euphratica . Six 25 rDNA loci were detected on the chromosomes of two wild triploid species (2n= 3x = 57) , P. tomentosa in section Leuce and P. euramericana cv . Wuhei-1 in section Aigeiros . While only one pair of 25 rDNA loci located at the smallest chromosome was detected in P. euphratica chromosomes. Coupled with the approach of common karoytype analysis, this article also discusses the variations of 25S rDNA distribution on Populus chromosomes .

Key words: Populus