Abstract: In order to identify the possible original ancestors of the allodecaploid Rosa praelucens Byhouwer，the cytological Fluorescence in situ Hybridization（FISH）with 45S rDNA and 5S rDNA as probes was used to study the karyotypes of R. praelucens and 20 other closely related and sympatrically distributed congeneric species. The results were as follows：(1) Among the studied 21 rose species，only the karyotype of R. praelucens was 2B，and that of the other 20 species all belonged to 2A. (2) The number of 45S rDNA hybridization sites in 20 rose species was identical to their ploidy number in interphase cells， except for that of the diploid R. banksiopsis，which had three 45S rDNA hybridization sites. The number of 45S rDNA sites equaled to its ploidy number in metaphase cells. Some 45S rDNA loci in 14 species， namely，R. graciliflora，R. omeiensis var. omeiensis，R. sericea，R. mairei，R. sertata，R. davidii，R. banksiopsis，R. persetosa，R. macrophylla，R. acicularis，R. moyesii，R. odorata var. pseudoindica， R. roxburghii and R. praelucens were fragile sites. (3) The number and location of 5S rDNA loci were uncertain. Diploid R. willmottiae var. glandulifera，R. odorata var. pseudoindica，R. helenae，R. brunonii and R. soulieana var. soulieana all had four 5S rDNA hybridization sites in both interphase and metaphase cells. In addition，there were four 5S rDNA hybridization sites in the interphase of diploids R. davidii and R. banksiopsis，but only two 5S rDNA hybridization sites could be observed in metaphase. The number of 5S rDNA hybridization loci of other 14 species in both interphase and metaphase were identical to their ploidy. These results indicated that FISH with 45S rDNA and 5S rDNA as probes could identify some of the chromosomes more accurately and thus could improve the results of karyological study of genus Rosa. Based on the mapping of rDNA in the chromosomes of R. praelucens and its 20 relative species，it could be postulated that R. graciliflora，R. moyesii and R. murielae were the possible original ancestors of R. praelucens.

Key words: Rosa, Rosa praelucens, FISH, 45S rDNA, 5S rDNA, hybrid origination, karyotype analysis