CHARACTERIZATION OF THE NUCLEOLAR ORGANIZING REGIONS DURING THE CELL-CYCLE IN 2 VARIETIES OF PETUNIA-HYBRIDA AS VISUALIZED BY FLUORESCENCE IN-SITU HYBRIDIZATION AND SILVER STAINING

The cell cycle-dependent spatial position, morphology and activity of the four nucleolar organising regions (NORs) of the Petunia hybrida cu ltivar Mitchell and the inbred line V26 have been analysed. Applicatio n of the silver staining technique and fluorescence in situ hybridisat ion on fixed root-tip material revealed that these interspecific hybri ds possess four NORs of which only those of chromosome 2 are active du ring interphase, which implies that the NOR activity is not of parenta l origin. However, at the end of mitosis, activity of all NOR regions could be detected, suggesting that the high demand for ribosomes at th is stage of the cell cycle requires temporal activity of all NORs. Usi ng actin DNA probes as markers in fluorescence in situ hybridisation e xperiments enabled the identification of the individual petunia chromo somes.