NUCLEOLAR ORGANIZER EXPRESSION IN ALLIUM-CEPA L CHROMOSOMES

Roots from Allium cepa L. (cv. Francesa) bulbs in which a maximum of t wo nucleoli per nucleus developed were selected for this study. Five r DNA clusters were detected by fluorescent in situ hybridization on chr omosomal squashes (2n = 16) with a rhodamine-labelled wheat rDNA repea t. The rDNA clusters were located on four chromosomes: the largest clu ster occurred on the small arm of a single homologue of the smallest p air 8. Its homologue showed two different small rDNA clusters, one nea r each telomere. The two homologues of the satellited chromosomes 6 al so showed different rDNA contents, which were intermediate to those fo und in pair 8. The same five well-differentiated hybridization signals were observed in interphase cells that were inactive in transcription because they were in dormant roots, or in proliferating ones in which the synthesis of the large rRNA precursor was prevented. After multip olarizing agent was applied in anaphase followed by inhibition of cyto kinesis, multinucleate autotetraploid cells were formed, which often c ontained more than four nucleoli. Thus, at least two of the three nucl eolar organizer regions that consistently failed to develop a nucleolu s in normal mononucleate cells were capable of developing nucleoli whe n segregated Into different nuclei in multinucleate cells.