Studies on nucleolar RNA synthesis in Drosophila melanogaster. II. The influence of conditions resulting in a bobbed phenotype on rate of synthesis and secondary constriction formation.

The influence of conditions resulting in bobbed phenotypes on nucleolar RNA synthesis and the formation of constrictions at nucleolus organizers was examined in larval tissues of Drosophila melanogaster. By means of [H]uridine incorporation and autoradiographic analysis, a mutation at the bobbed locus was shown to limit the rate of nucleolar RNA synthesis in salivary glands of XO larvae. The formation of constrictions at the organizer sites of a 4-nucleolusorganizer stock was monitored in dividing neuroblast cells stained with acridine orange. Loss of the ribosomal cistrons had been reported by other workers when such stocks were maintained for several generations. In the first generation in our work, constrictions were visible at only 2 of the 4 nucleolus organizers. This situation persisted until the fifth generation, when constrictions appeared at all 4 of the organizer sites. An increase in the rate of nucleolar RNA synthesis in the salivary glands was temporally correlated with the appearance of the extra constrictions. We interpret these observations to mean that 2 of the organizers of the 4nucleolus-organizer stock were caused to function through the loss of ribosomal RNA cistrons; thus the functional status of an organizer would appear to be subject to control.