The formation of ribonucleoprotein droplets in Chironomus salivary gland nuclei

The appearance of RNP droplets associated with specific regions of polytene chromosomes from Chironomus larvae has been studied, systematically by light microscopy using glutarladehyde fixation and lactic acid squashing of the salivary gland. During the fourth larval instar the highest frequency of nuclei which contained droplets (over 50%) was observed in late prepupae. Several treatments (heat and cold shock, cycloheximide) were found to stimulate the formation of a large number of droplets whose features under the light and electron microscopes seemed to be identical to those observed in untreated larvae. Droplets could be observed either in a free state in the nuclear sap or grouped around specific regions of the chromosome such as some Balbiani rings and puffs. Kinetic studies suggest that droplets first appear in a free state and later group themselves on specific chromosome regions. Transcriptional activity seems to be essential for the process of droplet accumulation in the chromosome, since it does not take place if RNA synthesis is inhibited. Cytochemical tests showed striking similarities between the staining reactions of droplets and those of the fibrillar component of the nucleolus. These observations indicate that droplets probably originate from, or share a common component with, nucleolar material and that this component has a preferential affinity for some active chromosomal loci, where it accumulates.