The nucleolus in primary spermatocytes of Drosophila hydei

Abstract

Transcribing ribosomal RNA genes in primary spermatocyte nucleoli of Drosophila hydei have been visualized by electron microscopy using a microspreading technique. The length of the transcribing unit is in agreement with the length of the ribosomal RNA precursor as determined by acrylamide gel electrophoresis (2.6×10⁶ daltons). The length of the non-transcribed spacer is approximately 1.0×10⁶ daltons. The maximum number of active cistrons in wild type (XY) males only approaches one half the estimated number of ribosomal cistrons of the replicated diploid genome (∼300) of D. hydei and is found to vary between 120 and 320 cistrons in different developmental stages of spermatocytes. There is also some variation between different males. In a few cases a variation in the transcriptional activity of different cistrons has been observed. The synthesis of ribosomal RNA therefore seems to be regulated primarily by the activation or inaetivation of varying numbers of ribosomal cistrons. Groups of adjacent cistrons seem to be under coordinated control. Inhibition of RNA synthesis by actinomycin, in contrast, follows a random pattern. The frequent observation of a bipartite nucleolus indicates that the nucleolus organizer regions of the two sex chromosomes are both active in transcription. The number of active ribosomal eistrons found in some X-Y translocation stocks and in XO males deviates considerably from that expected on the basis of DNA/RNA hybridization data. We conclude that, in agreement with the observations of other authors, a mechanism adjusting the number of ribosomal cistrons may be operating in these cases.