A stochastic mechanism controls the relative replication of equally competent ribosomal RNA gene sets in individual dipteran polyploid nuclei.

The endoreplication of the two nucleolar organizers (NOs) of the diploid genome has been examined in individual polyploid nuclei of the dipteran Calliphora erythrocephala. Crosses between two strains with diagnostic nontranscribed spacer polymorphisms in their rRNA genes were used to provide progeny with distinguishable NOs, and single nuclei of two highly polyploid cell types--salivary gland and nurse cells--were examined from individual F1 animals. Initially the representation of the two NOs in total polyploid tissue DNA was determined. This revealed that, although the NO regions present in one of the strains (Tom) were very similar in spacer composition, they displayed two types of behavior in the hybrids containing the single NO region typical of the second strain (Karla). In TW phenotype F1 progeny, very little replication of the Tom NO relative to the Karla NO occurred, whereas in TS phenotype progeny replication of the Tom and Karla NOs was approximately equivalent. When individual polyploid nuclei of the TS phenotype animals were examined, however, the relative replication of the Tom and Karla NOs was found not to be a fixed genetic property but to vary dramatically from cell to cell. This was true even for the nurse cell nuclei within a single ovarian follicle, which are the products of only four mitotic divisions of a single germ-line cell. These findings indicate that for NOs of similar replicative competence, a stochastic mechanism governs the relative usage of each NO for endoreplication and that the relative activity of the two NOs is not stably determined through the mitotic divisions preceding polyploidization. Stochastic selection after mitotic DNA replication could be a general phenomenon governing the relative usage (transcription) of different, but equally competent, alleles of any gene in individual cells, if the required factors are in short supply.