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

Abstract

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.Within the Diptera, the replication of the rRNA genes (rDNA) during polyploidization has long been known to be independent of the replication of the euchromatic genome (1-4). The mechanisms underlying this independent regulation of rDNA polyploidization are not understood but the studies of Spear and Gall (2, 3) have established that in polytene salivary gland tissue, the rDNA is autonomously replicated to a constant level that is independent of both the amount of rDNA initially present in the diploid genome and the number of nucleolar organizer (NO) regions within the genome in which this rDNA is distributed. It has been suggested that endoreplication of rDNA cistrons from only one NO may in part explain these observations (1, 5-7).Recent studies have established that, at the level of the total polyploid tissue DNA, the overall contribution of the two NOs to the polyploidization of the rDNA is a function of the pair of NOs examined (6,(8)(9)(10)(11)(12). In some cases, one NO is replicated to the virtual exclusion of the other, whereas for other NO pairs, cistrons from both NOs are equally well represented in the total polyploid tissue DNA...

Incomplete replication generates somatic DNA alterations within Drosophila polytene salivary gland cells

“…This predicted cell-to-cell variation has already been observed. The level of underreplication of particular classes of rDNA repeat varied stochastically from cell to cell in the salivary gland (Belikoff and Beckingham 1985). The copy number of the yellow gene when located in a UR region varied widely between individual salivary gland cells when assayed by in situ hybridization, and yellow expression variegated in polytene bristle cells (Karpen and Spradling 1990).…”

The Drosophila endocycle is controlled by Cyclin E and lacks a checkpoint ensuring S-phase completion.

“…There may be some stochastic variation in the period of CycE oscillation between different endocycles and between cells within a tissue. This could account for cell-to-cell heterogeneity in the content of specific rDNA repeats within individual polyploid nurse cells (Belikoff and Beckingham 1985).…”