Independent Control of Ribosomal Gene Replication in Polytene Chromosomes of Drosophila melanogaster

Abstract

The saturation values for ribosomal RNA hybridization have been determined for DNA from diploid and polytene tissues of Drosophila melanogaster. These values have been measured in XO and XX larvae which have, respectively, one and two nucleolus organizers in the diploid chromosome set. The results show that (1) in diploid cells the ribosomal (r)DNA is present in amounts proportional to the number of nucleolus organizers, (2) in polytene cells the rDNA is under-replicated with respect to the euchromatic DNA, and (3) in polytene cells the amount of rDNA is independent of the diploid number of nucleolus organizers. These observations suggest that somatic variations in rDNA content may involve independent polytenization of the nucleolus organizer without change in the number of ribosomal cistrons per organizer. The independent polytenization of rl)NA is proposed as the chromosomal explanation for the relative increase in rDNA in flies of the XO constitution.The chromosomes of Drosophila melanogaster, like those of most eukaryotes, contain two distinct types of chromatin, the euchromatin and the heterochromatin. The euchromatin is diffuse during interphase, is lightly staining in mitotic prophase, and contains almost all of the known genes (1). The heterochromatin, contrastingly, remains condensed during interphase, is consequently darkly staining in mitotic prophase, and contains very few genes. In addition to these genetic and cytological distinctions, the euchromatin consists mainly of unique DNA sequences, while most of the heterochromatin in diploid cells is highly repeated satellite DNA (2)(3)(4).During the formation of the giant polytene chromosomes of the Drosophila salivary gland, the differences between the euchromatin and heterochromatin become even more pronounced. The euchromatic arms proceed through a series of DNA replications resulting in as many as 1024 DNA fibers (5). The heterochromatic regions of the chromosomes, on the other hand, clump together forming the chromocenter and replicate little or not at all (2, 6). The resulting polytene nucleus has a large increase in unique DNA, but little or no increase in the heterochromatic satellite DNA (2, 7).In D. melanogaster the DNA that codes for ribosomal RNA is located at or near the nucleolus organizer, a region within the heterochromatin of the X and Y chromosomes (8). The ribosomal cistrons are repeated about 250 times in the nucleolus organizer of wild-type stocks (9). The properties of the ribosomal (r)DNA are quite different from those of the heterochromatic DNA that surrounds it. For instance, the rDNA is genetically active, transcribing rRNA and forming a prominent nucleolus in both diploid and polytene tissues. Unlike the satellite DNA, the rDNA replicates during the formation of polytene tissue. This has been demonstrated by thymidine incorporation in D. melanogaster (10, 28), and by rRNA hybridization to DNA from salivary glands of D. hydei (11, 12), Sciara (13), and Rhynchosciara (14).Because the rDNA in polytene nuclei is near the n...

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

“…Thus, satellite DNAs appear to be UR ;30-fold (about five rounds of replication) during salivary gland development, a somewhat smaller degree of underreplication than previously reported in Drosophila melanogaster (Rudkin 1969) or Drosophila virilis (Gall et al 1971). The read frequency of rDNA sequences in the salivary gland averaged 21% 6 1.0% that of the embryo (N = 3) (Supplemental Table S3), consistent with the fourfold underrepresentation previously reported (Spear and Gall 1973). 5S rDNA, which is encoded in a separate locus, replicated fully as expected (Hammond and Laird 1985).…”

Induction of Nucleolar and Mitochondrial DNA Replication in Tetrahymena pyriformis

“…DISCUSSION The results presented in this paper suggest that the replication of nucleolar and mitochondrial DNA is controlled by a mechanism which is separate from that which controls the replication of the bulk macronuclear DNA in Tetrahymena. Evidence for independent control of ribosomal gene replication has also been obtained in Drosophila during the formation of polytene chromosomes (12). However, the Tetrahymena system has the uniqueness that a selective replication of nucleolar as well as of mitochondrial DNA can be physiologically induced in a specific way in starved cells which are refed with a complex growth medium.…”

Ribosomal DNA copy number is coupled with gene expression variation and mitochondrial abundance in humans

“…On the other hand, upper limits placing boundaries of rDNA dosage might be less sharply defined. Rapid amplification of the locus has long been well documented in somatic tissues of fruitflies that inherit reduced rDNA arrays [44][45][46][47] , and an upwards of 10,000-fold amplification of the rDNA has been well described in amphibians oocytes 48,49 . These observations, along with others, indicate that somatic gene amplification and selection might contribute to shaping rDNA dosage 38 and that molecular models for the trans-regulatory effects of rDNA dosage need to be refined.…”