The genes for ribosomal RNA in diploid and polytene chromosomes of Drosophila melanogaster

The proportion of the Drosophila genome coding for ribosomal RNA was examined in DNA from both diploid and polytene tissues of Drosophila melanogaster by rRNA-DNA hybridization. Measurements were made on larvae with one, two, three and four nucleolus organizer regions per genome. In DNA from diploid tissues the percent rDNA (coding for 28S and 18S ribosomal DNA) was found to be in proportion to the number of nucleolus organizers present. The number of rRNA genes within a nucleolus organizer therefore does not vary in response to changes in the number of nucleolus organizers. On the other hand, in DNA from cells with polytene chromosomes the percent rDNA remained at a level of about 0.1% (two to six times lower than the diploid values), regardless of either the number of nucleolus organizers per genome or whether the nucleolus organizers were carried by the X or Y chromosomes. This independence of polytene rDNA content from the number of nucleolus organizers is presumably due to the autonomous polytenization of this region of the chromosome. When the rDNA content of DNA from whole flies is examined, both the rDNA additivity of the diploid cells and the rDNA independence of polytene cells will affect the results. This is a possible explanation for the relative rDNA increase known to occur in X0 flies, but probably not for the phenomenon of rDNA magnification. — In further studies on DNA from larval diploid tissues, the following findings were made: 1) the Ybb-chromosome carries no rDNA; 2) flies carrying four nucleolus organizers do not tend to lose rDNA, even after eleven generations, and 3) the nucleolus organizer on the wild type Y chromosome may have significantly less rDNA than does that on the corresponding X chromosome.     

The chromosomal location of ribosomal DNA in the mouse

In situ hybridization of I¹²⁵-labelled ribosomal RNA to mouse chromosomes was used to determine the location of the rDNA loci. The results demonstrate the presence of rDNA sites on chromosomes 15, 18 and 19.     

Localization of rDNA in the chimpanzee (Pan troglodytes) chromosome complement

In situ hybridization was used to identify the sites of rDNA in the chromosome complement of the chimpanzee (Pan troglodytes). The rDNA was present in the satellite regions of chimpanzee chromosomes 14, 15, 17, 22 and 23. Four of these (14, 15, 22, 23) are homologous to human chromosomes carrying rDNA: 13, 14, 21 and 22.     

Localization of rDNA in the chromosome complement of the Rhesus (Macaca mulatta)

In situ hybridization was used to identify the region of the Rhesus genome complementary to ribosomal RNA. The rDNA sites have been assigned to the achromatic region of chromosome 20.     

Light microscope autoradiography of ribosomal DNA amplification in Xenopus laevis

A technique for the isolation of very high molecular weight rDNA¹ from the ovary of Xenopus laevis is described. Tritiated rDNA was prepared by this method from ovaries at the amplification stage, and spread on slides for light microscope autoradiography. The average molecular weight of the spread DNA was greater than 180×10⁶ daltons. Unlike chromosomal DNA grain tracks, rDNA tracks after 2 or 4 hours of labelling were not tandemly arranged. By allowing ovaries to equilibrate gradually with exogenous precursors, tracks showing a single gradient of grain density were produced, indicating that replication was proceeding in one direction at these sites. Bidirectional initiations, if they occur at all during amplification, are rare. The rate of rDNA chain growth is 10.5 μ/hour at 23° C, which is the same as the rate for chromosomal DNA synthesis in X. laevis. After 24 hours some tracks are over 200 μ long, suggesting that replication at a site may be continuous for at least this period. Although they do not distinguish between several alternative mechanisms, the results are compatible with a rolling circle mechanism for gene amplification.     

A genetic and ultrastructural study of three clones of Porphyridium purpureum (Bory de Saint-Vincent, 1797) Drew et Ross, 1965 (Rhodophyta) from the marine microalgae collection of the Zhirmunsky institute of marine biology

In this paper, we present the results of a comparative genetic and ultrastructural study of three clones of the microalga Porphyridium purpureum (Rhodophyta) from the culture collection of marine microalgae of the Zhirmunsky Institute of Marine Biology. All clones, which have different geographical origins, showed a high similarity in terms of the ultramicroscopic structure and the nucleotide sequences of the nuclear ribosomal DNA genes (18S rDNA, ITS1-5.8S rDNA-ITS2, D1-D2 region of 28S rDNA). The obtained data are very helpful for the certification of two strains of P. purpureum that were isolated for the first time in the practice of Russian algological research.