Extensive deletions in the Q region of the mouse major histocompatibility complex

By use of Southern blot analyses and low copy number probes, the fine structure of the Q region of the mouse major histocompatibility complex was studied in more detail. With a probe recognizing the even-numbered genes Q4, Q6, and Q8, it was evident that Q4 and/or the regions flanking Q4 are polymorphic, whereas Q6 and Q8, and their flanking regions are nonpolymorphic. Perhaps the most noteworthy finding is that at least two strain haplotypes, H-2 ^k and H-2 ^f, possessed extensive deletions in the Q region. The most striking deletion was found in the H-2 ^f haplotype, where the QI through Q9 genes appear to be missing. Because of these extensive deletions the functional importance of the Q region is questioned.     

Determination of the region of rDNA involved in polytenization in salivary glands of Drosophila hydei

During the formation of polytene chromosomes in salivary glands of Drosophila hydei, the genes for ribosomal RNA (rDNA) are underreplicated relative to the rest of the genome. We have measured the number of rRNA genes with and without intervening sequences (ivs+ and ivs- genes) in polytene chromosomes of different genotypes. In the group of genotypes having a large number of ivs- rRNA genes polytenization only occurs within the cluster of ivs- genes. In each of these genotypes rDNA polytenization reaches a constant level of 150 ivs- genes per two chromatid sets (2C); X/X constitutions having two nucleolus organizers (NOs) in the diploid set polytenize the same amount of rDNA as X/O constitutions. In the group of genotypes with small ivs- gene numbers, the rDNA region involved in polytenization is longer and has an average length of 1,700 kb per NO, which is constant in these genotypes. Polytenization of rDNA is extended into the cluster of ivs+ genes, in spite of the fact that these genes appear to be nonfunctional. The smaller the number of ivs- genes, the greater the number of ivs+ genes that are polytenized in the NO. In these genotypes, X/X females replicate twice as much rDNA as X/O males, suggesting that both NOs of the diploid set are polytenized. A comparison of the pattern of spacer length heterogeneity in hybrids between different stocks also demonstrates that both NOs are replicated during polytenization.     

Pleiotropic effects associated with the deletion of heterochromatin surrounding rDNA on the X chromosome of Drosophila

In Drosophila melanogaster X chromosome heterochromatin (Xh) constitutes the proximal 40% of the X chromosome DNA and contains a number of genetic elements with homologous sites on the Y chromosome, one of which is well defined, namely, the bobbed locus, the repetitive structural locus for the 18S and 28S rRNAs. This report presents the localisation of specific repeated DNA sequences within Xh and the employment of this sequence map in constructing new chromosomes to analyse the nature of the heterochromatin surrounding the rDNA region. Repeated sequences were located relative to inversion breakpoints which differentiate Xh cytogenetically. When the rDNA region was manipulated to be in a position in the chromosome so that it was without the Xh which normally surrounds it, the following obser-vations were made, (i) The rDNA region of Xh is intrinsically hetero-chromatic, remaining genetically active and yet possessing major heterochromatic properties even in the absence of the flanking heterochromatin regions, (ii) The size of the deletion removing the portion of Xh normally located distal to the rDNA region affected the dominance relationship between the X and Y nucleolar organizers (activity/endoreduplication assayed in male salivary glands). The X rDNA without any flanking heterochromatin was dominant over Y rDNA while the presence of some Xh allowed both the X and Y rDNA to be utilized, (iii) Enhancement of the position effect variegation on the white locus was demonstrated to occur as a result of the Xh deletions generated. EMS mutagenesis studies argue that the regions of Xh flanking the rDNA region contain no vital loci despite the fact that they strongly effect gene expression in some genotypes. This is consistent with early studies using X-ray mutagenesis (Lindsley et al., 1960). The pleiotropic effects of deleting specific regions of Xh is discussed in relation to the possible influence of heterochromatin on the organisation of the functional interphase nucleus.     

The centric region of the X chromosome rDNA functions in male meiotic pairing in Drosophila melanogaster

In Drosophila melanogaster males, sex chromosome pairing at meiosis is ensured by so-called pairing site(s) located discretely in the centric heterochromatin. The property of the pairing sites is not well understood. Recently, an hypothesis has been proposed that 240 bp repeats in the nontranscribed spacer region of rDNA function as the pairing sites in male meiosis. However, considerable cytogenetic evidence exists that is contrary to this hypothesis. Hence, the question is whether the chromosomal rDNA clusters, in which a high copy number of 240 bp repeats exists, are involved in the pairing. In order to resolve the problem we X-rayed Drosophila carrying the X chromosome inversion In(1)sc ^V2L sc ^8R and generated free, mini-X chromosomes carrying a substantial amount of rDNA. We defined cytogenetically the size of the mini-chromosomes and studied their meiotic behavior. Our results demonstrate that the heterochromatin at the distal end of the inversion, whose length is approximately 0.4 times that of the fourth chromosome, includes a meiotic pairing site in the male. We discuss the cytological location of the pairing site and the possible role of rDNA in meiotic pairing.     

A unique nucleoprotein structure associated with the Drosophila melanogaster 18-28 S rDNA nontranscribed spacer

We have detected unique nucleoprotein particles specific for the 18-28 S rDNA nontranscribed spacer of Drosophila melanogaster. The particles migrate between di- and trinucleosomes on nucleoprotein gels, and are between mono- and dinucleosomal in DNA length. These migration properties suggest that the nontranscribed spacer particles could have a protein component larger than a histone core. The variant nucleoprotein structures map primarily within the nontranscribed spacer 235 base pair internal subrepeat, which is AT-rich and possesses a 50 base pair sequence homologous to the RNA polymerase I binding site.     

Bidirectional deletions associated with IS4

Summary A new type of chromosomal rearrangements associated with a transposable element has been described for IS4. These rearrangements are deletions, in which the transposable element IS4 and DNA adjacent to it on either side is lost. These deletions are at least ten times more frequent in the presence of IS4, than when the element is absent from that region. The formation of bidirectional deletions is more than 1,000 times more frequent than precise excision of IS4.     

Amplification of rDNA and type I sequences in Drosophila males deficient in rDNA

rDNA magnification is a heritable change in rDNA content that occurs in D. melanogaster males when chromosomes deficient in rDNA are placed together for several generations. We have examined the restriction endonuclease cleavage pattern of the rDNA from an X chromosome undergoing magnification, and find no evidence for the selective amplification of either uninterrupted rDNA units or those containing insertion sequences. In addition, we observe an amplification of rDNA in the first generation of extremely bobbed male progeny to a level exceeding that of wild-type flies, but that reduces to the wild-type level in subsequent generations. The type I rDNA insertion elements also occur as tandem arrays, independently of rDNA. Southern hybridizations indicate that the majority of these sequences are located in the heterochromatin surrounding the nucleolus organizer on the X chromosome, and we find that they, too, amplify transiently in the first generation of magnifying males.     

Xp deletions associated with autism in three females

We report eight females with small deletions of the short arm of the X chromosome, three of whom showed features of autism. Our results suggest that there may be a critical region for autism in females with Xp deletions between the pseudoautosomal boundary and DXS7103. We hypothesise that this effect might be due either to the loss of function of a specific gene within the deleted region or to functional nullisomy resulting from X inactivation of the normal X chromosome. Received: 6 April 1998 / Accepted: 4 November 1998     

Ring Chromosomes and rDNA Magnification in Drosophila

Tartof showed that ribosomal gene magnification in Drosophila was inhibited in a ring X chromosome. The present studies extend this observation by showing that ring X chromosomes are lost meiotically in male Drosophila undergoing ribosomal gene magnification as evidenced by the recovery of a lower number of ring-bearing progeny under magnifying conditions compared with nonmagnifying conditions. Associated with ring chromosome loss is a highly significant increase in the number of double-sized dicentric ring chromosomes in meiotic cells from magnifying males. These observations explain the failure of ring X chromosomes to magnify and imply that magnification in rod chromosomes occurs via a mechanism of unequal sister chromatid exchange. Our results support the hypothesis that the primary event of magnification is a sister chromatid exchange in the rDNA, that the frequency of sister strand exchanges is increased in magnifying flies, that a significant number of exchanges in magnifying flies occurs meiotically and that some of the exchanges are nonreciprocal. We have also found that autosomal mutations can affect both the frequency of abnormal ring structures and the ability of ring X chromosomes to magnify.     

Elastin region deletions in Williams syndrome

Williams syndrome (WS) is considered a contiguous gene syndrome, with most patients having a 1.5-Mb deletion of chromosome 7q11.23 containing the elastin gene and flanking genes. Studies of the frequency, extent, and origin of these deletions are ongoing in many labs to discover ultimately the molecular and pathogenetic basis for WS. An analysis of 9 sporadic WS families with typical phenotypes was performed by genotyping polymorphisms in the region. This study revealed deletions in all 9 patients, with one showing a novel deletion extending much further centromeric than any other WS deletions yet reported.     

Sigma viruses from three species of Drosophila form a major new clade in the rhabdovirus phylogeny

The sigma virus (DMelSV), which is a natural pathogen of Drosophila melanogaster, is the only Drosophila-specific rhabdovirus that has been described. We have discovered two new rhabdoviruses, D. obscura and D. affinis, which we have named DObsSV and DAffSV, respectively. We sequenced the complete genomes of DObsSV and DMelSV, and the L gene from DAffSV. Combining these data with sequences from a wide range of other rhabdoviruses, we found that the three sigma viruses form a distinct clade which is a sister group to the Dimarhabdovirus supergroup, and the high levels of divergence between these viruses suggest that they deserve to be recognized as a new genus. Furthermore, our analysis produced the most robustly supported phylogeny of the Rhabdoviridae to date, allowing us to reconstruct the major transitions that have occurred during the evolution of the family. Our data suggest that the bias towards research into plants and vertebrates means that much of the diversity of rhabdoviruses has been missed, and rhabdoviruses may be common pathogens of insects.     

Evidence for an excess of rDNA in the testis of Drosophila melanogaster during rDNA magnification.

Summary Hybridization of rRNA with DNA extracted from different tissues of different genotypes have been performed. The results show that: 1) in DNA extracted from the testis of premagnified males there exists an excess of rDNA, which is consistent with the model proposed by Ritossa (1972) and by us (1973) to explain the phenomenon of magnification. 2) in DNA extracted from diploid tissues of different genotypes the percent of rDNA is directly proportional to the number of ribosomal genes. 3) in polytene cells the percent of rDNA for all genotypes so far studied is less than that in diploid cells and is not significantly dependant on the genotype. This last result is consistent with those of Spear and Gall (1973).     

Genomic deletions of the Drosophila melanogaster Hsp70 genes

Homologous recombination can produce directed mutations in the genomes of a number of model organisms, including Drosophila melanogaster. One of the most useful applications has been to delete target genes to generate null alleles. In Drosophila, specific gene deletions have not yet been produced by this method. To test whether such deletions could be produced by homologous recombination in D. melanogaster we set out to delete the Hsp70 genes. Six nearly identical copies of this gene, encoding the major heat-shock protein in Drosophila, are found at two separate but closely linked loci. This arrangement has thwarted standard genetic approaches to generate an Hsp70-null fly, making this an ideal test of gene targeting. In this study, ends-out targeting was used to generate specific deletions of all Hsp70 genes, including one deletion that spanned approximately 47 kb. The Hsp70-null flies are viable and fertile. The results show that genomic deletions of varied sizes can be readily generated by homologous recombination in Drosophila.