Under-replication of intron+ rDNA cistrons in polyploid nurse cell nuclei of Calliphora erythrocephala

Differential magnification of rDNA gene types in bobbed mutants of Drosophila melanogaster

In Drosophila melanogaster a partial loss of ribosomal genes leads to the bobbed phenotype. Magnification is a heritable increase in rDNA that may occur in males carrying a deleted X chromosome with a strong bobbed phenotype. The restriction patterns of X chromosome total rDNA, insertions and spacers from magnified bobbed strains were compared with those of the original bobbed mutations. It was found that magnification modifies restriction patterns and differentially affects gene types, increasing specific genes lacking insertions (INS-). Increases in copy number of genes with type I insertions are generally lower than the total number of INS- genes while type II insertion genes are not perceptibly increased. The recovery of homogeneous progeny from a single premagnified male indicates that the magnification event might take place and become stable very early in the germ line, arguing against magnification being due to extrachromosomal amplification. Additionally, some gene types increase 3.5-fold while others are eliminated, indicating that they could not result from a single unequal cross-over. These results are in good agreement with the existence of partial clustering of rDNA genes according to type, and suggest that magnification could result from local amplification of genes.

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

Early during Drosophila oogenesis the 16 interconnected cells of each germ-line cyst choose between two alternative fates. The single future oocyte enters meiosis, arrests, and becomes transcriptionally quiescent. The remaining 15 cells initiate a series of polyploid cell cycles to prepare for their role as nurse cells. Like many other polyploid and polytene cells, during nurse cell growth the major satellite DNAs become highly under-represented by a mechanism that has remained obscure. We implicate the cell-cycle regulator cyclin E in DNA under-representation by identifying a hypomorphic, female sterile cycE mutation, cycE01672, that increases the amount of satellite DNA propagated in nurse cells. In mutant but not wild-type endomitotic nurse cells, "late S" patterns of bromodeoxyuridine incorporation are observed similar to those in mitotic cells. CycE protein still cycles in cycE01672 germ-line cysts but at reduced levels, and it is found throughout a longer fraction of the cell cycle. Our experiments support the view that oscillating levels of CycE control the polyploid S phase. Moreover, they indicate that a checkpoint linking the presence of unreplicated DNA to the CycE oscillator is lacking, leading to incomplete replication of late-replicating sequences such as satellite DNAs. Unexpectedly, two to three of the 16 cells in cycE01672 cysts frequently differentiate as oocytes, implicating cell-cycle programming in oocyte determination.

Polytene chromosomes of monogenic and amphogenic Chrysomya species (Calliphoridae, Diptera): analysis of banding patterns and in situ hybridization with Drosophila sex determining gene sequences

Standard maps for the five banded polytene chromosomes found in trichogen cell nuclei of the monogenic blowfly Chrysomya rufifacies and the amphogenic Chrysomya pinguis are presented. The chromosomes are highly homologous in the two species; differences in banding patterns are predominantly caused by one pericentric and ten paracentric inversions. In chromosome 5 of the amphogenic Chrysomya phaonis, also analysed in this paper, an additional paracentric inversion was observed. The distribution of species specific inversions indicates that the monogenic C. rufifacies is phylogenetically older than the amphogenic species. The maternal sex realizer locus F'/f on polytene chromosome 5 of C. rufifacies is not associated with a structural heterozygosity. Chromosome pair 6 of C. rufifacies and the sex chromosome pair of C. pinguis are under-replicated in polytene nuclei; they consist of irregular chromatin granules, frequently associated with nucleolus material. Evolution of heteromorphic sex chromosomes in Chrysomya is probably correlated with heterochromatin accumulation. A search for sex determining genes in Chrysomya was initiated using sex determining sequences from Drosophila melanogaster for in situ hybridization. The polytene band 41A1 on chromosome 5 of monogenic and amphogenic Chrysomya species contains sequences homologous to the maternal sex determining gene daughterless (da). Homology to the zygotic gene Sex-lethal (Sxl) of Drosophila is detected in band 39A1 on chromosome 5 of C. rufifacies. The findings reported here are the first evidence for a possible homology between the da gene of Drosophila and the maternal sex realizer F' of C. rufifacies. An hypothesis for the evolution of the maternal effect sex determination of C. rufifacies is proposed.

Isolation and characterization of ribosomal DNA variants from Sciara coprophila

The ribosomal RNA multigene family in the fungus fly Sciara coprophila contains a total of only 65 to 70 repeat units. We explored the types and frequencies of variant repeats in this small multigene family by characterizing different cloned rDNA variants from Sciara. Although we did not observe any intergenic spacer length variants in Sciara, we found a variant due to the insertion of a putative mobile element (lambda Bc11), and variants containing ribosomal insertion elements. By DNA sequence analysis of rDNA/non-rDNA junctions, there are three distinct types of ribosomal insertion elements found in Sciara rDNA: two correspond to the R1 and R2 insertion elements found in other dipterans (clones lambda Bc5 and pBc1L1, respectively), and one is a novel class of ribosomal insertion elements (R3, exemplified by clone pBc6D6) which so far is unique to Sciara. Together, the several different rDNA variants make up from 12 to 20% of the rDNA in Sciara. These results are discussed in the context of evolution of the ribosomal RNA multigene family.

Genes with specific functions in the ovarian follicles of Calliphora erythrocephala (Diptera)

Working with the large dipteran Calliphora erythrocephala, we have performed differential screening to identify genes actively expressed in the previtellogenic and early vitellogenic stages of oogenesis but silent during the early stages of embryogenesis. Clones containing sequences homologous to four such genes have been characterized. Two clones are homologous to the yolk protein 1 gene of Drosophila melanogaster. These two clones are expressed not only in the columnar follicle cells surrounding the oocyte but also in the border cells--a highly specialized subgroup of the follicle cells. This indicates a new function for these cells though previously to contribute mainly in the formation of the micropyle. A third clone, which is related to the D. melanogaster vitelline membrane protein genes of the cluster at chromosomal locus 26A, is expressed only in the perioocyte follicle cells and not the border cell population. The fourth clone encodes a sequence of unknown function which is abundantly expressed in the germ line cells of the follicle. Transcripts homologous to this clone persist into the mature follicle and initially appear concentrated at the anterior pole of the oocyte. The distribution of repetitious DNA within these four clones indicates that the C. erythrocephala genome has a short interspersion arrangement of repetitive DNA.

GATA tandem repeats detect minisatellite regions in blowfly DNA (Diptera: Calliphoridae)

A DNA probe containing GATA tandem repeats detected numerous dispersed minisatellite regions in the genomes of the blowflies Chrysomya rufifacies and Calliphora erythrocephala. These regions seemed to be actively transcribed into poly(A)+ RNA in a tissue-specific manner. When genomic DNA of blastoderm embryos was compared with adult genomic DNA some loci hybridizing to GATA displayed a marked stage-specific variation in length. In Calliphora, a small sex-linked dimorphism of GATA mini-satellite-associated restriction fragments was observed.

3B55: a repetitious sequence family which is transcribed and proportionately replicated in germ-line polyploid nuclei of Calliphora erythrocephala

The chromosomes of dipteran polyploid nurse cell nuclei are functionally analogous to the oocyte lampbrush chromosomes of the Amphibia. In investigating the transcriptional and replicative activity of these nuclei in Calliphora erythrocephala we have identified a cloned highly repetitious DNA fragment which shows enhanced transcriptional activity in these cells and different replicative behavior in these germ-line polyploid nuclei as opposed to somatic polytene nuclei. The clone, 3B55, contains a 6.8-kb insert which consists primarily of tandemly repeated 200-bp sequences defined by RsaI sites. From Southern hybridizations to diploid (embryonic) genomic DNA, 3B55-related DNA was calculated to represent a significant fraction of the haploid genome (0.8% or 5000 kb). In situ hybridizations established that these sequences are present in the pericentric regions of four of the six chromosomes. Thus the 3B55 sequences have the properties of a satellite-type DNA family. Quantitation of the 3B55 200-bp monomer (which represents approximately 60% of the genomic 3B55 DNA sequences in all tissues examined) revealed that in somatic polytene salivary gland nuclei, 3B55 DNA is highly under-replicated to give a monomer representation of only 48 +/- 11 kb per haploid genome. However, in germ-line nurse cell nuclei, 3B55 DNA is proportionately replicated to give a monomer genomic representation (3560 +/- 344 kb) equivalent to that of diploid DNA (3011 +/- 202 kb). Transcripts complementary to 3B55 sequences are at least 25 times more abundant in total nurse cell nuclear RNA than in total embryonic nuclear RNA. These findings suggest an association of the 3B55 sequence family with some germ-line specific function.

Different chromatin states of the intron- and type 1 intron+ rRNA genes of Calliphora erythrocephala

In most species of dipteran fly examined, a fraction of the rDNA cistrons are interrupted by introns. These dipteran intron+ rRNA genes are unique in that they are transcriptionally inactive. Previous studies have investigated the mechanism underlying this transcriptional repression for rRNA genes carrying the best characterized sequence family of such introns, the so-called type 1 introns first identified in Drosophila melanogaster. These studies have established that cloned examples of both intron-free and type 1 intron+ rRNA genes will support transcription in a cell-free system and suggest therefore that a difference in the chromatin state of the two gene types must underlie their very different potential for in vivo transcription. We have examined this possibility for the type 1 intron+ rDNA cistrons of Calliphora erythrocephala by in situ hybridization studies using the polytene chromosome complement of the pupal bristle-forming (trichogen) cells. These studies show that the chromatin configuration of the two gene types is strikingly different. The intron-free genes are preferentially localized in the actively transcribed fibrillar center of the nucleolus. The intron+ genes are preferentially condensed in the blocks of heterochromatin attached to the nucleolus.

Allocyclic behaviour and underreplication of the nucleolus chromosome in Pseudodiamesa (Chironomidae)

The chironomid midge Pseudodiamesa branickii shows three polytene, banded elements in salivary gland and Malpighian tubule cells, none of which carry a nucleolar organizer. The present investigation revealed four chromosome pairs in mitotic metaphases, the nucleolus was localized in the tiny chromosome 4. Microphotometric determinations of the DNA contents in mitotic and salivary gland nuclei of Ps. branickii yielded a 2C value of 0.23 pg DNA and a maximum level of 12 endoreplications. In both salivary glands and Malpighian tubules, chromosome 4 appeared as a nonbanded network of chromatin in Feulgen preparations. Indirect immunofluorescent staining with antibodies specific for RNA X DNA hybrids indicated that this structure was transcriptionally active. Its reaction to heavy metal staining suggested that it included the nucleolar organizer, and this was conclusively demonstrated by in situ hybridization with 125I-rRNA. Chromosome pair 4 comprises about 11.4% of the total DNA in metaphase, but only 3.7% in the highly polytene salivary gland complement. According to a numerical simulation, the degree of underrepresentation suggests that about 70% of chromosome 4 is precluded from polytenization. The allocyclic aspect is probably due to this underreplication, with the nucleolar activity causing a structural loosening of the rest of the chromosome. The possible nature of the underreplicated sequences is discussed in the light of recent work on molecular aspects of rDNA structure in various Diptera.