Three euchromatic DNA sequences under-replicated in polytene chromosomes of Drosophila are localized in constrictions and ectopic fibers

Arp2/3 and Unc45 maintain heterochromatin stability in Drosophila polytene chromosomes

Repairing DNA double-strand breaks is particularly challenging in pericentromeric heterochromatin, where the abundance of repeated sequences exacerbates the risk of ectopic recombination. In Drosophila Kc cells, accurate homologous recombination repair of heterochromatic double-strand breaks relies on the relocalization of repair sites to the nuclear periphery before Rad51 recruitment and strand invasion. This movement is driven by Arp2/3-dependent nuclear actin filaments and myosins’ ability to walk along them. Conserved mechanisms enable the relocalization of heterochromatic repair sites in mouse cells, and defects in these pathways lead to massive ectopic recombination in heterochromatin and chromosome rearrangements. In Drosophila polytene chromosomes, extensive DNA movement is blocked by a stiff structure of chromosome bundles. Repair pathways in this context are poorly characterized, and whether heterochromatic double-strand breaks relocalize in these cells is unknown. Here, we show that damage in heterochromatin results in relaxation of the heterochromatic chromocenter, consistent with a dynamic response. Arp2/3, the Arp2/3 activator Scar, and the myosin activator Unc45, are required for heterochromatin stability in polytene cells, suggesting that relocalization enables heterochromatin repair also in this tissue. Together, these studies reveal critical roles for actin polymerization and myosin motors in heterochromatin repair and genome stability across different organisms and tissue types.

Centromere-Proximal Meiotic Crossovers in Drosophila melanogaster Are Suppressed by Both Highly Repetitive Heterochromatin and Proximity to the Centromere

Crossovers are essential in meiosis of most organisms to ensure the proper segregation of chromosomes, but improper placement of crossovers can result in nondisjunction and aneuploidy in progeny. In particular, crossovers near the centromere can cause nondisjunction. Centromere-proximal crossovers are suppressed by what is termed the centromere effect, but the mechanism is unknown. Here, we investigate contributions to centromere-proximal crossover suppression in Drosophila melanogaster We mapped a large number of centromere-proximal crossovers, and find that crossovers are essentially absent from the highly repetitive (HR)-heterochromatin surrounding the centromere but occur at a low frequency within the less-repetitive (LR)-heterochromatic region and adjacent euchromatin. Previous research suggested that flies that lack the Bloom syndrome helicase (Blm) lose meiotic crossover patterning, including the centromere effect. Mapping of centromere-proximal crossovers in Blm mutants reveals that the suppression within the HR-heterochromatin is intact, but the distance-dependent centromere effect is lost. We conclude that centromere-proximal crossovers are suppressed by two separable mechanisms: an HR-heterochromatin effect that completely suppresses crossovers in the HR-heterochromatin, and the centromere effect, which suppresses crossovers with a dissipating effect with distance from the centromere.

DNA Replication Control During Drosophila Development: Insights into the Onset of S Phase, Replication Initiation, and Fork Progression

Proper control of DNA replication is critical to ensure genomic integrity during cell proliferation. In addition, differential regulation of the DNA replication program during development can change gene copy number to influence cell size and gene expression. Drosophila melanogaster serves as a powerful organism to study the developmental control of DNA replication in various cell cycle contexts in a variety of differentiated cell and tissue types. Additionally, Drosophila has provided several developmentally regulated replication models to dissect the molecular mechanisms that underlie replication-based copy number changes in the genome, which include differential underreplication and gene amplification. Here, we review key findings and our current understanding of the developmental control of DNA replication in the contexts of the archetypal replication program as well as of underreplication and differential gene amplification. We focus on the use of these latter two replication systems to delineate many of the molecular mechanisms that underlie the developmental control of replication initiation and fork elongation.

Chromatin Heterogeneity and Distribution of Regulatory Elements in the Late-Replicating Intercalary Heterochromatin Domains of Drosophila melanogaster Chromosomes

Late-replicating domains (intercalary heterochromatin) in the Drosophila genome display a number of features suggesting their organization is quite unique. Typically, they are quite large and encompass clusters of functionally unrelated tissue-specific genes. They correspond to the topologically associating domains and conserved microsynteny blocks. Our study aims at exploring further details of molecular organization of intercalary heterochromatin and has uncovered surprising heterogeneity of chromatin composition in these regions. Using the 4HMM model developed in our group earlier, intercalary heterochromatin regions were found to host chromatin fragments with a particular epigenetic profile. Aquamarine chromatin fragments (spanning 0.67% of late-replicating regions) are characterized as a class of sequences that appear heterogeneous in terms of their decompactization. These fragments are enriched with enhancer sequences and binding sites for insulator proteins. They likely mark the chromatin state that is related to the binding of cis-regulatory proteins. Malachite chromatin fragments (11% of late-replicating regions) appear to function as universal transitional regions between two contrasting chromatin states. Namely, they invariably delimit intercalary heterochromatin regions from the adjacent active chromatin of interbands. Malachite fragments also flank aquamarine fragments embedded in the repressed chromatin of late-replicating regions. Significant enrichment of insulator proteins CP190, SU(HW), and MOD2.2 was observed in malachite chromatin. Neither aquamarine nor malachite chromatin types appear to correlate with the positions of highly conserved non-coding elements (HCNE) that are typically replete in intercalary heterochromatin. Malachite chromatin found on the flanks of intercalary heterochromatin regions tends to replicate earlier than the malachite chromatin embedded in intercalary heterochromatin. In other words, there exists a gradient of replication progressing from the flanks of intercalary heterochromatin regions center-wise. The peculiar organization and features of replication in large late-replicating regions are discussed as possible factors shaping the evolutionary stability of intercalary heterochromatin.

Preferential Breakpoints in the Recovery of Broken Dicentric Chromosomes in Drosophila melanogaster

We designed a system to determine whether dicentric chromosomes in Drosophila melanogaster break at random or at preferred sites. Sister chromatid exchange in a Ring-X chromosome produced dicentric chromosomes with two bridging arms connecting segregating centromeres as cells divide. This double bridge can break in mitosis. A genetic screen recovered chromosomes that were linearized by breakage in the male germline. Because the screen required viability of males with this X chromosome, the breakpoints in each arm of the double bridge must be closely matched to produce a nearly euploid chromosome. We expected that most linear chromosomes would be broken in heterochromatin because there are no vital genes in heterochromatin, and breakpoint distribution would be relatively unconstrained. Surprisingly, approximately half the breakpoints are found in euchromatin, and the breakpoints are clustered in just a few regions of the chromosome that closely match regions identified as intercalary heterochromatin. The results support the Laird hypothesis that intercalary heterochromatin can explain fragile sites in mitotic chromosomes, including fragile X. Opened rings also were recovered after male larvae were exposed to X-rays. This method was much less efficient and produced chromosomes with a strikingly different array of breakpoints, with almost all located in heterochromatin. A series of circularly permuted linear X chromosomes was generated that may be useful for investigating aspects of chromosome behavior, such as crossover distribution and interference in meiosis, or questions of nuclear organization and function.

Banding patterns in Drosophila melanogaster polytene chromosomes correlate with DNA-binding protein occupancy

The most enigmatic feature of polytene chromosomes is their banding pattern, the genetic organization of which has been a very attractive puzzle for many years. Recent genome-wide protein mapping efforts have produced a wealth of data for the chromosome proteins of Drosophila cells. Based on their specific protein composition, the chromosomes comprise two types of bands, as well as interbands. These differ in terms of time of replication and specific types of proteins. The interbands are characterized by their association with "active" chromatin proteins, nucleosome remodeling, and origin recognition complexes, and so they have three functions: acting as binding sites for RNA pol II, initiation of replication and nucleosome remodeling of short fragments of DNA. The borders and organization of the same band and interband regions are largely identical, irrespective of the cell type studied. This demonstrates that the banding pattern is a universal principle of the organization of interphase polytene and non-polytene chromosomes.

Late replication domains in polytene and non-polytene cells of Drosophila melanogaster

In D. melanogaster polytene chromosomes, intercalary heterochromatin (IH) appears as large dense bands scattered in euchromatin and comprises clusters of repressed genes. IH displays distinctly low gene density, indicative of their particular regulation. Genes embedded in IH replicate late in the S phase and become underreplicated. We asked whether localization and organization of these late-replicating domains is conserved in a distinct cell type. Using published comprehensive genome-wide chromatin annotation datasets (modENCODE and others), we compared IH organization in salivary gland cells and in a Kc cell line. We first established the borders of 60 IH regions on a molecular map, these regions containing underreplicated material and encompassing ∼12% of Drosophila genome. We showed that in Kc cells repressed chromatin constituted 97% of the sequences that corresponded to IH bands. This chromatin is depleted for ORC-2 binding and largely replicates late. Differences in replication timing between the cell types analyzed are local and affect only sub-regions but never whole IH bands. As a rule such differentially replicating sub-regions display open chromatin organization, which apparently results from cell-type specific gene expression of underlying genes. We conclude that repressed chromatin organization of IH is generally conserved in polytene and non-polytene cells. Yet, IH domains do not function as transcription- and replication-regulatory units, because differences in transcription and replication between cell types are not domain-wide, rather they are restricted to small “islands” embedded in these domains. IH regions can thus be defined as a special class of domains with low gene density, which have narrow temporal expression patterns, and so displaying relatively conserved organization.

Drosophila D1 overexpression induces ectopic pairing of polytene chromosomes and is deleterious to development

Eukaryotic genomes function in the context of chromatin, but the roles of most nonhistone chromosomal proteins are far from understood. The D1 protein of Drosophila is an example of a chromosomal protein that has been fairly well characterized biochemically, but has nevertheless eluded functional description. To this end, we have undertaken a gain-of-function genetical analysis of D1, utilizing the GAL4-UAS system. We determined that ubiquitous overexpression of D1 using the Act5C- or tubP-GAL4 drivers was lethal to the organism during larval growth. We also ectopically expressed D1 in a tissue-limited manner using other GAL4 drivers. In general, ectopic D1 was observed to inhibit differentiation and/or development. We observed effects on pattern formation of the adult eye, bristle morphogenesis, and spermatogenesis. These phenotypes may be the consequence of misregulation of D1 target genes. A surprising result was obtained when D1 was overexpressed in the third instar salivary gland. The polytene chromosomes exhibited numerous ectopic associations such that spreading of the chromosome arms was precluded. We mapped the sites of ectopic pairing along the polytene chromosome arms, and found a correlation with sites of intercalary heterochromatin. We speculate that these sites comprise the natural targets of D1 protein activity and that D1 is involved in the ectopic pairing observed for wild-type chromosomes. Together, our data suggest that D1 may influence multiple biochemical activities within the nucleus.

Localization and characteristics of DNA underreplication zone in the 75C region of intercalary heterochromatin in Drosophila melanogaster polytene chromosomes

In Drosophila polytene chromosomes, regions of intercalary heterochromatin are scattered throughout the euchromatic arms. Here, we present data on the first fine analysis of the individual intercalary heterochromatin region, 75C1-2, located in the 3L chromosome. By using electron microscopy, we demonstrated that this region appears as three closely adjacent condensed bands. Mapping of the region on the physical map by means of the chromosomal rearrangements with known breakpoints showed that the length of the region is about 445 kb. Although it seems that the SUUR protein binds to the whole 75C1-2 region, the proximal part of the region is fully polytenized, so the DNA underreplication zone is asymmetric and located in the distal half of the region. Finally, we speculate that intercalary heterochromatin regions of Drosophila polytene chromosomes are organized into three different types with respect to the localization of the underreplication zone.

Genetic and cytogenetic analysis of the fruit fly Rhagoletis cerasi (Diptera: Tephritidae)

The European cherry fruit fly, Rhagoletis cerasi, is a major agricultural pest for which biological, genetic, and cytogenetic information is limited. We report here a cytogenetic analysis of 4 natural Greek populations of R. cerasi, all of them infected with the endosymbiotic bacterium Wolbachia pipientis. The mitotic karyotype and detailed photographic maps of the salivary gland polytene chromosomes of this pest species are presented here. The mitotic metaphase complement consists of 6 pairs of chromosomes, including one pair of heteromorphic sex chromosomes, with the male being the heterogametic sex. The analysis of the salivary gland polytene complement has shown a total of 5 long chromosomes (10 polytene arms) that correspond to the 5 autosomes of the mitotic nuclei and a heterochromatic mass corresponding to the sex chromosomes. The most prominent landmarks of each polytene chromosome, the "weak points", and the unusual asynapsis of homologous pairs of polytene chromosomes at certain regions of the polytene elements are also presented and discussed.

Polytene Chromosomes: 70 Years of Genetic Research

Polytene chromosomes were described in 1881 and since 1934 they have served as an outstanding model for a variety of genetic experiments. Using the polytene chromosomes, numerous biological phenomena were discovered. First the polytene chromosomes served as a model of the interphase chromosomes in general. In polytene chromosomes, condensed (bands), decondensed (interbands), genetically active (puffs), and silent (pericentric and intercalary heterochromatin as well as regions subject to position effect variegation) regions were found and their features were described in detail. Analysis of the general organization of replication and transcription at the cytological level has become possible using polytene chromosomes. In studies of sequential puff formation it was found for the first time that the steroid hormone (ecdysone) exerts its action through gene activation, and that the process of gene activation upon ecdysone proceeds as a cascade. Namely on the polytene chromosomes a new phenomenon of cellular stress response (heat shock) was discovered. Subsequently chromatin boundaries (insulators) were discovered to flank the heat shock puffs. Major progress in solving the problems of dosage compensation and position effect variegation phenomena was mainly related to studies on polytene chromosomes. This review summarizes the current status of studies of polytene chromosomes and of various phenomena described using this successful model.

The Drosophila suppressor of underreplication protein binds to late-replicating regions of polytene chromosomes

In many late-replicating euchromatic regions of salivary gland polytene chromosomes, DNA is underrepresented. A mutation in the SuUR gene suppresses underreplication and leads to normal levels of DNA polytenization in these regions. We identified the SuUR gene and determined its structure. In the SuUR mutant stock a 6-kb insertion was found in the fourth exon of the gene. A single SuUR transcript is present at all stages of Drosophila development and is most abundant in adult females and embryos. The SuUR gene encodes a protein of 962 amino acids whose putative sequence is similar to the N-terminal part of SNF2/SWI2 proteins. Staining of salivary gland polytene chromosomes with antibodies directed against the SuUR protein shows that the protein is localized mainly in late-replicating regions and in regions of intercalary and pericentric heterochromatin.

Polycomb group repression reduces DNA accessibility

The Polycomb group proteins are responsible for long-term repression of a number of genes in Drosophila melanogaster, including the homeotic genes of the bithorax complex. The Polycomb protein is thought to alter the chromatin structure of its target genes, but there has been little direct evidence for this model. In this study, the chromatin structure of the bithorax complex was probed with three separate assays for DNA accessibility: (i) activation of polymerase II (Pol II) transcription by Gal4, (ii) transcription by the bacteriophage T7 RNA polymerase (T7RNAP), and (iii) FLP-mediated site-specific recombination. All three processes are restricted or blocked in Polycomb-repressed segments. In contrast, control test sites outside of the bithorax complex permitted Gal4, T7RNAP, and FLP activities throughout the embryo. Several P insertions in the bithorax complex were tested, providing evidence that the Polycomb-induced effect is widespread over target genes. This accessibility effect is similar to that seen for SIR silencing in Saccharomyces cerevisiae. In contrast to SIR silencing, however, episomes excised from Polycomb-repressed chromosomal sites do not show an altered superhelix density.

The Bithorax Complex of Drosophila melanogaster: Underreplication and morphology in polytene chromosomes

The level of polyteny of the Drosophila salivary gland chromosomes was determined throughout the chromosome region 89E1-4, the locus of the Bithorax Complex. A zone of underreplication spans the 300 kb of DNA from the Ubx to Abd-B loci. From the centromere proximal end of the complex, a 70-kb-long gradual decrease of polytenization starts with the Ubx transcription unit and, after a floor corresponding to the abd-A locus, raises gradually back to the maximum over 70 kb in the region of the Abd-B transcription unit. In flies carrying the mutation Suppressor of DNA Underreplication [Su(UR)ES], the underreplication of the Bithorax Complex is fully suppressed. In the wild type, the Bithorax Complex forms a weak point featuring thinner bands separated by clefts or constrictions. In Su(UR)ES strain in contrast, the 89E1-4 band looks like a single solid band consisting of homogenous dense material. We speculate that the wild-type Su(UR)ES protein hampers DNA replication of silenced domains and leads to their underreplication in salivary gland polytene chromosomes.

The bithorax complex of Drosophila melanogaster: Underreplication and morphology in polytene chromosomes

The level of polyteny of the Drosophila salivary gland chromosomes was determined throughout the chromosome region 89E1-4, the locus of the Bithorax Complex. A zone of underreplication spans the 300 kb of DNA from the Ubx to Abd-B loci. From the centromere proximal end of the complex, a 70-kb-long gradual decrease of polytenization starts with the Ubx transcription unit and, after a floor corresponding to the abd-A locus, raises gradually back to the maximum over 70 kb in the region of the Abd-B transcription unit. In flies carrying the mutation Suppressor of DNA Underreplication [Su(UR)ES], the underreplication of the Bithorax Complex is fully suppressed. In the wild type, the Bithorax Complex forms a weak point featuring thinner bands separated by clefts or constrictions. In Su(UR)ES strain in contrast, the 89E1-4 band looks like a single solid band consisting of homogenous dense material. We speculate that the wild-type Su(UR)ES protein hampers DNA replication of silenced domains and leads to their underreplication in salivary gland polytene chromosomes.

Su(UR)ES: a gene suppressing DNA underreplication in intercalary and pericentric heterochromatin of Drosophila melanogaster polytene chromosomes

A genetic locus suppressing DNA underreplication in intercalary heterochromatin (IH) and pericentric heterochromatin (PH) of the polytene chromosomes of Drosophila melanogaster salivary glands, has been described. Found in the In(1)scV2 strain, the mutation, designated as Su(UR)ES, was located on chromosome 3L at position 34. 8 and cytologically mapped to region 68A3-B4. A cytological phenotype was observed in the salivary gland chromosomes of larvae homozygous and hemizygous for Su(UR)ES: (i) in the IH regions, that normally are incompletely polytenized and so they often break to form "weak points," underreplication is suppressed, breaks and ectopic contacts disappear; (ii) the degree of polytenization in PH grows higher. That is why the regions in chromosome arm basements, normally beta-heterochromatic, acquire a distinct banding pattern, i. e., become euchromatic by morphological criteria; (iii) an additional bulk of polytenized material arises between the arms of chromosome 3 to form a fragment with a typical banding pattern. Chromosome 2 PH reveals additional alpha-heterochromatin. Su(UR)ES does not affect the viability, fertility, or morphological characters of the imago, and has semidominant expression in the heterozygote and distinct maternal effect. The results obtained provide evidence that the processes leading to DNA underreplication in IH and PH are affected by the same genetic mechanism.

A 1.5 kb repeat sequence flanks the suppressor of forked gene at the euchromatin-heterochromatin boundary of the Drosophila melanogaster X chromosome

A 1.5 kilobasepair repeated DNA sequence is duplicated in direct orientation so as to flank the suppressor of forked gene in the euchromatin-heterochromatin transition region on the X chromosome of Drosophila melanogaster. These two copies are almost identical, but DNA blotting, analysis of cloned sequences and database searches show that elsewhere in the genome, homologous sequences are poorly conserved. They are often associated with other repeats, suggesting that they may belong to a scrambled and clustered middle repetitive DNA family. The sequences do not appear to be related to transposable elements and their location in different strains is conserved. In situ hybridization to metaphase chromosomes shows that homologous sequences are concentrated in the pericentric regions of the autosomes and the X chromosome. The sequences are not significantly under-represented in DNA from polytene tissue and must lie in the replicated regions of polytene chromosomes. The almost perfect conservation of the two repeats around suppressor of forked in D. melanogaster suggests they arose by duplication or gene conversion. Suppression of recombination in this chromosomal region presumably allows this unusual organization to be stably maintained. In the X-ray induced allele, suppressor of forked-L26, the sequence between the repeats, including the gene, and one copy of the repeat have been deleted.

The Afrotropical Drosophila montium subgroup: Balbiani ring 1, polytene chromosomes, and heat shock response of Drosophila vulcana

A detailed photographic map of the salivary gland polytene chromosomes of Drosophila vulcana, an Afrotropical species of the montium subgroup of the melanogaster group, is presented, along with chromosomal rearrangements, such as reverse tandem duplications and inversions, the well-formed Balbiani ring 1, and the most prominent puffs during normal larval and white prepupal development and after ecdysone treatment. In addition, the heat inducible protein and puffing pattern and the loci of the major heat shock genes, namely, hsp70, hsp83, the "small" hsps, and a putative hsp68, of this species were studied. In the light of the data revealed by the above studies, phylogenetic relationship among the montium subgroup species are attempted.

In vivo assay for protein-protein interactions using Drosophila chromosomes

The ability of a chimeric HP1-Polycomb (Pc) protein to bind both to heterochromatin and to euchromatic sites of Pc protein binding was exploited to detect stable protein-protein interactions in vivo. Previously, we showed that endogenous Pc protein was recruited to ectopic heterochromatic binding sites by the chimeric protein. Here, we examine the association of other Pc group (Pc-G) proteins. We show that Posterior sex combs (Psc) protein also is recruited to heterochromatin by the chimeric protein, demonstrating that Psc protein participates in direct protein-protein interaction with Pc protein or Pc-associated protein. In flies carrying temperature-sensitive alleles of Enhancer of zeste[E(z)] the general decondensation of polytene chromosomes that occurs at the restrictive temperature is associated with loss of binding of endogenous Pc and chimeric HP1-Polycomb protein to euchromatin, but binding of HP1 and chimeric HP1-Polycomb protein to the heterochromatin is maintained. The E(z) mutation also results in the loss of chimera-dependent binding to heterochromatin by endogenous Pc and Psc proteins at the restrictive temperature, suggesting that interaction of these proteins is mediated by E(z) protein. A myc-tagged full-length Suppressor 2 of zeste [Su(z)2] protein interacts poorly or not at all with ectopic Pc-G complexes, but a truncated Su(z)2 protein is strongly recruited to all sites of chimeric protein binding. Trithorax protein is not recruited to the heterochromatin by the chimeric HP1-Polycomb protein, suggesting either that this protein does not interact directly with Pc-G complexes or that such interactions are regulated. Ectopic binding of chimeric chromosomal proteins provides a useful tool for distinguishing specific protein-protein interactions from specific protein-DNA interactions important for complex assembly in vivo.

Cell proliferation and DNA replication defects in a Drosophila MCM2 mutant

The yeast MCM2, MCM3, and MCM5/CDC46 genes are required for DNA replication and have been proposed to act as factors that license the DNA for one and only one round of replication per cell cycle. We have identified a Drosophila gene, DmMCM2, that is highly homologous to MCM2. A P-element insertion into this gene, which prevents its transcription, inhibits proliferation of cells in the imaginal discs and central nervous system (CNS) and causes an apparent prolongation of S phase in the embryonic and larval CNS. DmMCM2 is expressed in the embryo in a pattern corresponding to that of S-phase cells. These results suggest that DmMCM2 plays a role in the regulation of DNA replication analogous to that of its yeast counterpart.

Polytene chromosomes from ovarian pseudonurse cells of the Drosophila melanogaster otu mutant. II. Photographic map of the X chromosome

The banding pattern of the polytene chromosomes of the ovarian pseudonurse cells (PNC) of the Drosophila melanogaster otu mutant were compared with larval salivary gland (SG) polytene chromosomes. The X chromosome was studied and no significant differences were found in the banding pattern between these functionally very different tissues. Most of the differences result from differential puffing activity. In situ hybridisation with five different DNA probes located along the X chromosome was used to cross-check the results obtained by morphological mapping. The constrictions present in the SG chromosomes were found to be absent in the germ line derived PNC chromosomes. There are prominent puffs in the PNC chromosomes at certain locations where genes known to be transcriptionally active in the germ line reside. This suggests that at least some of the genes active in the wild-type nurse cells may also be active in the PNC cells.

Comparison of the polytene chromosomes of the salivary gland, the fat body and the midgut nuclei of Drosophila auraria

Photo-maps of the fat body and midgut polytene chromosomes of Drosophila auraria were constructed. These photo-maps are compared with a new, more detailed photo-map of the salivary gland chromosomes of the same species. Seven, not previously described inverted tandem-duplications were detected, raising the number of such structures found in this species to 31. The constancy of the banding pattern based on the analysis of the above chromosomes is discussed.

Heteropycnosis of an underreplicating chromosome

Drosophila nasutoides has an extraordinary genome since 62% of its DNA resides in chromosome 4. This element mainly consists of constitutive heterochromatin which does not polytenize. Earlier studies of heterochromatin attributed little attention to the fact that "condensed" chromosomes often vary in condensation. This paper reports that chromosomes of the same complement display different degrees and kinetics of condensation. In D. nasutoides, even sex specific differences can be observed. The results of a comparative microphotometric study on neuroblast metaphases in both sexes revealed the following picture. The process of chromosome condensation is not restricted to mitotic prophase but continues into the metaphase. The mean condensation is not equal for all chromosomes. In the metaphase of the female, Feulgen density increases from the X chromosome, via 3 and 2, to chromosome 4. In the male, the order is X, 2, 3, Y, and 4. During the metaphase of the male, chromosomes condense with similar kinetics. In contrast, chromosomes of the female display asynchrony as monitored by area and length determinations. The X chromosomes of the female probably have enhanced shortening during prophase. This would explain the metaphase of the female where the X chromosomes shorten less than the autosomes, and why each of the X chromosomes is 15% shorter than the X chromosome in the metaphase of the male. Further differences were observed in the longitudinal and lateral compaction of the chromosomes in males and females. The sex chromosomes and chromosome 3 condense by shortening, while chromosome 2 and 4 preferentially reduce their diameter. The large amount of DNA engaged in heteropycnosis and the isochromosome nature allow the identification of chromosome 4 during interphase. At this stage, a new category of extreme DNA packaging was detected. The interphase density of chromosome 4 can exceed that of metaphase by a factor of up to 8. Two events account for this high degree of condensation: (1) the homologues are particularly associated due to somatic pairing and (2) the arms are further tightened as a result of pericentric folding. The features of the isochromosome suggest that the interaction of chromatids during interphase is essentially caused by specific DNA sequences. The data confirm that heteropycnosis not only interferes with gene expression but also strongly inhibits DNA synthesis in endocycles.

Chromosome structure at interfaces between major chromatin types: alpha- and beta-heterochromatin

The chromocenter of Drosophila polytene chromosomes, which consists of two major chromatin types, has long been a troublesome region in molecular terms. The recent microcloning of part of this region, the isolation of a monoclonal antibody to a beta-heterochromatin binding protein, and new in situ studies now shed a little more light on this chromosomal region.

Position-effect variegation and intercalary heterochromatin: a comparative study

The behaviour of IH (intercalary heterochromatin) regions of Drosophila melanogaster polytene chromosomes was compared with that of euchromatin condensed as a result of position-effect variegation. Normally replicating regions, when subject to such an effect, were found to become among the last regions in the genome to replicate. It is shown that the factors which enhance position effect (low temperature, the removal of the Y chromosome, genetic enhancers of position effect) increase the weak point frequency in the IH, i.e. enhance DNA underreplication in these regions. We suggest that the similarity in the properties of IH, CH (centromeric heterochromatin) and the dense blocks induced by position effect is due to strong genetic inactivation and supercondensation caused by specific proteins in early development. The primary DNA structure is not likely to play a key role in this process.

Towards an understanding of position effect variegation

Most variegating position effects are a consequence of placing a euchromatic gene adjacent to alpha-heterochromatin. In such rearrangements, the affected locus is inactivated in some cells, but not others, thereby giving rise to a mosaic tissue of mutant and wild-type cells. A detailed examination of the molecular structure of three variegating white mottled mutations of Drosophila melanogaster, all of which are inversions of the X chromosome, reveals that their euchromatic breakpoints are clustered and located approximately 25 kb downstream of the white promoter and that the heterochromatic sequences to which the white locus is adjoined are transposons. An analysis of three revertants of the wm4 mutation, created by relocating white to another euchromatic site, demonstrates that they also carry some heterochromatically derived sequences with them upon restoration of the wild-type phenotype. This suggests that variegation is not controlled from a heterochromatic sequence immediately adjacent to the variegating gene but rather from some site more internal to the heterochromatic domain itself. As a consequence of this observation we have proposed a boundary model for understanding how heterochromatic domains may be formed. It has been recognized for many years that the phenotype of variegating position effects may be altered by the presence of trans-acting dominant mutations that act to either enhance or suppress variegation. Using P-element mutagenesis, we have induced and examined 12 dominant enhancers of variegation that represent four loci on the second and third chromosomes. Most of these mutations are cytologically visible duplications or deficiencies. They exert their dominant effects through changes in the copy number of wild-type genes and can be divided into two reciprocally acting classes. Class I modifiers are genes that act as enhancers of variegation when duplicated and as suppressors when mutated or deficient. Conversely, class II modifiers are genes that enhance when mutated or deleted and suppress when duplicated. The available data indicate that, in Drosophila, there are 20-30 loci capable of dominantly modifying variegation. Of these, most appear to be of the class I type whereas only two class II modifiers have been identified so far.(ABSTRACT TRUNCATED AT 400 WORDS)

Intercalary heterochromatin in Drosophila. III. Homology between DNA sequences from the Y chromosome, bases of polytene chromosome limbs, and chromosome 4 of D. melanogaster

Molecular and cytogenetic characteristics are given of a 2846 bp DNA sequence from the YDm12 clone, previously derived from the long arm of the Drosophila melanogaster Y chromosome. Sequence analysis revealed within it a 1176 bp fragment with 37 bp terminal inverted repeats, flanked by 6 bp direct repeats. This fragment (called "element 1360") appeared to be A-T rich, and was saturated with short direct and inverted repeats of different degrees of homology and consensus sequences for transcription, potential Z-DNA transition and autonomous replication. After in situ hybridization to polytene chromosomes, the element 1360 exhibited variable, strain-specifics location in the euchromatic parts of the chromosome arms, but constant heavy labelling of the X chromosome region 12E1-2, autosomal regions 42B1-3, 52A1-2, 62A1-2, 75B, 82C1-3, chromosome bases, the chromocentre and numerous sites of chromosome 4. The possible role of element 1360 in heterochromatin organization is discussed.

Molecular studies on interspersed repetitive and unique sequences in the region of the complementation group uncoordinated on the X chromosome of Drosophila melanogaster

The technique of chromosome walking was used to isolate approximately 60 kb of DNA from the region containing the complementation group uncoordinated of Drosophila melanogaster, located in that part of the X chromosome which spans the euchromatin-heterochromatin junction. The cloned DNA can be divided into two distinct regions. The first contains sequences that are low copy number or unique and are largely conserved between strains. The second region is characterized by units repeated in tandem arrays and is polymorphic within, and between, strains. Each repetitive unit is separated by a member of an abundant sequence family, part of which is homologous to the ribosomal type 1 insertion sequence of D. melanogaster. The molecular organization of the cloned DNA was compared with that of sequences isolated from regions of intercalary heterochromatin and also with genes which have been characterized from more conventional euchromatic regions.

Intercalary heterochromatin of Drosophila as a potential model for human fragile sites

We summarize our proposal that "intercalary heterochromatin" of Drosophila is a useful model for human fragile sites. Comparison with Drosophila site 11A suggests that the normal allele of fragile site Xq27 is a meiotic pairing site.

Aneuploidy and the fragile X syndrome

The possibility that female carriers of the fragile X gene(s) are at increased risk for nondisjunctional events leading to aneuploid offspring has been suggested by several investigators. To better address this question we analyzed pedigrees of 117 families in which the fragile X syndrome is segregating. The 117 pedigrees, originally collected for segregation analyses, included 236 females with offspring whose carrier status was determined by cytogenetic or pedigree analysis or by analyses using flanking DNA markers. These 236 females have had 931 offspring including one 47,XXY and 6 trisomy 21 individuals (1/155). Statistical analysis suggested that the observed rate of trisomy 21 was significantly higher than expected (Fisher's exact test, p less than or equal to 0.05). Assuming a Poisson distribution to calculate the confidence interval for the observed rate of trisomy 21 individuals, we found that the expected rate of 1.6/1000 in this sample fell outside the 99% confidence limits of our observed rate of 1/155. Additional data from a larger sample are needed to replicate these findings.

Developmental genetics of the gastrulation defective locus in Drosophila melanogaster

The fs(1)gastrulation defective (dg) locus is one of the dorsal-group genes of Drosophila. Maternal expression of this gene is required for gastrulation movements and the differentiation of structures along the embryonic dorso-ventral axis. Twelve alleles of gd displayed a complex pattern of complementation, suggesting a direct interaction between subunits of a multimeric protein. Essential expression of the gd locus was strictly maternal with no zygotic contribution by the paternally derived allele. Clonal analysis revealed that expression of the gd locus was required in the germ line and that extreme dorsalization represented the null gd phenotype. Temperature-sensitive (ts) alleles displayed a ts period that included the last 4-5 hr of oogenesis and the first 1.5-2 hr of embryogenesis. Eggs from one ts allelic combination displayed reduced hatching when retained in the ovary at permissive temperatures, suggesting the loss of a labile egg component. This lability may also be responsible for the variable phenotypes displayed by offspring from individual females.