Germline stem cell division and egg chamber development in transplanted Drosophila germaria

Germline and somatic stem cells reside within the anterior region (or "germarium") of each ovariole in the Drosophila ovary. When individual germaria were dissected free of developing eggs and sheath tissue and transplanted into the abdominal cavity of a host fly, they regenerated ovariole-like structures and continuously supported the entire process of oogenesis, indicating that the stem cells remained functional. This system allowed us to measure the duration of several stages in oogenesis and to analyze the role of specific germarial cells in providing stem cell function. Laser ablation of presumptive germline stem cells near the apical tip prior to transplantation blocked the production of new germline cysts, but allowed previously initiated cysts to complete development. This confirmed the location of germline stem cells and showed that subsequent development of preexisting cysts did not require continued cyst production. Ablation of a distinct group of somatic cells lying close to the germline stem cells ("the terminal filament") increased the rate of oogenesis by approximately 40%, suggesting that the terminal filament may negatively regulate stem cell division.

Preferential transposition of Drosophila P elements to nearby chromosomal sites

Two different schemes were used to demonstrate that Drosophila P elements preferentially transpose into genomic regions close to their starting sites. A starting element with weak rosy+ marker gene expression was mobilized from its location in the subtelomeric region of the 1,300-kb Dp1187 minichromosome. Among progeny lines with altered rosy+ expression, a much higher than expected frequency contained new insertions on Dp1187. Terminal deficiencies were also recovered frequently. In a second screen, a rosy(+)-marked element causing a lethal mutation of the cactus gene was mobilized in male and female germlines, and viable revertant chromosomes were recovered that still contained a rosy+ gene due to an intrachromosomal transposition. New transpositions recovered using both methods were mapped between 0 and 128 kb from the starting site. Our results suggested that some mechanism elevates the frequency 43-67-fold with which a P element inserts near its starting site. Local transposition is likely to be useful for enhancing the rate of insertional mutation within predetermined regions of the genome.

Efficient and dispersed local P element transposition from Drosophila females

We have investigated how Drosophila P element insertions are distributed in the chromosomal region near their starting site. A single P element residing in the euchromatin of minichromosome Dp1187 was mobilized following a cross to the delta 2-3 (99B) strain, and progeny bearing transpositions were identified with a minimum of bias by performing Southern blots on progeny. Approximately 1-2% of all progeny minichromosomes contained new insertions. Many of these "local transpositions" landed very close to or within the starting P element; however, nearly 1% of all progeny chromosomes contained new insertions 1-180 kb from the donor element. More local insertions were observed in the progeny of females than from male parents, and most occurred in a preferred orientation relative to the starting element. These observations suggested that donor elements are frequently excised and reinserted locally without ever dissociating from a transposition complex. The high frequency and diverse distribution of local transpositions recovered from females suggested that the efficiency of insertional mutagenesis can be significantly enhanced by using a starting P element(s) located near the target of interest.

Replication forks are not found in a Drosophila minichromosome demonstrating a gradient of polytenization

Differential DNA replication is widely held to influence polytene chromosome structure by causing the dramatic reductions in heterochromatic DNA content that are characteristic of most endopolyploid cells. The "underreplication model" of heterochromatic sequence underrepresentation predicts that replication intermediates should populate regions of DNA between fully polytenized euchromatic sequences and underpolytenized heterochromatic sequences. We directly tested this prediction using Dp1187, a 1300 kb Drosophila minichromosome containing well-defined heterochromatic regions. DNA from a euchromatic/heterochromatic junction region of Dp1187, demonstrating a significant gradient of underrepresentation in larval salivary glands, lacked the stalled replication forks predicted by the underreplication model. We consider an alternative mechanism leading to heterochromatic sequence underrepresentation involving a process of DNA elimination.

hu-li tai shao, a gene required for ring canal formation during Drosophila oogenesis, encodes a homolog of adducin

Drosophila females bearing mutations in a previously undescribed gene, hu-li tai shao [(hts) too little nursing], produced egg chambers that contained fewer than the normal 15 nurse cells and that usually lacked an oocyte. The cytoplasmic bridges (ring canals) interconnecting nurse cells and the oocyte appeared abnormal, and lacked associated actin rings. The hts locus was found to encode a homolog of the mammalian membrane skeletal protein adducin. During oogenesis, hts mRNA became localized at the anterior of the oocyte and was subsequently expressed in a variety of embryonic tissues. These studies suggested that Drosophila adducin is needed to assemble actin at specialized regions of cell-cell contact in developing egg chambers and may also function at other times during the Drosophila life cycle.

Analysis of subtelomeric heterochromatin in the Drosophila minichromosome Dp1187 by single P element insertional mutagenesis

We investigated whether single P element insertional mutagenesis could be used to analyze heterochromatin within the Drosophila minichromosome Dp1187. Forty-five insertions of the P[lacZ,rosy+] element onto Dp1187 (recovered among 7,825 transpositions) were highly clustered. None was recovered in centromeric heterochromatin, but 39 occurred about 40 kb from the distal telomere within a 4.7-kb hotspot containing tandem copies of a novel 1.8-kb repetitive DNA sequence. The DNA within and distal to this region lacked essential genes and displayed several other properties characteristic of heterochromatin. The rosy+ genes within the inserted transposons were inhibited by position-effect variegation, and the subtelomeric region was underrepresented in polytene salivary gland cells. These experiments demonstrated that P elements preferentially transpose into a small subset of heterochromatic sites, providing a versatile method for studying the structure and function of these chromosome regions. This approach revealed that a Drosophila chromosome contains a large region of subtelomeric heterochromatin with specific structural and genetic properties.

slow border cells, a locus required for a developmentally regulated cell migration during oogenesis, encodes Drosophila C/EBP

During Drosophila oogenesis six to ten follicle cells, the border cells, undergo a dramatic and stereotypic migration through the developing egg chamber. We identified four independent P element insertion mutations that specifically blocked border cell migration. They defined a single, novel locus that was named slow border cells (slbo), because hypomorphic alleles caused delayed onset of the migration. Laser ablation of the border cells, or failure of their migration, caused improper morphogenesis of the micropyle, the egg-shell structure through which the sperm enters at fertilization. The slbo locus was found to encode a product homologous to the CCAAT/enhancer-binding protein (C/EBP), a basic region-leucine zipper transcription factor. Drosophila C/EBP may be required for the expression of gene products mediating border cell migration.

Laser ablation studies of the role of the Drosophila oocyte nucleus in pattern formation

Somatic and germline cells interact during oogenesis to establish the pattern axes of the Drosophila eggshell and embryo. The role of the oocyte nucleus in pattern formation was tested with the use of laser ablation. Ablation in stage 6 to 9 egg chambers caused partial or complete ventralization of the eggshell, phenotypes similar to those of eggs produced by gurken or torpedo females. Accumulation of vasa protein at the posterior pole of treated oocytes was also disrupted. Thus the oocyte nucleus is required as late as stage 9 for dorsoventral patterning within the follicle cells and for polar plasm assembly in the oocyte.

Studies on the rate and site-specificity of P element transposition

A single genetically marked P element can be efficiently mobilized to insertionally mutagenize the Drosophila genome. We have investigated how the structure of the starting element and its location along the X chromosome influenced the rate and location of mutations recovered. The structure of two P[rosy+] elements strongly affected mobilization by the autonomous "Jumpstarter-1" element. Their average transposition rates differed more than 12-fold, while their initial chromosomal location had a smaller effect. The lethal and sterile mutations induced by mobilizing a P[rosy+] element from position 1F were compared with those identified previously using a P[neoR] element at position 9C. With one possible exception, insertion hotspots for one element were frequently also targets of the other transposon. These experiments suggested that the genomic location of a P element does not usually influence its target sites on nonhomologous chromosomes. During the course of these experiments, Y-linked insertions expressing rosy+ were recovered, suggesting that marked P elements can sometimes insert and function at heterochromatic sites.

bag-of-marbles: a Drosophila gene required to initiate both male and female gametogenesis

In Drosophila, male and female gametes begin development when a stem cell divides to produce a cyst precursor. Subsequently, four special divisions give rise to a cluster of 16 interconnected cystocytes that develop into a single egg or 64 sperm. We identified and characterized a gene, bag-of-marbles (bam), that disrupts cyst formation in both sexes. An apparent null mutation causes abnormal cysts to form containing an excess number of cells that cannot differentiate into gametes. bam function resides within a simple 2.2-kb transcription unit encoding a single 442-amino-acid protein that shows similarity to the product of the ovarian tumor gene. The specific expression of bam RNA within female cystoblasts suggested that it might be involved in the specific cell-cycle alterations that occur during cystocyte divisions.

Reduced DNA polytenization of a minichromosome region undergoing position-effect variegation in Drosophila

Molecular analysis of a Drosophila minichromosome, Dp(1;f)1187, revealed a relationship between position-effect variegation and the copy number reductions of heterochromatic sequences that occur in polytene cells. Heterochromatin adjacent to a defined junction with euchromatin underpolytenized at least 60-fold. Lesser reductions were observed in euchromatic sequences up to 103 kb from the breakpoint. The copy number changes behaved in all respects like the expression of yellow, a gene located within the affected region. Both copy number and yellow expression displayed a cell-by-cell mosaic pattern of reduction, and adding a Y chromosome, a known suppressor of variegation, increased both substantially. We discuss the possibility that changes in replication alter copy number locally and also propose an alternative model of position-effect variegation based on the somatic elimination of heterochromatic sequences.

The orthodenticle gene encodes a novel homeo domain protein involved in the development of the Drosophila nervous system and ocellar visual structures

The orthodenticle (otd) locus of Drosophila is required for embryonic development, and null mutations of otd cause defects in head development and segmental patterning. We show here that otd is necessary for the formation of the embryonic central nervous system (CNS). otd mutations result in the formation of an abnormal neuropil and in the disappearance of identified neurons associated with the midline of the CNS. In addition, otd is allelic to ocelliless (oc), a mutation that causes the deletion of the ocelli of the adult fly. We have identified a transcription unit corresponding to the otd locus and find that it is expressed early in a stripe near the anterior pole of the cellular blastoderm and later in the region of the CNS from which these neurons normally arise. The predicted otd protein contains a well-conserved homeo domain and is therefore likely to be a transcriptional regulator involved in specifying cell fate both in the embryonic CNS and in the ocelli.

Constructing deletions with defined endpoints in Drosophila

Chromosomes bearing small deletions are valuable tools in Drosophila genetics. We have investigated a method for efficiently constructing precise chromosomal deficiencies. Two P transposable elements were positioned within a progenitor strain at the sites of the desired deletion endpoints. Deletions spanning the two transposons were recovered at high frequency when P element transposase was expressed in these flies, but only if the flanking P elements were in a cis rather than a trans configuration. Appropriate progenitor strains can now be constructed to delete virtually any chromosomal region by utilizing an extensive collection of lines containing single P element insertions throughout the Drosophila genome.

Multiple replication origins are used during Drosophila chorion gene amplification

DNA from Drosophila egg chambers undergoing chorion gene amplification was analyzed using the two-dimensional gel technique of Brewer and Fangman. At stage 10, 34% of DNA molecules from the maximally amplified region of the third chromosome chorion gene cluster contained replication forks or bubbles. These nonlinear forms were intermediates in the process of amplification; they were confined to follicle cells, and were found only within the replicating region during the time of amplification. Multiple origins gave rise to these intermediates, since three separate regions of the third chromosome chorion locus contained replication bubbles. However, initiation was nonrandom; the majority of initiations appeared to occur near the Bgl II site located between the s18 and s15 chorion genes. The P[S6.9] chorion transposon also contained abundant replication intermediates in follicle cells from a transformed line. Initiation within P[S6.9] occurred near two previously defined cis-regulatory elements, one near the same Bgl II site (in the AER-d region) and one near the ACE3 element.

The role of ACE3 in Drosophila chorion gene amplification

ACE3, an amplification control element for the third chromosome chorion cluster of Drosophila melanogaster, was identified previously as a cis-regulatory element for amplification of transposons containing the three chorion genes s18, s15 and s19. The deletion defining ACE3, located from -620 to -190 bp upstream of s18, disrupted both amplification and s18 transcription, suggesting that ACE3 might contain a transcription enhancer that regulated replication, as had been observed in a number of eukaryotic viruses. We show here that transcription control can be separated from replication control in delineating ACE3 to a 320 bp region. Addition of heterologous enhancers fails to activate amplification in tissues other than the follicle cells. Therefore ACE3 does not appear to be analogous to a transcription enhancer. However, further deletions within the ACE3 region revealed that it contains multiple functional domains. In addition, ACE3 functions independently of orientation with respect to other chorion sequences, and can be moved 1.5 kb away from other chorion sequences without eliminating amplification.

Amplification of the X-linked Drosophila chorion gene cluster requires a region upstream from the s38 chorion gene

Genomic sequences controlling follicle cell-specific amplification of the X-linked Drosophila chorion gene cluster were mapped by P element-mediated transformation. Several DNA fragments containing the s38 gene and flanking sequences induced tissue-specific amplification, although replication levels were subject to position effects. Deletion analysis identified a 467-bp region upstream from the s38 transcription start site that contained sequences essential in cis for amplification. The essential region shared 32 bp of imperfect sequence homology with a previously identified region necessary for third chromosome chorion gene cluster amplification. This homologous segment contained a repetitive motif consisting of perfect and imperfect AATAC repeats; it was localized near the boundary of the essential domain since most, but not all, the repeats could be deleted without eliminating transposon-induced amplification. The repetitive region was not required for developmentally regulated s38 transcription, therefore our results identified at least one element required for amplification but not for chorion gene transcription. The homologous repetitive sequences within the amplification-essential regions may constitute part of the replication origins used to differentially replicate the two chorion domains during oogenesis.

Drosophila chorion gene amplification requires an upstream region regulating s18 transcription

A cluster of Drosophila melanogaster chorion genes at locus 66D on the third chromosome amplifies 60-fold in the ovarian follicle cells prior to the onset of gene expression. A 3.8-kilobase (kb) region of the gene cluster can induce tissue-specific amplification in transformants. Previous models postulated that amplification is activated in follicle cells by transcription of one of the two chorion genes (s15 and s18) located within the 3.8-kb essential region. In this study, we showed that neither s15 nor s18 chorion gene transcription was required for amplification. However, a 510-bp region upstream from s18 contained sequences essential for both amplification and s18 transcription. No other region within the 3.8-kb fragment was required for amplification. We propose that upstream transcription control elements rather than transcription per se are involved in controlling amplification during development.

Developmentally regulated expression of Drosophila chorion genes introduced at diverse chromosomal positions

Drosophila chorion genes are organized into two clusters that are selectively amplified in the ovarian follicle cells. During oogenesis the transcription of individual genes is temporally regulated, resulting in distinct, stage-specific profiles of chorion mRNA accumulation. P element-mediated gene transfer was used to study the regulation of genes encoding the major chorion proteins s15-1 and s38-1. Transformed chorion genes integrated at diverse chromosomal locations exhibited proper tissue-specific and stage-specific expression, despite separation from the gene clusters. Qualitatively normal expression was not dependent on the ability of the inserted DNA to undergo amplification. However, chromosome position quantitatively influenced the RNA produced by the transformed genes. The level of RNA per gene copy produced by individual transformed genes varied approximately tenfold, after correction for differences in gene dosage due to the amplification of some inserted sequences. Transformation experiments with an s38-1-lacZ fusion gene demonstrated that cis-regulatory sequences sufficient for the stage-specific program of s38-1 expression were confined to a 1.3 X 10(3) base-pair segment between -748 and +573 relative to the s38-1 initiation site. Finally, egg chamber-specific amplification was induced at the site of two s38-1 insertions, suggesting that an amplification control element resides near this gene.

Localization of a cis-acting element responsible for the developmentally regulated amplification of Drosophila chorion genes

Late in oogenesis two clusters of Drosophila chorion genes and flanking DNA sequences undergo specific amplification in ovarian follicle cells. Lines were constructed using P-element-mediated transformation in which DNA segments derived from the chorion gene cluster at 66D on chromosome III had been inserted at new chromosomal locations. Only transposons that contained a specific 3.8 kb genomic segment derived from the cluster underwent amplification during oogenesis, which occurred with apparently normal tissue and temporal specificity. Adjacent nonchorion sequences also underwent amplification. However, the ability of a transposon to replicate differentially was subject to position effect. These studies provide evidence for the existence of a specific, cis-acting element controlling chorion gene amplification, which includes an origin for disproportionate DNA replication. Attempts to induce amplification with subfragments of the 3.8 kb segment were unsuccessful, suggesting that much of this fragment may be required for amplification.

Vectors for P element-mediated gene transfer in Drosophila

We have constructed and tested several new vectors for P element-mediated gene transfer. These vectors contain restriction sites for cloning a wide variety of DNA fragments within a small, non-autonomous P element and can be used to efficiently transduce microinjected DNA sequences into the germ line chromosomes of D. melanogaster. The P element in one vector also carries the rosy gene which serves as an easily scored marker to facilitate the transfer of DNA fragments that do not themselves confer a recognizable phenotype. The failure of certain P element constructs to function as vectors suggests that P element sequences, in addition to the 31 bp inverse terminal repeats, are required in cis for transposition. Moreover, removal of the first 38 bp of the autonomous 2.9 kb P element appears to destroy its ability to provide a trans-acting factor (s) required for the transposition of non-autonomous P elements. Finally, we describe a genomic sequence arrangement that apparently arose by the transposition of a 54 kb composite P element from a tetramer plasmid.

The effect of chromosomal position on the expression of the Drosophila xanthine dehydrogenase gene

Thirty-six isogenic D. melanogaster strains that differed only in the chromosomal location of a 7.2 or an 8.1 kb DNA segment containing the (autosomal) rosy gene were constructed by P-element-mediated gene transfer. Since the flies were homozygous for a rosy- allele, rosy gene function in these indicated the influence of flanking sequences on gene expression. The tissue distribution of XDH activity in all the strains was normal. Each line exhibited a characteristic level of adult XDH-specific activity. The majority of these values were close to wild-type levels; however, the total variation in specific activity among the lines was nearly fivefold. Thus position effects influence expression of the rosy gene quantitatively but do not detectably alter tissue specificity. X-linked rosy insertions were expressed on average 1.6 times more activity in males than in females. Hence the gene acquires at least partial dosage compensation upon insertion into the X chromosome.

Genetic transformation of Drosophila with transposable element vectors

Exogenous DNA sequences were introduced into the Drosophila germ line. A rosy transposon (ry1), constructed by inserting a chromosomal DNA fragment containing the wild-type rosy gene into a P transposable element, transformed germ line cells in 20 to 50 percent of the injected rosy mutant embryos. Transformants contained one or two copies of chromosomally integrated, intact ry1 that were stably inherited in subsequent generations. These transformed flies had wild-type eye color indicating that the visible genetic defect in the host strain could be fully and permanently corrected by the transferred gene. To demonstrate the generality of this approach, a DNA segment that does not confer a recognizable phenotype on recipients was also transferred into germ line chromosomes.

Transposition of cloned P elements into Drosophila germ line chromosomes

Recombinant DNA carrying the 3-kilobase transposable element was injected into Drosophila embryos of a strain that lacked such elements. Under optimum conditions, half of the surviving embryos showed evidence of P element-induced mutations in a fraction of their progeny. Direct analysis of the DNA of strains derived from such flies showed them to contain from one to five intact 3-kilobase P elements located at a wide variety of chromosomal sites. DNA sequences located outside the P element on the injected DNA were not transferred. Thus P elements can efficiently and selectively transpose from extrachromosomal DNA to the DNA of germ line chromosomes in Drosophila embryos. These observations provide the basis for efficient DNA-mediated gene transfer in Drosophila.

The organization and amplification of two chromosomal domains containing Drosophila chorion genes

Drosophila chorion genes are located in two clusters, one on the X chromosome at 7F1-2 of the polytene chromosome map and a second on the third chromosome at 66D11-15. Genes in both regions undergo amplification in ovarian follicle cells prior to their expression late in oogenesis. Analysis of cloned genomic segments derived from these chromosomal sites revealed that each cluster contains two tandemly transcribed chorion protein genes separated by only 1-2 kb. At least two other regions complementary to ovary RNA are located within 5 kb of these genes. During oogenesis, the transcribed sequences within each cluster, as well as the spacer sequences that separate them, are amplified equally. Sequences adjacent to the transcribed regions also replicate differentially but to a lesser extent, giving rise to gradients of decreasing amplification involving 40-50 kb of flanking chromosomal sequences. Differences between the restriction maps of unamplified and amplified DNA could not be detected in genomic DNA within either of the 90-100 kb domains of amplification. These observations suggest a model of amplification in which additional rounds of replication are specifically initiated within the central gene-containing regions, followed by bidirectional replication in the absence of discrete termination sites.

Amplification of genes for chorion proteins during oogenesis in Drosophila melanogaster

The endochorion and exochorion of Drosophila eggs are synthesized by the ovarian follicle cells during a brief period of about 5 hr. In this terminal phase of egg chamber development, the structural genes for several abundant chorion proteins are expressed at high levels according to a temporally regulated program. The female-sterile mutation ocelliless maps at the site of the genes for two of these proteins, the 36,000- and 38,000-dalton chorion proteins (c36 and c38), which are closely linked. The mutation results in a cis-acting reduction in the amounts of c36 and c38 that accumulate in late-stage egg chambers. We have investigated the mechanism that underlies this decreased production by using cDNA clones complementary to these gene sequences. Unexpectedly, it was found that, in normal females, the genes for c36, c38, and at least one other chorion protein are specifically amplified more than 10-fold in the DNA of late-stage egg chambers. The extra replication involves at least some adjacent chromosomal sequences and begins prior to the onset of mRNA and protein synthesis. The additional DNA remains stable after gene expression has ceased. The behavior of these genes is thus reminiscent of the properties of the DNA puffs that have been described in several groups of Diptera. The extent of amplification of c36 and c38, but not of the 18,000-dalton chorion protein c18 (which is unlinked), was decreased in the egg chambers of flies homozygous for ocelliless, suggesting that altered gene dosage may be responsible for the decreased synthesis of chorion proteins in the mutant.