Molecular organization of the vestigial region in Drosophila melanogaster

Polycomb safeguards imaginal disc specification through control of the Vestigial-Scalloped complex

A fundamental goal of developmental biology is to understand how cell and tissue fates are specified. The imaginal discs of Drosophila are excellent model systems for addressing this paradigm as their fate can be redirected when discs regenerate after injury or when key selector genes are misregulated. Here, we show that when Polycomb expression is reduced, the wing selector gene vestigial is ectopically activated. This leads to the inappropriate formation of the Vestigial-Scalloped complex, which forces the eye to transform into a wing. We further demonstrate that disrupting this complex does not simply block wing formation or restore eye development. Instead, immunohistochemistry and high-throughput genomic analysis show that the eye-antennal disc unexpectedly undergoes hyperplastic growth with multiple domains being organized into other imaginal discs and tissues. These findings provide insight into the complex developmental landscape that tissues must navigate before adopting their final fate.

Polycomb safeguards imaginal disc specification through control of the Vestigial-Scalloped complex

A fundamental goal of developmental biology is to understand how cell and tissue fates are specified. The imaginal discs of Drosophila are excellent model systems for addressing this paradigm as their fate can be redirected when discs regenerate after injury or when key selector genes are mis-regulated. Here, we show that when Polycomb expression is reduced, the wing selector gene vestigial is ectopically activated. This leads to the inappropriate formation of the Vestigial-Scalloped complex which forces the eye to transform into a wing. We further demonstrate that disrupting this complex does not simply block wing formation or restore eye development. Instead, immunohistochemistry and high throughput genomic analysis show that the eye-antennal disc unexpectedly undergoes hyperplastic growth with multiple domains being organized into other imaginal discs and tissues. These findings provide insight into the complex developmental landscape that tissues must navigate before adopting their final fate.

Summary Statement

Here we describe a novel mechanism by which Pc promotes an eye fate during normal development and how the eye is reprogrammed into a wing in its absence.

How well do you know your mutation? Complex effects of genetic background on expressivity, complementation, and ordering of allelic effects

For a given gene, different mutations influence organismal phenotypes to varying degrees. However, the expressivity of these variants not only depends on the DNA lesion associated with the mutation, but also on factors including the genetic background and rearing environment. The degree to which these factors influence related alleles, genes, or pathways similarly, and whether similar developmental mechanisms underlie variation in the expressivity of a single allele across conditions and among alleles is poorly understood. Besides their fundamental biological significance, these questions have important implications for the interpretation of functional genetic analyses, for example, if these factors alter the ordering of allelic series or patterns of complementation. We examined the impact of genetic background and rearing environment for a series of mutations spanning the range of phenotypic effects for both the scalloped and vestigial genes, which influence wing development in Drosophila melanogaster. Genetic background and rearing environment influenced the phenotypic outcome of mutations, including intra-genic interactions, particularly for mutations of moderate expressivity. We examined whether cellular correlates (such as cell proliferation during development) of these phenotypic effects matched the observed phenotypic outcome. While cell proliferation decreased with mutations of increasingly severe effects, surprisingly it did not co-vary strongly with the degree of background dependence. We discuss these findings and propose a phenomenological model to aid in understanding the biology of genes, and how this influences our interpretation of allelic effects in genetic analysis.

Different mutations in a gene, or in genes with related functions, can have effects of varying severity. Studying sets of mutations and analyzing how they interact are essential components of a geneticist's toolkit. However, the effects caused by a mutation depend not only on the mutation itself, but on additional genetic variation throughout an organism's genome and on the environment that organism has experienced. Therefore, identifying how the genomic and environmental context alter the expression of mutations is critical for making reliable inferences about how genes function. Yet studies on this context dependence have largely been limited to single mutations in single genes. We examined how the genomic and environmental context influence the expression of multiple mutations in two related genes affecting the fruit fly wing. Our results show that the genetic and environmental context generally affect the expression of related mutations in similar ways. However, the interactions between two different mutations in a single gene sometimes depended strongly on context. In addition, cell proliferation in the developing wing and adult wing size were not affected by the genetic and environmental context in similar ways in mutant flies, suggesting that variation in cell growth cannot fully explain how mutations affect wings. Overall, our findings show that context can have a big impact on the interpretation of genetic experiments, including how we draw conclusions about gene function and cause-and-effect relationships.

The activity of the Drosophila Vestigial protein is modified by Scalloped-dependent phosphorylation

The Drosophila vestigial gene is required for proliferation and differentiation of the adult wing and for differentiation of larval and adult muscle identity. Vestigial is part of a multi-protein transcription factor complex, which includes Scalloped, a TEAD-class DNA binding protein. Binding Scalloped is necessary for translocation of Vestigial into the nucleus. We show that Vestigial is extensively post-translationally modified and at least one of these modifications is required for proper function during development. We have shown that there is p38-dependent phosphorylation of Serine 215 in the carboxyl-terminal region of Vestigial. Phosphorylation of Serine 215 occurs in the nucleus and requires the presence of Scalloped. Comparison of a phosphomimetic and non-phosphorylatable mutant forms of Vestigial shows differences in the ability to rescue the wing and muscle phenotypes associated with a null vestigial allele.

Delivery of chitosan/dsRNA nanoparticles for silencing of wing development vestigial (vg) gene in Aedes aegypti mosquitoes

RNA interference (RNAi) has been used as a gene silencing strategy by the introduction of long double stranded RNA (dsRNA) for the control of pest insects. The aim of the present study was to examine whether the expression of vg gene which is responsible for wing development, can be repressed by chitosan/dsRNA based nanoparticles in Aedes aegypti. The vestigial gene (vg) was amplified from adult mosquito and cloned in pLitmus28i vector. Genetically engineered recombinant plasmid was transformed into RNase III deficient strain for synthesis of bacterially expressed dsRNA. Nanoparticles were prepared via electrostatic interaction between cationic polymer chitosan and anionic nucleic acids (dsRNA). The formation of chitosan/dsRNAnanoparticles and their size were confirmed by Atomic force microscopy (AFM). Chitosan/dsRNA mediated knockdown of Enhanced Green Fluorescence Protein (EGFP) was demonstrated in Sf21 cells. Further, we tested whether such an approach could be used to target vg gene in Ae. aegypti. The results showed that chitosan/dsRNA caused significant mortality, delayed growth development and caused adult wing-malformation. A qRT-PCR analysis confirmed that the chitosan/dsRNA mediated transcriptional level was downregulated. Our findings suggest that vg gene intervention strategies through RNAi can emerge as viable option for pest control.

The diaphanous gene of Drosophila interacts antagonistically with multiple wing hairs and plays a key role in wing hair morphogenesis

The Drosophila wing is covered by an array of distally pointing hairs that has served as a key model system for studying planar cell polarity (PCP). The adult cuticular hairs are formed in the pupae from cell extensions that contain extensive actin filaments and microtubules. The importance of the actin cytoskeleton for hair growth and morphogenesis is clear from the wide range of phenotypes seen in mutations in well-known actin regulators. Formin proteins promote the formation of long actin filaments of the sort thought to be important for hair growth. We report here that the formin encoding diaphanous (dia) gene plays a key role in hair morphogenesis. Both loss of function mutations and the expression of a constitutively active Dia led to cells forming both morphologically abnormal hairs and multiple hairs. The conserved frizzled (fz)/starry night (stan) PCP pathway functions to restrict hair initiation and activation of the cytoskeleton to the distal most part of wing cells. It also ensures the formation of a single hair per cell. Our data suggest that the localized inhibition of Dia activity may be part of this mechanism. We found the expression of constitutively active Dia greatly expands the region for activation of the cytoskeleton and that dia functions antagonistically with multiple wing hairs (mwh), the most downstream member of the fz/stan pathway. Further we established that purified fragments of Dia and Mwh could be co-immunoprecipitated suggesting the genetic interaction could reflect a direct physical interaction.

An intact RNA interference pathway is required for expression of the mutant wing phenotype of vg(21-3), a P-element-induced allele of the vestigial gene in Drosophila

We have determined that two P elements, P[21-3] and P[21r36], residing in the 5'-UTR of the vestigial wing gene, encode functional repressors in eye tissue. However, neither element fits a previous categorization of repressor-making elements as Type I or II. Both elements encode polypeptides that are shorter than the canonical elements they most closely resemble. DNA sequencing reveals that P[21r36] encodes an intact THAP domain that is missing in the P[21] element, which does not encode a functional repressor. Recovery of P[21-3] at sites other than vestigial (where it causes the wing mutant, vg(21-3)) reveals that the element can make repressor in wing tissue of sufficient activity to repress the mutant phenotype of vg(21-3). Why the P[21-3] element fails to produce repressor when located at vestigial may be explained by our observation that three different mutants in the RNA interference pathway cause a partial reversion of vg(21-3). We speculate that the vg and P-initiated transcripts that arise at the vg locus in the vg(21-3) mutant trigger an RNA interference response that results in the mutual degradation of both transcripts.

Novel events associated with phenotypic reversion of a P element mutant in Drosophila melanogaster

Transposable P elements have been used extensively for Drosophila mutagenesis. While their mutagenic activity has long been recognized, the mechanisms by which P elements cause mutations are varied and not completely understood. We describe here an experiment to replace a P element at vestigial (vg) that caused a strong mutant phenotype (P[21-3]) with a P element (P[21]) known to produce a very weak phenotype when inserted at vg. In addition to testing the feasibility of P element replacements at vg, our investigation led to the production of 7 new vg alleles and 1 apparent second site suppressor. All the vg21-3 revertants that we recovered had a P element inserted into the first exon of vg at the same location and in the same orientation as the original element in vg21-3, providing a unique opportunity to study the mechanism of transposon mutagenesis. A majority of the revertants arose from a previously described event: internal deletion of P sequences, including the P promoter. In addition, 3 novel reversions of the vg21-3 wing phenotype were recovered. The wings of homozygous vg21r36 flies were normal. However, vg21r36 in combination with a deletion of the vg locus exhibited a strong mutant wing phenotype. This was surprising, because the P element insertion in vg21r36 was very similar to that found in the vg21 allele, which showed only slight nicking of the wings in combination with a deletion. In vg21r4, reversion was caused by a tandem insertion of P[21] and the original P[21-3] element present in vg21-3. Finally, the vg21r7 revertant had a P[21-3] insert at vg and 3 additional P elements elsewhere in the genome. We hypothesize that reversion in the 3 novel cases might be caused by P repressor produced by an element at vg or, in the case of vg21r7, elsewhere in the genome. This raises an interesting aspect of P element evolution. While P transposons produce mutations that might prove deleterious to their host, their success in invading the genome of D. melanogaster may be explained by their ability to silence those same mutations by a range of repressor-producing elements.

Transvection at the vestigial locus of Drosophila melanogaster

Transvection is a phenomenon wherein gene expression is effected by the interaction of alleles in trans and often results in partial complementation between mutant alleles. Transvection is dependent upon somatic pairing between homologous chromosome regions and is a form of interallelic complementation that does not occur at the polypeptide level. In this study we demonstrated that transvection could occur at the vestigial (vg) locus by revealing that partial complementation between two vg mutant alleles could be disrupted by changing the genomic location of the alleles through chromosome rearrangement. If chromosome rearrangements affect transvection by disrupting somatic pairing, then combining chromosome rearrangements that restore somatic pairing should restore transvection. We were able to restore partial complementation in numerous rearrangement trans-heterozygotes, thus providing substantial evidence that the observed complementation at vg results from a transvection effect. Cytological analyses revealed this transvection effect to have a large proximal critical region, a feature common to other transvection effects. In the Drosophila interphase nucleus, paired chromosome arms are separated into distinct, nonoverlapping domains. We propose that if the relative position of each arm in the nucleus is determined by the centromere as a relic of chromosome positions after the last mitotic division, then a locus will be displaced to a different territory of the interphase nucleus relative to its nonrearranged homolog by any rearrangement that links that locus to a different centromere. This physical displacement in the nucleus hinders transvection by disrupting the somatic pairing of homologous chromosomes and gives rise to proximal critical regions.

An in vivo analysis of the vestigial gene in Drosophila melanogaster defines the domains required for Vg function

Considerable evidence indicates an obligate partnership of the Drosophila melanogaster Vestigial (VG) and Scalloped (SD) proteins within the context of wing development. These two proteins interact physically and a 56-amino-acid motif within VG is necessary and sufficient for this binding. While the importance of this SD-binding domain has been clearly demonstrated both in vitro and in vivo, the remaining portions of VG have not been examined for in vivo function. Herein, additional regions within VG were tested for possible in vivo functions. The results identify two additional domains that must be present for optimal VG function as measured by the loss of ability to rescue vg mutants, to induce ectopic sd expression, and to perform other normal VG functions when they are deleted. An in vivo study such as this one is fundamentally important because it identifies domains of VG that are necessary in the cellular context in which wing development actually occurs. The results also indicate that an additional large portion of VG, outside of these two domains and the SD-binding domain, is dispensable in the execution of these normal VG functions.

The mutant phenotype associated with P-element alleles of the vestigial locus in Drosophila melanogaster may be caused by a readthrough transcript initiated at the P-element promoter

We report here the isolation of a new P-element-induced allele of the vestigial locus vg(2a33), the molecular characterization of which allows us to propose a unifying explanation of the phenotypes of the large number of vestigial P-element alleles that now exists. The first P-element allele of vestigial to be isolated was vg(21), which results in a very weak mutant wing phenotype that is suppressed in the P cytotype. By destabilizing vg(2a33) in a dysgenic cross, we isolated the vg(2a33) allele, which exhibits a moderate mutant wing phenotype and is not suppressed by the P cytotype. The new allele is characterized by a 46-bp deletion that removes the 3'-proximal copy of the 11-bp internal repeat from the P element of vg(21). To understand how this subtle difference between the two alleles leads to a rather pronounced difference in their phenotypes, we mapped both the vg and P-element transcription units present in wild type and mutants. Using both 5'-RACE and S1 protection, we found that P-element transcription is initiated 19 bp farther upstream than previously thought. Using primer extension, the start of vg transcription was determined to lie 435 bp upstream of the longest cDNA recovered to date and upstream of the P-element insertion site. Our discovery that the P element is situated within the first vg exon has prompted a reassessment of the large body of genetic data on a series of alleles derived from vg(21). Our current hypothesis to explain the degree of variation in the mutant phenotypes and their response to the P repressor invokes a critical RNA secondary structure in the vg transcript, the formation of which is hindered by a readthrough transcript initiated at the P-element promoter.

bicaudal encodes the Drosophila beta NAC homolog, a component of the ribosomal translational machinery*

bicaudal was the first Drosophila mutation identified as producing mirror-image pattern duplications along the anteroposterior axis of the embryo. However the mutation has been little studied due to its low penetrance and suppressibility. We undertook cloning of the bicaudal locus together with studies of the mutation's effects on key elements of the posterior embryonic patterning pathway. Our mapping studies place the bicaudal mutation within a approximately 2 kb region, 3′ to the protein coding sequence of the Drosophila homolog of beta NAC, a subunit of Nascent polypeptide Associated Complex (NAC). Genomic DNA encoding beta NAC completely rescues the bicaudal phenotype. The lethal phenotype of Enhancer of Bicaudal, E(Bic), a mutation hypothesized to affect the bicaudal locus, is also completely rescued by the beta NAC locus. We further demonstrate that the E(Bic) mutation is caused by a P element insertion into the transcribed region of the beta NAC gene. NAC is among the first ribosome-associated entities to bind the nascent polypeptide after peptide bond formation. In contrast to other bicaudal-embryo-producing mutations, bicaudal leads to ectopic translation of mRNA for the posterior determinant nanos, without affecting the localization of mRNA for its upstream regulator, oskar, in the embryo. These findings suggest that repression of nanos mRNA translation occurs on the ribosome and involves a role for beta NAC.

Reciprocal transplantation of wing discs between a wing deficient mutant (fl) and wild type of the silkworm, Bombyx mori

The wingless mutant flügellos (fl) of the silkworm lacks all four wings. Although wing discs of the fl seem to develop normally until the fourth larval instar, wing morphogenesis stops after the fourth larval ecdysis, probably caused by aberrant expression of an unidentified factor, referred to as fl. To characterize factor fl, the wing discs dissected from the wild-type (WT) and fl larvae were transplanted into other larvae and developmental changes of the discs were examined. When the wing disc from a WT larva was transplanted into another WT larva and allowed to grow until emergence, a small wing appeared that was covered with scales. Thus, the transplanted wing discs can develop autonomously, form scales and evert from adult skin. The WT wing discs transplanted into the fl larvae also developed at a high rate. However, the fl wing discs transplanted into the WT larvae did not develop during the larval to pupal developmental stages. These data suggest that the fl gene product (factor fl) works in the wing disc cells during wing morphogenesis. Its function cannot be complemented by hemolymph in the WT larva. It is also implied that the level of humoral factors and hormones required for wing morphogenesis are normally maintained in the fl larva.

The effect of dominant vestigial alleles upon vestigial-mediated wing patterning during development of Drosophila melanogaster

The vestigial gene product is required for the completion of wing development in Drosophila melanogaster. In the absence of vestigial gene expression, cells within the larval wing and haltere imaginal discs fail to proliferate normally thus producing adults with severely reduced wings. Of a large number of vestigial mutations that have been characterized, only two are currently known to exist, vestigial(U) and vestigial(W), which manifest a significant dominant phenotype. Both are associated with chromosomal inversions that fuse the majority of the vestigial coding regions to other genes; mastermind in vestigial(U) and invected in vestigial(W) Examination of vestigial expression in the presence of these dominant alleles shows alterations in the disc-specific expression of vestigial during later stages of larval development. These patterning disruptions are specific to cells of the wing imaginal disc, as significant suppression of total levels of vestigial expression within entire larvae could not be detected. This dominant interference of vestigial patterning appears to be mediated in part by the vestigial coding sequences that are within the gene fusions. Further evidence that the dominant phenotype is the result of disrupted vestigial patterning comes from observations that the dominant alleles can be partially suppressed by mutations within the Drosophila-epidermal growth factor receptor gene. Mutagenesis of vestigial(U) and vestigial(W) produced a series of alleles with partially dominant phenotypes that restored various amounts of the adult wing. These phenotypes can be correlated with alterations in specific portions of the vestigial sequences associated with the dominant alleles. In the presence of these partially dominant alleles, wing imaginal discs have significantly more cells which express vestigial compared with the number associated with the original dominant phenotype. Additionally, eliminating some of the dominant effect causes alterations in the patterns of early stage apoptotic cell death associated with dominant vestigial alleles. Utilizing these new vestigial alleles, it is possible to correlate the consequence of altered vestigial expression to subsequent changes in patterning of the wing disc.

Mutations in Drosophila DP and E2F distinguish G1-S progression from an associated transcriptional program

The E2F transcription factor, a heterodimer of E2F and DP subunits, is capable of driving the G1-S transition of the cell cycle. However, mice in which the E2F-1 gene had been disrupted developed tumors, suggesting a negative role for E2F in controlling cell proliferation in some tissues. The consequences of disrupting the DP genes have not been reported. We screened for mutations that disrupt G1-S transcription late in Drosophila embryogenesis and identified five mutations in the dDP gene. Although mutations in dDP or dE2F nearly eliminate E2F-dependent G1-S transcription, S-phase still occurs. Cyclin E has been shown to be essential for S-phase in late embryogenesis, but in dDP and dE2F mutants the peaks of G1-S transcription of cyclin E are missing. Thus, greatly reduced levels of cyclin E transcript suffice for DNA replication until late in development. Both dDP and dE2F are necessary for viability, and mutations in the genes cause lethality at the late larval/pupal stage. The mutant phenotypes reveal that both genes promote progression of the cell cycle.

Vestigial gene expression in Drosophila melanogaster is modulated by the dTMP pool

The vestigial (vg) gene of Drosophila melanogaster encodes a nuclear protein which plays a key role in wing formation but is also involved in other developmental processes. We have previously shown that depletion of the dTMP pool by aminopterin, an inhibitor of the enzyme dihydrofolate reductase, or by fluorodeoxyuridine, an inhibitor of thymidylate synthetase, induces nicks in the wings of wild-type flies and a strong vg phenotype in vgBG/+ flies and also in individuals heterozygous for a deficiency of the vg locus (vgB/+). Furthermore, specific alterations of the vg locus, caused by intronic insertions, are associated with resistance to these drugs. In this paper, we show that: (1) depletion of the dTMP pool by aminopterin leads to a decrease in the amount of vg transcripts; (2) insertion of the retrotransposon 412 in the vgBG mutant, which is resistant to aminopterin, leads to the formation of a truncated transcript that is prematurely terminated in the long terminal repeat of this transposable element; and (3) aminopterin also affects the level of this truncated transcript. These results indicate that alterations of the wing by inhibitors of dTMP synthesis are caused by an effect of these drugs on levels of vg transcripts; the resistance to such agents observed for the vgBG strain is not due to a qualitatively different effect of this drug on the vg transcript but, rather, is related to the expression of a modified Vg protein encoded by a truncated transcript. These results are compatible with a role for vestigial in modulating cell proliferation.

Wingless induces transdetermination in developing Drosophila imaginal discs

Drosophila imaginal discs, the precursors of the adult fly appendages, have been the subject of intensive developmental studies, particularly on cell determination. Cultured disc fragments are recognized not only for the ability to maintain their determined state through extra cell divisions but also for the ability to transdetermine, or switch to the determined state of a different disc. An understanding of transdetermination at a molecular level will provide further insight into the requirements for maintaining cell determination. We find that ectopic expression of the Drosophila gene wingless induces transdetermination of foreleg imaginal disc cells to wing cells. This transdetermination occurs in foreleg discs of developing larvae without disc fragmentation. The in situ-transdetermining cells localize to the dorsal region of the foreleg disc. This wingless-induced transdetermination event is remarkably similar to the leg-to-wing switch that occurs after leg disc culture. Thus we have identified a new approach to a molecular dissection of transdetermination.

A genetic analysis of the Suppressor 2 of zeste complex of Drosophila melanogaster

The zeste1 (z1) mutation of Drosophila melanogaster produces a mutant yellow eye color instead of the wild-type red. Genetic and molecular data suggest that z1 achieves this change by altering expression of the wild-type white gene in a manner that exhibits transvection effects. There exist suppressor and enhancer mutations that modify the z1 eye color, and this paper summarizes our studies of those belonging to the Suppressor 2 of zeste complex [Su(z)2-C]. The Su(z)2-C consists of at least three subregions called Psc (Posterior sex combs), Su(z)2 and Su(z)2D (Distal). The products of these subregions are proposed to act at the level of chromatin. Complementation analyses predict that the products are functionally similar and interacting. The alleles of Psc define two overlapping phenotypic classes, the hopeful and hapless. The distinctions between these two classes and the intragenic complementation seen among some of the Psc alleles are consistent with a multidomain structure for the product of Psc. Psc is a member of the homeotic Polycomb group of genes. A general discussion of the Polycomb and trithorax group of genes, position-effect variegation, transvection, chromosome pairing and chromatin structure is presented.

The putative Drosophila NMDARA1 gene is located on the second chromosome and is ubiquitously expressed in embryogenesis

We describe the cDNA sequence of a Drosophila gene located at 49D on the second chromosome. The predicted protein sequence has extensive homology (46% identity, 76% similarity) along its entire length to the central portion of the mammalian brain-specific NMDARA1 glutamate binding protein. While RNA expression studies of the Drosophila protein shown that it is ubiquitously expressed throughout embryogenesis, there appears to be a selective accumulation of transcript in the precursor for the larval brain. The similarity in both expression and sequence suggests that the two proteins may share a conserved function.

Protected P-element termini suggest a role for inverted-repeat-binding protein in transposase-induced gap repair in Drosophila melanogaster

P-element transposition is thought to occur by a cut-and paste mechanism that generates a double-strand break at the donor site, the repair of which can lead to internally deleted elements. We have generated a series of both phenotypically stronger and weaker allelic derivatives of vg21, a vestigial mutant caused by a P-element insertion in the 5' region of the gene. Virtually all of the new alleles arose by internal deletion of the parental element in vg21, and we have characterized a number of these internally deleted P elements. Depending upon the selection scheme used, we see a very different spectrum of amount and source of P-element sequences in the resultant derivatives. Strikingly, most of the breakpoints occur within the inverted-repeats such that the last 15-17 bp of the termini are retained. This sequence is known to bind the inverted-repeat-binding protein (IRBP). We propose that the IRBP may act to preserve the P-element ends when transposition produces a double-strand gap. This allows the terminus to serve as a template upon which DNA synthesis can act to repair the gap. Filler sequences found at the breakpoints of the internally deleted P elements resemble short stretches, often in tandem arrays, of these terminal sequences. The structure of the filler sequences suggests replication slippage may occur during the process of gap repair.

Targeted transposition at the vestigial locus of Drosophila melanogaster

Targeted transposition is the replacement of one P element with another. We are exploiting this unique property of P elements to study the complex regulatory domain of the Dopa decarboxylase (Ddc) gene in Drosophila melanogaster. P element constructs targeted to the same site in the genome will be subjected to the same position effect. This allows the subtle effects typical of most mutations in the Ddc regulatory region to be measured in the absence of the variable influences of position effects which are associated with the current method of germline transformation. We have investigated some of the parameters affecting targeted transposition of a Ddc transposon, P[Ddc], into a P element allele at the vestigial locus. These events were detected by an increased mutant vg phenotype. The location of the donor transposon in cis or in trans to the target had little effect on the frequency of targeting. Likewise, the mobility of different donor elements, as measured by their rate of transposition to a different chromosome, varied nearly 20-fold, while the rate of targeted transposition was very similar between them. All targeted alleles were precise replacements of the target P element by P[Ddc], but in several cases the donor was inserted in the opposite orientation. The targeted alleles could be described as the result of a replicative, conversion-like event.

Pattern formation and eyespot determination in butterfly wings

Butterfly wings display pattern elements of many types and colors. To identify the molecular processes underlying the generation of these patterns, several butterfly cognates of Drosophila appendage patterning genes have been cloned and their expression patterns have been analyzed. Butterfly wing patterns are organized by two spatial coordinate systems. One system specifies positional information with respect to the entire wing field and is conserved between fruit flies and butterflies. A second system, superimposed on the general system and involving several of the same genes, operates within each wing subdivision to elaborate discrete pattern elements. Eyespots, which form from discrete developmental organizers, are marked by Distal-less gene expression. These circular pattern elements appear to be generated by a process similar to, and perhaps evolved from, proximodistal pattern formation in insect appendages.

Organization of wing formation and induction of a wing-patterning gene at the dorsal/ventral compartment boundary

The appendages of arthropods and vertebrates possess a third, proximodistal patterning axis that is established after the primary anteroposterior and dorsoventral body axes by mechanisms that are largely unknown. The vestigial gene is required for formation of the entire Drosophila wing, and the dorsal/ventral boundary is shown to organize wing formation and vestigial gene expression. Interactions between dorsal and ventral cells in the growing imaginal disc induce vestigial gene expression through a discrete, extraordinarily conserved imaginal disc-specific enhancer. The link between dorsal/ventral compartmentalization and wing formation distinguishes the development of this sheet-like appendage from that of legs and antennae.

The vestigial locus of Drosophila melanogaster is involved in resistance to inhibitors of dTMP synthesis

The vestigal (vg) gene encodes a nuclear protein which plays a major role in the formation of the wing of Drosophila. Resistance or sensitivity to aminopterin, an inhibitor of the dihydrofolate reductase enzyme in D. melanogaster, seems to be associated with a specific alteration in vg gene function. Wild-type and vg mutant strains selected for growth on increasing concentrations of aminopterin display changes in physiological and biochemical parameters such as viability on normal and aminopterin-containing media, duration of development, wing phenotype, dihydrofolate reductase activity, and cross-resistance to fluorodeoxyuridine (FUdR) and to methotrexate. Our results indicate that the mechanisms of resistance differ in the wild-type and mutant strains. The vg83b27 mutant, in which the major part of intron 2 of the vg gene is deleted, is associated with a high rate of resistance to FUdR, an inhibitor of thymidylate synthetase. Moreover, vg83b27/vgBG heterozygotes, which are wild type when grown on normal medium, display a strong vg phenotype when grown on aminopterin. Our results indicate a role for the vestigial locus in mediating resistance to inhibitors of dTMP synthesis.

A deleted hobo element is involved in the unstable thermosensitive vg(al) mutation at the vestigial locus in Drosophila melanogaster

We have described a new unstable mutant of the vestigial locus isolated from a natural population. From this mutant, vestigial(almost) (vg(al)), wild-type (vg(al+)), and extreme (vg(ext)), alleles arose spontaneously. The molecular analysis of vg(al) shows that the mutation is due to a 1874 bp hobo element inserted in a vestigial intron. Two distinct kinds of events lead a wild-type phenotype. Three independent vg(al+) alleles result from an excision of the hobo element and two other vg(al+) alleles have further deletions of hobo sequence. The sequence of one of them shows a 1516 bp hobo insertion at the same place and in the same orientation as the 1874 bp insertion. In the vg(ext) alleles, we found a 5' or 3' variably sized deletion of vg sequences. One of them, which has been cloned and sequenced, has a deletion finishing exactly at the left terminal repeat' hobo element. The genetic implications of these different genetic structures are discussed.

Related chromosome binding sites for zeste, suppressors of zeste and Polycomb group proteins in Drosophila and their dependence on Enhancer of zeste function

Polycomb group genes are necessary for maintaining homeotic genes repressed in appropriate parts of the body plan. Some of these genes, e.g. Psc, Su(z)2 and E(z), are also modifiers of the zeste-white interaction. The products of Psc and Su(z)2 were immunohistochemically detected at 80-90 sites on polytene chromosomes. The chromosomal binding sites of these two proteins were compared with those of zeste protein and two other Polycomb group proteins, Polycomb and polyhomeotic. The five proteins co-localize at a large number of sites, suggesting that they frequently act together on target genes. In larvae carrying a temperature sensitive mutation in another Polycomb group gene, E(z), the Su(z)2 and Psc products become dissociated from chromatin at non-permissive temperatures from most but not all sites, while the binding of the zeste protein is unaffected. The polytene chromosomes in these mutant larvae acquire a decondensed appearance, frequently losing characteristic constrictions. These results suggest that the binding of at least some Polycomb group proteins requires interactions with other members of the group and, although zeste can bind independently, its repressive effect on white involves the presence of at least some of the Polycomb group proteins.

Pattern formation in a secondary field: a hierarchy of regulatory genes subdivides the developing Drosophila wing disc into discrete subregions

The legs and wings of insects and vertebrates develop from secondary embryonic fields that arise after the primary body axes have been established. In order to understand how the insect imaginal wing field is patterned, we have examined in detail the temporal and spatial expression patterns of, and epistatic relationships between, four key regulatory genes that are specifically required for wing formation in Drosophila. The wingless protein, in a role surprisingly distinct from its embryonic segment polarity function, appears to be the earliest-acting member of the hierarchy and crucial for distinguishing the notum/wing subfields, and for the compartmentalization of the dorsal and ventral wing surfaces. The wingless product is required to restrict the expression of the apterous gene to dorsal cells and to promote the expression of the vestigial and scalloped genes that demarcate the wing primordia and act in concert to promote morphogenesis.

vestigial suppressor genes and resistance to aminopterin in Drosophila melanogaster

We have shown that the vestigial (vg) mutant of D. melanogaster has a perturbed nucleotide metabolism compared to various wild-type strains. The mutant is particularly spontaneously resistant to aminopterin. The resistance seems to correlate with an increase in dihydrofolate reductase (DHFR) activity and quantity. The DHRF is a target enzyme of aminopterin. Our results suggest that the vg+ gene could be a regulatory gene acting on the DHFR gene. The wing mutant phenotype being due to a decrease in the thymidylate pool (dTMP) (Silber et al., 1989). In order to understand better the action of the mutant gene on nucleotide metabolism, we have induced suppressor genes of the mutant phenotype by mutagenesis with ethyl methanesulfonate (EMS) and bromouridine (BUR). The suppressor strains obtained display a phenotype intermediate between wild-type and vg phenotype. The action of three independent suppressor genes on eight parameters of nucleotide metabolism is reported here [three enzyme activities, resistance to aminopterin and to fluorodeoxyuridine (FUdR), auxotrophy test and the ability to use exogenous thymidine and uridine]. In comparison to the original vg strain, major changes for the parameters tested are observed. The most striking effects are obtained with the vgBUR27 strain, which is highly sensitive to aminopterin and to fluorodeoxyuridine and didplas the highest thymidine kinase (TK) and DHFR activities within the strains tested. The potential actions of suppressor genes on the vg mutant are discussed.

Genetic and molecular analyses of vgal: a spontaneous and unstable mutation at the vestigial locus in Drosophila melanogaster

We describe herein, a new unstable mutant of the vestigial locus, isolated from a French natural population. From this mutant vestigial almost (vgal) wild-type flies (vgal+) and extreme vg phenotypes (vge) arose spontaneously without genomic shock. The occurrence of vgal+ or vge alleles depends mostly on the breeding temperature; vgal+ revertants arose principally at low temperature (21 degrees C) and vge at 28 degrees C. These events occur mainly in the male germ line and the phenomenon appears to be premeiotic. Our results with in situ hybridization experiments and Southern blots show that the vgal mutation is due to a 2 kb DNA insertion, which is a deleted hobo element. Genetic and molecular analyses show that two distinct events may underly the wild-type revertants. One is the excision of the resident hobo element, the other a further deletion (about 300 bp in the example characterized herein). The vge mutation is probably due to a deletion of vestigial sequences flanking the hobo insertion.

Genetic and molecular analysis of vgU and vgW: two dominant vg alleles associated with gene fusions in Drosophila

In the absence of a vg+ gene, extensive cell death occurs in third instar imaginal discs, which results in a complete loss of adult wing margin structures. Essentially all molecularly characterized vg alleles are associated with deletions or insertions of DNA into the vg locus. These alterations reduce or eliminate a 3.8-kb vg-specific transcript, resulting in recessive loss of function alleles. We report here the analysis of two dominant vg alleles which have been identified (vgU and vgW). The vgU allele is associated with a chromosomal inversion which splits the vg locus, resulting in a gene fusion between vg and the mastermind (mam) neurogenic locus. Reversion analysis of vgU indicates that sequences from the mam locus are required for vgU dominance. The vgW allele is also the result of a chromosomal inversion, in this case resulting in a gene fusion between vg and the homeobox-containing invected (inv) gene. It is also associated with novel dominant homeotic transformations. Revertant analysis indicates that sequences from inv are required for the dominant wing and dominant homeotic effects of vgW. The vg dominance does not appear to be mediated through a reduction of vg expression or a novel fusion transcript in either vgU or vgW. The results are consistent with a model in which inappropriate expression of inv causes the dominant homeotic effects seen in vgW.

Interactions among modifiers of retrotransposon-induced alleles of the white locus of Drosophila melanogaster

Mutations in five loci that modify the phenotype of whiteapricot (wa), caused by the retrotransposon, copia, were examined in two-way combinations to determine whether their effects were additive or epistatic. All two-way combinations of mutations in these five loci, mottler of white (mw), suppressor of forked (su(f], suppressor of white apricot (su(wa], Enhancer of whiteapricot, (E(wa] and Darkener of apricot (Doa), are additive in their effects on wa, implying that each second-site modifier locus affects a different process. Three other copia-induced mutations, HwUa, whd81b25 and ctns were also examined for responsiveness to mutations in these modifier loci. None clearly responded. Mutations associated with B104 insertions, including Gl, vgni, ctn and wric were also examined for responsiveness to mw mutations, which have specificity for this element as well. Both vgni and wric respond to mutations in mw. The former interaction demonstrates that mw is capable of interacting with B104 elements in loci other than white. The significance of the results with respect to the nature of second-site modifier loci is discussed.

The functional organization of the vestigial locus in Drosophila melanogaster

Vestigial mutants are associated with imaginal disc cell death which results in the deletion of adult wing and haltere structures. The vestigial locus has previously been cloned, and mutational lesions associated with a number of vg alleles were mapped within a 19 kb DNA region defined as essential for vg function. Herein we report the identification and characterization of a developmentally regulated 3.8 kb vg transcript which is spliced from exons distributed throughout the essential interval defined above. All the characterized classical alleles have predictable effects on this transcription unit, and the severity of this effect is directly proportional to the severity of the wing phenotype. A repetitive domain within this transcription unit was identified and may serve as a tag to isolate other genes with functions related to vg. We also report an exceptional vg allele (vg83b27) that produces an extreme wing and haltere phenotype, but which defines a second vg complementation unit. This allele is associated with a 4 kb deletion entirely within a 4.5 kb vg intron as defined by the 3.8 kb transcription unit. Molecular and genetic evidence indicates that the vg83b27 mutation has a functional 3.8 kb transcription unit, thus accounting for its ability to complement classical alleles. The results indicate that sequences within a vg intron are essential for normal wing and haltere development.

Vestigial mutants of Drosophila melanogaster live better in the presence of aminopterin: increased level of dihydrofolate reductase in a mutant

Vestigial (vg) mutants of Drosophila melanogaster are characterized by atrophied wings. In this paper we show that: (1) aminopterin an inhibitor of dihydrofolate reductase (DHFR) and fluorodeoxyuridine (FUdR), an inhibitor of thymidylate synthetase induce nicks in the wings of wild-type flies and phenocopies of the vg mutant phenotype when vg/+ and vgB/+ flies are reared on these substances (vgB is a deficiency of the vg locus). Only thymidine and thymidylate can rescue the flies from the effect of aminopterin. We propose that the vg phenotype is due to a decrease in the dTMP pool in the wings. (2) Mutant vg strains yield more offspring on medium containing aminopterin than on normal medium. The resistance of vg larvae to the inhibitor seems specific to the gene. This is the first case of aminopterin resistance in living eucaryotes. In contrast sensitivity of the vg larvae to FUdR is observed. (3) An increase in the activity and amount of DHFR is observed in mutant strains as compared with the wild-type flies. Our data suggest that the vg+ gene is a regulatory gene acting on the DHFR gene or a structural gene involved in the same metabolic pathway.