Aberrant splicing and transcription termination caused by P element insertion into the intron of a Drosophila gene

Unconventional translation initiation factor EIF2A is required for Drosophila spermatogenesis

BACKGROUND

EIF2A is an unconventional translation factor required for initiation of protein synthesis from non-AUG codons from a variety of transcripts, including oncogenes and stress related transcripts in mammalian cells. Its function in multicellular organisms has not been reported.

RESULTS

Here, we identify and characterize mutant alleles of the CG7414 gene, which encodes the Drosophila EIF2A ortholog. We identified that CG7414 undergoes sex-specific splicing that regulates its male-specific expression. We characterized a Mi{Mic} transposon insertion that disrupts the coding regions of all predicted isoforms and is a likely null allele, and a PBac transposon insertion into an intron, which is a hypomorph. The Mi{Mic} allele is homozygous lethal, while the viable progeny from the hypomorphic PiggyBac allele are male sterile and female fertile. In dEIF2A mutant flies, sperm failed to individualize due to defects in F-actin cones and failure to form and maintain cystic bulges, ultimately leading to sterility.

CONCLUSIONS

These results demonstrate that EIF2A is essential in a multicellular organism, both for normal development and spermatogenesis, and provide an entrée into the elucidation of the role of EIF2A and unconventional translation in vivo.

The evolution and functional significance of nested gene structures in Drosophila melanogaster

Nearly 10% of the genes in the genome of Drosophila melanogaster are in nested structures, in which one gene is completely nested within the intron of another gene (nested and including gene, respectively). Even though the coding sequences and untranslated regions of these nested/including gene pairs do not overlap, their intimate structures and the possibility of shared regulatory sequences raise questions about the evolutionary forces governing the origination and subsequent functional and evolutionary impacts of these structures. In this study, we show that nested genes experience weaker evolutionary constraint, have faster rates of protein evolution, and are expressed in fewer tissues than other genes, while including genes show the opposite patterns. Surprisingly, despite completely overlapping with each other, nested and including genes are less likely to display correlated gene expression and biological function than the nearby yet nonoverlapping genes. Interestingly, significantly fewer nested genes are transcribed from the same strand as the including gene. We found that same-strand nested genes are more likely to be single-exon genes. In addition, same-strand including genes are less likely to have known lethal or sterile phenotypes than opposite-strand including genes only when the corresponding nested genes have introns. These results support our hypothesis that selection against potential erroneous mRNA splicing when nested and including genes are on the same strand plays an important role in the evolution of nested gene structures.

Genetic basis for developmental homeostasis of germline stem cell niche number: a network of Tramtrack-Group nuclear BTB factors

The potential to produce new cells during adult life depends on the number of stem cell niches and the capacity of stem cells to divide, and is therefore under the control of programs ensuring developmental homeostasis. However, it remains generally unknown how the number of stem cell niches is controlled. In the insect ovary, each germline stem cell (GSC) niche is embedded in a functional unit called an ovariole. The number of ovarioles, and thus the number of GSC niches, varies widely among species. In Drosophila, morphogenesis of ovarioles starts in larvae with the formation of terminal filaments (TFs), each made of 8–10 cells that pile up and sort in stacks. TFs constitute organizers of individual germline stem cell niches during larval and early pupal development. In the Drosophila melanogaster subgroup, the number of ovarioles varies interspecifically from 8 to 20. Here we show that pipsqueak, Trithorax-like, batman and the bric-à-brac (bab) locus, all encoding nuclear BTB/POZ factors of the Tramtrack Group, are involved in limiting the number of ovarioles in D. melanogaster. At least two different processes are differentially perturbed by reducing the function of these genes. We found that when the bab dose is reduced, sorting of TF cells into TFs was affected such that each TF contains fewer cells and more TFs are formed. In contrast, psq mutants exhibited a greater number of TF cells per ovary, with a normal number of cells per TF, thereby leading to formation of more TFs per ovary than in the wild type. Our results indicate that two parallel genetic pathways under the control of a network of nuclear BTB factors are combined in order to negatively control the number of germline stem cell niches.

Starch-branching enzyme IIa is required for proper diurnal cycling of starch in leaves of maize

Starch-branching enzyme (SBE), a glucosyl transferase, is required for the highly regular pattern of α-1,6 bonds in the amylopectin component of starch. In the absence of SBEIIa, as shown previously in the sbe2a mutant of maize (Zea mays), leaf starch has drastically reduced branching and the leaves exhibit a severe senescence-like phenotype. Detailed characterization of the maize sbe2a mutant revealed that SBEIIa is the primary active branching enzyme in the leaf and that in its absence plant growth is affected. Both seedling and mature sbe2a mutant leaves do not properly degrade starch during the night, resulting in hyperaccumulation. In mature sbe2a leaves, starch hyperaccumulation is greatest in visibly senescing regions but also observed in green tissue and is correlated to a drastic reduction in photosynthesis within the leaf. Starch granules from sbe2a leaves observed via scanning electron microscopy and transmission electron microscopy analyses are larger, irregular, and amorphous as compared with the highly regular, discoid starch granules observed in wild-type leaves. This appears to trigger premature senescence, as shown by an increased expression of genes encoding proteins known to be involved in senescence and programmed cell death processes. Together, these results indicate that SBEIIa is required for the proper diurnal cycling of transitory starch within the leaf and suggest that SBEIIa is necessary in producing an amylopectin structure amenable to degradation by starch metabolism enzymes.

Anti-lipopolysaccharide factors from the black tiger shrimp, Penaeus monodon, are encoded by two genomic loci

Different isoforms of the ALF homologues (ALFPm1-5) have been previously identified from Penaeus monodon expressed sequence tag (EST) database (http://pmonodon.biotec.or.th). The nucleotide and amino acid sequences of the P. monodon ALF homologues were analyzed and categorized into two groups, ALFPm1 and 2 in group A and ALFPm3-5 in group B. The genomic sequences of the two ALF gene groups were obtained by using the PCR and genome walking techniques. The ALF group A gene consisted of three exons interrupted by two introns whereas the ALF group B gene contained four exons interrupted by three introns. The alignment of genomic sequences with the ALF cDNA sequences revealed that different transcripts in both groups were generated by alternative RNA splicing of the pre-mRNA transcripts. The 5' upstream sequences of the two ALF groups contained the putative cis-regulatory elements, including the activator protein 1, the Octamer, the GATA, the nuclear factor-kappaB, and the GAAA motifs, which possibly promoted transcription in response to infection as in other antimicrobial peptide genes. The RT-PCR analysis revealed that although all ALF isoforms were expressed in individual shrimp, the ALFPm2 and 3 were the major or authentic ALFs in the hemocytes. The expression of both ALFPm2 and 3 were increased in response to Vibrio harveyi infection indicating the important function of the ALFs against bacterial invasion.

The first deltex null mutant indicates tissue-specific deltex-dependent Notch signaling in Drosophila

Notch (N) is a single-pass transmembrane receptor. The N signaling pathway is an evolutionarily conserved mechanism that controls various cell-specification processes. Drosophila Deltex (Dx), a RING-domain E3 ubiquitin ligase, binds to the N intracellular domain, promotes N's endocytic trafficking to late endosomes, and was proposed to activate Suppressor of Hairless [Su(H)]-independent N signaling. However, it has been difficult to evaluate the importance of dx, because no null mutant of a dx family gene has been available in any organism. Here, we report the first null mutant allele of Drosophila dx. We found that dx was involved only in the subsets of N signaling, but was not essential for it in any developmental context. A strong genetic interaction between dx and Su(H) suggested that dx might function in Su(H)-dependent N signaling. Our epistatic analyses suggested that dx functions downstream of the ligands and upstream of activated Su(H). We also uncovered a novel dx activity that suppressed N signaling downstream of N.

A complementary transposon tool kit for Drosophila melanogaster using P and piggyBac

With the availability of complete genome sequence for Drosophila melanogaster, one of the next strategic goals for fly researchers is a complete gene knockout collection. The P-element transposon, the workhorse of D. melanogaster molecular genetics, has a pronounced nonrandom insertion spectrum. It has been estimated that 87% saturation of the approximately 13,500-gene complement of D. melanogaster might require generating and analyzing up to 150,000 insertions. We describe specific improvements to the lepidopteran transposon piggyBac and the P element that enabled us to tag and disrupt genes in D. melanogaster more efficiently. We generated over 29,000 inserts resulting in 53% gene saturation and a more diverse collection of phenotypically stronger insertional alleles. We found that piggyBac has distinct global and local gene-tagging behavior from that of P elements. Notably, piggyBac excisions from the germ line are nearly always precise, piggyBac does not share chromosomal hotspots associated with P and piggyBac is more effective at gene disruption because it lacks the P bias for insertion in 5' regulatory sequences.

blistery encodes Drosophila tensin protein and interacts with integrin and the JNK signaling pathway during wing development

Tensin is an actin-binding protein that is localized in focal adhesions. At focal adhesion sites, tensin participates in the protein complex that establishes transmembrane linkage between the extracellular matrix and cytoskeletal actin filaments. Even though there have been many studies on tensin as an adaptor protein, the role of tensin during development has not yet been clearly elucidated. Thus, this study was designed to dissect the developmental role of tensin by isolating Drosophila tensin mutants and characterizing its role in wing development. The Drosophila tensin loss-of-function mutations resulted in the formation of blisters in the wings, which was due to a defective wing unfolding process. Interestingly, by(1)-the mutant allele of the gene blistery (by)-also showed a blistered wing phenotype, but failed to complement the wing blister phenotype of the Drosophila tensin mutants, and contains Y62N/T163R point mutations in Drosophila tensin coding sequences. These results demonstrate that by encodes Drosophila tensin protein and that the Drosophila tensin mutants are alleles of by. Using a genetic approach, we have demonstrated that tensin interacts with integrin and also with the components of the JNK signaling pathway during wing development; overexpression of by in wing imaginal discs significantly increased JNK activity and induced apoptotic cell death. Collectively, our data suggest that tensin relays signals from the extracellular matrix to the cytoskeleton through interaction with integrin, and through the modulation of the JNK signal transduction pathway during Drosophila wing development.

Genetic and molecular characterization of a noveliab-8regulatory domain in theAbdominal-Bgene ofDrosophila melanogaster

Homeotic (or Hox) genes are key determinants in specifying the anteroposterior axis of most animals. The temporal and spatial expression of these genes requires the presence of large and complex cis-regulatory regions. The Abdominal-B Hox gene of the bithorax complex ofDrosophila is regulated by several infraabdominal domains,which determine Abdominal-B expression in abdominal segments A5 to A9(parasegments 10 to 14). Some of the infraabdominal domains have been characterized, including an infraabdominal-8 domain, which has been located 3′ to the Abdominal-B transcription unit. We have analyzed the expression and mutant phenotype of a P-lacZ element inserted close to the Abdominal-B m origin of transcription and of derivatives of this transposon. Some of these derivatives represent a particular class of mutations in the bithorax complex, because they transform the eighth and ninth abdominal segments without affecting more anterior metameres. The analysis of these mutations and of transformants carrying sequences upstream the Abdominal-B m transcription unit has allowed us to define a new infraabdominal-8 regulatory region, located 5′ to the Abdominal-B transcription unit, and has helped to characterize better the complex regulation of the Abdominal-Bgene.

pipsqueak encodes a factor essential for sequence-specific targeting of a polycomb group protein complex

The Polycomb (Pc) group (Pc-G) of repressors is essential for transcriptional silencing of homeotic genes that determine the axial development of metazoan animals. It is generally believed that the multimeric complexes formed by these proteins nucleate certain chromatin structures to silence promoter activity upon binding to Pc-G response elements (PRE). Little is known, however, about the molecular mechanism involved in sequence-specific binding of these complexes. Here, we show that an immunoaffinity-purified Pc protein complex contains a DNA binding activity specific to the (GA)n motif in a PRE from the bithoraxoid region. We found that this activity can be attributed primarily to the large protein isoform encoded by pipsqueak (psq) instead of to the well-characterized GAGA factor. The functional relevance of psq to the silencing mechanism is strongly supported by its synergistic interactions with a subset of Pc-G that cause misexpression of homeotic genes.

Thebric à braclocus consists of two paralogous genes encoding BTB/POZ domain proteins and acts as a homeotic and morphogenetic regulator of imaginal development inDrosophila

The bric à brac (bab) locus acts as a homeotic and morphogenetic regulator in the development of ovaries, appendages and the abdomen. It consists of two structurally and functionally related genes, bab1 and bab2, each of which encodes a single nuclear protein. Bab1 and Bab2 have two conserved domains in common, a BTB/POZ domain and a Psq domain, a motif that characterizes a subfamily of BTB/POZ domain proteins in Drosophila. The tissue distribution of Bab1 and Bab2 overlaps, with Bab1 being expressed in a subpattern of Bab2. Analysis of a series of mutations indicates that the two bab genes have synergistic, distinct and redundant functions during imaginal development. Interestingly, several reproduction-related traits that are sexually dimorphic or show diversity among Drosophila species are highly sensitive to changes in the bab gene dose, suggesting that alterations in bab activity may contribute to evolutionary modification of sex-related morphology.

Drosophila rhino encodes a female-specific chromo-domain protein that affects chromosome structure and egg polarity

Here we describe our analyses of Rhino, a novel member of the Heterochromatin Protein 1(HP1) subfamily of chromo box proteins. rhino (rhi) is expressed only in females and chiefly in the germline, thus providing a new tool to dissect the role of chromo-domain proteins in development. Mutations in rhi disrupt eggshell and embryonic patterning and arrest nurse cell nuclei during a stage-specific reorganization of their polyploid chromosomes, a mitotic-like state called the "five-blob" stage. These visible alterations in chromosome structure do not affect polarity by altering transcription of key patterning genes. Expression levels of gurken (grk), oskar (osk), bicoid (bcd), and decapentaplegic (dpp) transcripts are normal, with a slight delay in the appearance of bcd and dpp mRNAs. Mislocalization of grk and osk transcripts, however, suggests a defect in the microtubule reorganization that occurs during the middle stages of oogenesis and determines axial polarity. This defect likely results from aberrant Grk/Egfr signaling at earlier stages, since rhi mutations delay synthesis of Grk protein in germaria and early egg chambers. In addition, Grk protein accumulates in large, actin-caged vesicles near the endoplasmic reticulum of stages 6-10 egg chambers. We propose two hypotheses to explain these results. First, Rhi may play dual roles in oogenesis, independently regulating chromosome compaction in nurse cells at the end of the unique endoreplication cycle 5 and repressing transcription of genes that inhibit Grk synthesis. Thus, loss-of-function mutations arrest nurse cell chromosome reorganization at the five-blob stage and delay production or processing of Grk protein, leading to axial patterning defects. Second, Rhi may regulate chromosome compaction in both nurse cells and oocyte. Loss-of-function mutations block nurse cell nuclear transitions at the five-blob stage and activate checkpoint controls in the oocyte that arrest Grk synthesis and/or inhibit cytoskeletal functions. These functions may involve direct binding of Rhi to chromosomes or may involve indirect effects on pathways controlling these processes.

Ras1 interacts with multiple new signaling and cytoskeletal loci in Drosophila eggshell patterning and morphogenesis

Little is known about the genes that interact with Ras signaling pathways to regulate morphogenesis. The synthesis of dorsal eggshell structures in Drosophila melanogaster requires multiple rounds of Ras signaling followed by dramatic epithelial sheet movements. We took advantage of this process to identify genes that link patterning and morphogenesis; we screened lethal mutations on the second chromosome for those that could enhance a weak Ras1 eggshell phenotype. Of 1618 lethal P-element mutations tested, 13 showed significant enhancement, resulting in forked and fused dorsal appendages. Our genetic and molecular analyses together with information from the Berkeley Drosophila Genome Project reveal that 11 of these lines carry mutations in previously characterized genes. Three mutations disrupt the known Ras1 cell signaling components Star, Egfr, and Blistered, while one mutation disrupts Sec61beta, implicated in ligand secretion. Seven lines represent cell signaling and cytoskeletal components that are new to the Ras1 pathway; these are Chickadee (Profilin), Tec29, Dreadlocks, POSH, Peanut, Smt3, and MESK2, a suppressor of dominant-negative Ksr. A twelfth insertion disrupts two genes, Nrk, a "neurospecific" receptor tyrosine kinase, and Tpp, which encodes a neuropeptidase. These results suggest that Ras1 signaling during oogenesis involves novel components that may be intimately associated with additional signaling processes and with the reorganization of the cytoskeleton. To determine whether these Ras1 Enhancers function upstream or downstream of the Egf receptor, four mutations were tested for their ability to suppress an activated Egfr construct (lambdatop) expressed in oogenesis exclusively in the follicle cells. Mutations in Star and l(2)43Bb had no significant effect upon the lambdatop eggshell defect whereas smt3 and dock alleles significantly suppressed the lambdatop phenotype.

Sensitivity to diethylether anesthesia of fruit flies primarily depends on the genotypes of the sodium channel gene rather than the states of the membranes and the mechanisms might be different from heat-induced paralysis

The para locus of Drosophila melanogaster encodes the alpha subunit of a voltage-sensitive sodium channel. Many of the mutants develop paralysis at the high temperature (37 degrees C) and are hypersensitive to diethylether anesthesia. We examined whether the two aspects of the phenotype are mediated by a same mechanism that involves the sodium channel molecule by investigating properties of the three para alleles (para(hd838), para(ts1) and para(ts3)). Larvae of the all para strains showed almost normal sensitivities to diethylether anesthesia while adult flies of them showed hypersensitivities to that in the following manner: para(hd838)<==para(ts1)<para(ts3)<Canton-S. Larvae of the two para strains showed hypersensitivities to heat-induced paralysis in the following manner: para(ts1)=para(ts3)<para(hd838)=Canton-S, while adult flies of the all para strains showed hypersensitivities to that in the following manner: para(ts1)=para(ts3)<para(hd838)<Canton-S. The distinct phenotype of para(hd838) from para(ts1) or para(ts3) observed in the larval and adult heat-induced paralysis, would be the reflection of the difference of the mutation sites between in para(hd838) and para(ts1) or para(ts3). In addition, because the rank of sensitivity in the adult anesthesia and heat-induced paralysis were reversed between para(hd838) and para(ts1) or para(ts3), the mechanisms of diethylether anesthesia and heat-induced paralysis are not the same. There would be the anesthesia-specific mechanisms because the sensitivity to anesthesia became more remarkable in para(hd838) than in para(ts1) or para(ts3). para(hd838) has an insertion of P-element, one of a transposable element, in the second intron of the para sodium channel gene. Excision of the P-element from the para locus of the para(hd838) conferred the flies the wild-typic phenotype and reduction of the para gene in para(hd838) dramatically enhanced the hypersensitivity to anesthesia, suggesting that the phenotype of the strain was caused exclusively by the mutation of the para locus. The susceptibility to anesthesia also depends on the temperature at which the flies were assayed, not at which they were cultured, but the dependence on the assay temperature was smaller than that on the genotype. By these findings we could assure that the sensitivity to diethylether anesthesia of para(hd838) primarily depends on the genotypes of the sodium channel gene rather than other reasons such as the fluidity of the membranes.

Domina (Dom), a new Drosophila member of the FKH/WH gene family, affects morphogenesis and is a suppressor of position-effect variegation

Domina (Dom) is a novel member of the FKH/WH transcription factor gene family of Drosophila. Two alternatively polyadenylated Dom transcripts of 2.9 and 3.9 kb encode a 719-amino-acid protein with a FKH/WH domain and a putative acidic transactivation domain. Dom is mainly expressed in the central and peripheral nervous system. Homozygous mutants show rough eyes, irregular arrangement of bristles, extended wings, defective posterior wing margins, and a severely diminished vitality and fertility. Heterozygous Dom flies are morphologically wild type but show suppression of position-effect variegation. Consistently with this chromatin effect DOM protein is accumulated in the chromocenter and, as expected from a transcription factor, is found at specific euchromatic loci. Sequence comparison suggests that DOM of Drosophila is homologous to the chordate WHN proteins. The chromatin modifying capability of DOM is probably based on the FKH/WH domain, which shows a remarkable structural similarity to the winged-helix structures of H1 and the central globular domain of H5.

Aberrant splicing and altered spatial expression patterns in fruitless mutants of Drosophila melanogaster

The fruitless (fru) gene functions in Drosophila males to establish the potential for male sexual behaviors. fru encodes a complex set of sex-specific and sex-nonspecific mRNAs through the use of multiple promoters and alternative pre-mRNA processing. The male-specific transcripts produced from the distal (P1) fru promoter are believed to be responsible for its role in specifying sexual behavior and are only expressed in a small fraction of central nervous system (CNS) cells. To understand the molecular etiology of fruitless mutant phenotypes, we compared wild-type and mutant transcription patterns. These experiments revealed that the fru(2), fru(3), fru(4), and fru(sat) mutations, which are due to P-element inserts, alter the pattern of sex-specific and sex-nonspecific fru RNAs. These changes arise in part from the P-element insertions containing splice acceptor sites that create alternative processing pathways. In situ hybridization revealed no alterations in the locations of cells expressing the P1-fru-promoter-derived transcripts in fru(2), fru(3), fru(4), and fru(sat) pharate adults. For the fru(1) mutant (which is due to an inversion breakpoint near the P1 promoter), Northern analyses revealed no significant changes in fru transcript patterns. However, in situ hybridization revealed anomalies in the level and distribution of P1-derived transcripts: in fru(1) males, fewer P1-expressing neurons are found in regions of the dorsal lateral protocerebrum and abdominal ganglion compared to wild-type males. In other regions of the CNS, expression of these transcripts appears normal in fru(1) males. The loss of fruitless expression in these regions likely accounts for the striking courtship abnormalities exhibited by fru(1) males. Thus, we suggest that the mutant phenotypes in fru(2), fru(3), fru(4), and fru(sat) animals are due to a failure to appropriately splice P1 transcripts, whereas the mutant phenotype of fru(1) animals is due to the reduction or absence of P1 transcripts within specific regions of the CNS.

The Drosophila pumilio gene encodes two functional protein isoforms that play multiple roles in germline development, gonadogenesis, oogenesis and embryogenesis

The pumilio (pum) gene plays an essential role in embryonic patterning and germline stem cell (GSC) maintenance during oogenesis in Drosophila. Here we report on a phenotypic analysis using pum(ovarette) mutations, which reveals multiple functions of pum in primordial germ cell proliferation, larval ovary formation, GSC division, and subsequent oogenic processes, as well as in oviposition. Specifically, by inducing pum(-) GSC clones at the onset of oogenesis, we show that pum is directly involved in GSC division, a function that is distinct from its requirement in primordial germ cells. Furthermore, we show that pum encodes 156- and 130-kD proteins, both of which are functional isoforms. Among pum(ovarette) mutations, pum(1688) specifically eliminates the 156-kD isoform but not the 130-kD isoform, while pum(2003) and pum(4277) specifically affect the 130-kD isoform but not the 156-kD isoform. Normal doses of both isoforms are required for the zygotic function of pum, yet either isoform alone at a normal dose is sufficient for the maternal effect function of pum. A pum cDNA transgene that contains the known open reading frame encodes only the 156-kD isoform and rescues the phenotype of both pum(1688) and pum(2003) mutants. These observations suggest that the 156- and 130-kD isoforms can compensate for each other's function in a dosage-dependent manner. Finally, we present molecular evidence suggesting that the two PUM isoforms share some of their primary structures.

wing blister, a new Drosophila laminin alpha chain required for cell adhesion and migration during embryonic and imaginal development

We report the molecular and functional characterization of a new alpha chain of laminin in Drosophila. The new laminin chain appears to be the Drosophila counterpart of both vertebrate alpha2 (also called merosin) and alpha1 chains, with a slightly higher degree of homology to alpha2, suggesting that this chain is an ancestral version of both alpha1 and alpha2 chains. During embryogenesis, the protein is associated with basement membranes of the digestive system and muscle attachment sites, and during larval stage it is found in a specific pattern in wing and eye discs. The gene is assigned to a locus called wing blister (wb), which is essential for embryonic viability. Embryonic phenotypes include twisted germbands and fewer pericardial cells, resulting in gaps in the presumptive heart and tracheal trunks, and myotubes detached from their target muscle attachment sites. Most phenotypes are in common with those observed in Drosophila laminin alpha3, 5 mutant embryos and many are in common with those observed in integrin mutations. Adult phenotypes show blisters in the wings in viable allelic combinations, similar to phenotypes observed in integrin genes. Mutation analysis in the eye demonstrates a function in rhabdomere organization. In summary, this new laminin alpha chain is essential for embryonic viability and is involved in processes requiring cell migration and cell adhesion.

Y-Linked male sterile mutations induced by P element in Drosophila melanogaster

The Y chromosome in Drosophila melanogaster is composed of highly repetitive sequences and is essential only in the male germ line. We employed P-element insertional mutagenesis to induce male sterile mutations in the Y chromosome. By using a combination of two modifiers of position effect variegation, adding an extra Y chromosome and increasing temperature, we isolated 61 P(ry+) elements in the Y chromosome. Six of these Y-linked insertions (approximately 10%) induced male sterile mutations that are mapped to two genes on the long and one on the short arms of the Y chromosome. These mutations are revertible to the wild type in a cell-autonomous and germ-line-dependent manner, consistent with previously defined Y-linked gene functions. Phenotypes associated with these P-induced mutations are similar to those resulting from deletions of the Y chromosome regions corresponding to the male fertility genes. Three alleles of the kl-3 gene on the Y long arm result in loss of the axonemal outer dynein arms in the spermatid tail, while three ks-2 alleles on the Y short arm induce defects at early postmeiotic stages. The recovery of the ms(Y) mutations induced by single P-element insertions will facilitate our effort to understand the structural and functional properties of the Y chromosome.

Genes expressed in the ring gland, the major endocrine organ of Drosophila melanogaster

We have used an enhancer-trap approach to begin characterizing the function of the Drosophila endocrine system during larval development. Five hundred and ten different lethal PZ element insertions were screened to identify those in which a reporter gene within the P element showed strong expression in part or all of the ring gland, the major site of production and release of developmental hormones, and which had a mutant phenotype consistent with an endocrine defect. Nine strong candidate genes were identified in this screen, and eight of these are expressed in the lateral cells of the ring gland that produce ecdysteroid molting hormone (EC). We have confirmed that the genes detected by these enhancer traps are expressed in patterns similar to those detected by the reporter gene. Two of the genes encode proteins, protein kinase A and calmodulin, that have previously been implicated in the signaling pathway leading to EC synthesis and release in other insects. A third gene product, the translational elongation factor EF-1alpha F1, could play a role in the translational regulation of EC production. The screen also identified the genes couch potato and tramtrack, previously known from their roles in peripheral nervous system development, as being expressed in the ring gland. One enhancer trap revealed expression of the gene encoding the C subunit of vacuolar ATPase (V-ATPase) in the medial cells of the ring gland, which produce the juvenile hormone that controls progression through developmental stages. This could reveal a function of V-ATPase in the response of this part of the ring gland to adenotropic neuropeptides. However, the gene identified by this enhancer trap is ubiquitously expressed, suggesting that the enhancer trap is detecting only a subset of its control elements. The results show that the enhancer trap approach can be a productive way of exploring tissue-specific genetic functions in Drosophila.

Functional dissection of YA, an essential, developmentally regulated nuclear lamina protein in Drosophila melanogaster

The Drosophila YA protein is a nuclear lamina component whose function is essential to initiate embryonic development. To identify regions of YA required for its action in its normal cellular context, we made targeted mutations in the YA protein and tested their consequences in flies and embryos in vivo. We found that critical amino acids are distributed along the length of the YA molecule, with functionally important regions including the N- and the C-terminal ends, the cysteine residues in YA's two potential zinc fingers, a serine/threonine-rich region, and a potential maturation-promoting factor or mitogen-activated protein kinase phosphorylation target site, ITPIR. In addition, several Ya mutations showed intragenic complementation, with N-terminal mutations complementing C-terminal mutations, suggesting that YA proteins interact with one another. In support of this interaction, we demonstrated by immunoprecipitation that YA molecules are present in complexes with each other. Finally, we showed that the C-terminal 179 amino acids of YA are necessary to target, or retain, YA in the nuclear envelope.

Drosophila kelch is an oligomeric ring canal actin organizer

Drosophila kelch has four protein domains, two of which are found in kelch-family proteins and in numerous nonkelch proteins. In Drosophila, kelch is required to maintain ring canal organization during oogenesis. We have performed a structure-function analysis to study the function of Drosophila kelch. The amino-terminal region (NTR) regulates the timing of kelch localization to the ring canals. Without the NTR, the protein localizes precociously and destabilizes the ring canals and the germ cell membranes, leading to dominant sterility. The amino half of the protein including the BTB domain mediates dimerization. Oligomerization through the amino half of kelch might allow cross-linking of ring canal actin filaments, organizing the inner rim cytoskeleton. The kelch repeat domain is necessary and sufficient for ring canal localization and likely mediates an additional interaction, possibly with actin.

The Drosophila G-protein-coupled receptor kinase homologue Gprk2 is required for egg morphogenesis

G protein signaling is a widely utilized form of extracellular communication that is mediated by a family of serpentine receptors containing seven transmembrane domains. In sensory neurons, cardiac muscle and other tissues, G protein-coupled receptors are desensitized through phosphorylation by a family of kinases, the G protein-coupled receptor kinases (GRKs). Desensitization allows a cell to decrease its response to a given signal, in the continued presence of that signal. We have identified a Drosophila mutant, gprk2(6936) that disrupts expression of a putative member of the GRK family, the G protein-coupled receptor kinase 2 gene (Gprk2). This mutation affects Gprk2 gene expression in the ovaries and renders mutant females sterile. The mutant eggs contain defects in several anterior eggshell structures that are produced by specific subsets of migratory follicle cells. In addition, rare eggs that become fertilized display gross defects in embryogenesis. These observations suggest that developmental signals transduced by G protein-coupled receptors are regulated by receptor phosphorylation. Based on the known functions of G protein-coupled receptor kinases, we speculate that receptor desensitization assists cells that are migrating or undergoing shape changes to respond rapidly to changing external signals.

Differential activity of Ras1 during patterning of the Drosophila dorsoventral axis

In Drosophila, the Ras1 gene is required downstream of receptor tyrosine kinases for correct eye development, embryonic patterning, wing vein formation, and border cell migration. Here we characterize a P-element allele of Ras1, Ras1(5703), that affects viability, eye morphogenesis, and early and late stages of oogenesis. Flies transheterozgyous for Ras1(5703) and existing EMS-induced Ras1 alleles are viable and exhibit a range of eye and eggshell defects. Differences in the severity of these phenotypes in different tissues suggest that there are allele-specific effects of Ras1 in development. Analysis of rescue constructs demonstrates that these differential phenotypes are due to loss of function in Ras1 alone and not due to effects on neighboring genes. Females mutant at the Ras1 locus lay eggs with reduced or missing dorsal eggshell structures. We observe dominant interactions between Ras1 mutants and other dorsoventral pathway mutants, including Egfr(top) and gurken. Ras1 is also epistatic to K10. Unlike Egfr(top) and gurken mutants, however, Ras1 females are moderately fertile, laying eggs with ventralized eggshells that can hatch normal larvae. These results suggest that Ras1 may have a different requirement in the patterning of the eggshell axis than in the patterning of the embryonic axis during oogenesis.

The Drosophila pipsqueak gene encodes a nuclear BTB-domain-containing protein required early in oogenesis

Mutations at the pipsqueak locus affect early patterning in the Drosophila egg and embryo. We have cloned pipsqueak and found that it is a large and complex gene, encoding multiple transcripts and protein isoforms. One protein, PsqA, is absent in all of the mutants that we have examined. We show that PsqA is a nuclear protein present in the germ cells and somatically derived follicle cells throughout oogenesis and that it is required prior to stage one of oogenesis. PsqA contains a BTB (POZ) domain at its amino terminus; additionally, we have identified an evolutionarily conserved motif of unknown function present four times in tandem at the C terminus of the protein. PZ pipsqueak mutants produce a putative fusion protein containing the pipsqueak BTB domain fused to sequences resident on the PZ element (H. Horowitz and C. Berg, 1995 Genetics 139, 327–335). We demonstrate here that expression of this fusion protein in wild-type flies has a dominant effect, resulting in infertility and eggshell defects. These dominant phenotypes are discussed in light of current theories on the role of the BTB domain in protein-protein interactions.

Aberrant pre-mRNA maturation is caused by LINE insertions into introns of the white gene of Drosophila melanogaster

Insertional mutagenesis screens have provided thousands of mutant alleles for analysing genes of varied functions in Drosophila melanogaster. We here document mechanisms of insertional mutagenesis by a LINE element, the I factor, by determining the molecular structure of RNAs produced from two alleles of the white gene of D.melanogaster, wIR1 and wIR6. These alleles result from insertion of the I factor into introns of the gene. We show that sequences present within the element direct aberrant splicing and termination events. When the I factor is inserted within the white first intron it may lead to the use of a cryptic 3' splice site which does not contain the dinucleotide AG. This splicing gives rise to a chimeric messenger RNA whose synthesis is controlled differently in tissues where the mutated gene is expressed. When the I factor is inserted within the white last intron it induces synthesis of truncated mRNAs. These results provide, for the first time, mechanisms for I factor insertional mutagenesis. They are discussed in the more general context of RNA processing in Drosophila and the evolution of eukaryotic gene introns.