A genetic screen in Drosophila reveals an unexpected role for the KIP1 ubiquitination-promoting complex in male fertility

A unifying feature of polycystin-2 channels is their localization to both primary and motile cilia/flagella. In Drosophila melanogaster, the fly polycystin-2 homologue, Amo, is an ER protein early in sperm development but the protein must ultimately cluster at the flagellar tip in mature sperm to be fully functional. Male flies lacking appropriate Amo localization are sterile due to abnormal sperm motility and failure of sperm storage. We performed a forward genetic screen to identify additional proteins that mediate ciliary trafficking of Amo. Here we report that Drosophila homologues of KPC1 and KPC2, which comprise the mammalian KIP1 ubiquitination-promoting complex (KPC), form a conserved unit that is required for the sperm tail tip localization of Amo. Male flies lacking either KPC1 or KPC2 phenocopy amo mutants and are sterile due to a failure of sperm storage. KPC is a heterodimer composed of KPC1, an E3 ligase, and KPC2 (or UBAC1), an adaptor protein. Like their mammalian counterparts Drosophila KPC1 and KPC2 physically interact and they stabilize one another at the protein level. In flies, KPC2 is monoubiquitinated and phosphorylated and this modified form of the protein is located in mature sperm. Neither KPC1 nor KPC2 directly interact with Amo but they are detected in proximity to Amo at the tip of the sperm flagellum. In summary we have identified a new complex that is involved in male fertility in Drosophila melanogaster.

Minotaur is critical for primary piRNA biogenesis

Piwi proteins and their associated small RNAs are essential for fertility in animals. In part, this is due to their roles in guarding germ cell genomes against the activity of mobile genetic elements. piRNA populations direct Piwi proteins to silence transposon targets and, as such, form a molecular code that discriminates transposons from endogenous genes. Information ultimately carried by piRNAs is encoded within genomic loci, termed piRNA clusters. These give rise to long, single-stranded, primary transcripts that are processed into piRNAs. Despite the biological importance of this pathway, neither the characteristics that define a locus as a source of piRNAs nor the mechanisms that catalyze primary piRNA biogenesis are well understood. We searched an EMS-mutant collection annotated for fertility phenotypes for genes involved in the piRNA pathway. Twenty-seven homozygous sterile strains showed transposon-silencing defects. One of these, which strongly impacted primary piRNA biogenesis, harbored a causal mutation in CG5508, a member of the Drosophila glycerol-3-phosphate O-acetyltransferase (GPAT) family. These enzymes catalyze the first acylation step on the path to the production of phosphatidic acid (PA). Though this pointed strongly to a function for phospholipid signaling in the piRNA pathway, a mutant form of CG5508, which lacks the GPAT active site, still functions in piRNA biogenesis. We have named this new biogenesis factor Minotaur.

Drosophila rae1 is required for male meiosis and spermatogenesis

Previous studies of RAE1, a conserved WD40 protein, in Schizosaccharomyces pombe and mouse revealed a role in mRNA export and cell cycle progression in mitotic cells. Studies of RAE1 in Drosophila showed that the protein localizes to the nuclear envelope and is required for progression through the G1 phase of the cell cycle but not RNA export in tissue culture cells. Drosophila RAE1 also plays an essential developmental role, as it is required for viability and synaptic growth regulation as a component of an E3 ubiquitin ligase complex. Here we describe characterization of a new Drosophila rae1 mutant that is viable but results in male sterility. The mutant showed striking defects in primary spermatocyte nuclear integrity, meiotic chromosome condensation, segregation, and spindle morphology. These defects led to a failure to complete meiosis but allowed several aspects of spermatid differentiation to proceed, including axoneme formation and elongation. A GFP-RAE1 fusion protein that rescued most of the cytological defects showed a dynamic localization to the nuclear envelope, chromatin and other structures depending on the stage of spermatogenesis. A role for RAE1 in male meiosis, as well as mitotic cells, was also indicated by the defects induced by expression of rae1-RNAi. These studies in Drosophila provide the first evidence for an essential meiotic role of RAE1, and thus define RAE1 as a protein required for both meiotic and mitotic cell cycles.

Facilitating long-term changes in student approaches to learning science

Undergraduates entering science curricula differ greatly in individual starting points and learning needs. The fast pace, high enrollment, and high stakes of introductory science courses, however, limit students' opportunities to self-assess and modify learning strategies. The University of Washington's Biology Fellows Program (BFP) intervenes through a 20-session, premajors course that introduces students to the rigor expected of bioscience majors and assists their development as science learners. This study uses quantitative and qualitative approaches to assess whether the 2007-2009 BFP achieved its desired short- and long-term impacts on student learning. Adjusting for differences in students' high school grade point average and Scholastic Aptitude Test scores, we found that participation in the BFP was associated with higher grades in two subsequent gateway biology courses, across multiple quarters and instructors. Two to 4 yr after participating in the program, students attributed changes in how they approached learning science to BFP participation. They reported having learned to "think like a scientist" and to value active-learning strategies and learning communities. In addition, they reported having developed a sense of belonging in bioscience communities. The achievement of long-term impacts for a short-term instructional investment suggests a practical means to prepare diverse students for the rigors of science curricula.

Molecular landscape of modified histones in Drosophila heterochromatic genes and euchromatin-heterochromatin transition zones

Constitutive heterochromatin is enriched in repetitive sequences and histone H3-methylated-at-lysine 9. Both components contribute to heterochromatin's ability to silence euchromatic genes. However, heterochromatin also harbors hundreds of expressed genes in organisms such as Drosophila. Recent studies have provided a detailed picture of sequence organization of D. melanogaster heterochromatin, but how histone modifications are associated with heterochromatic sequences at high resolution has not been described. Here, distributions of modified histones in the vicinity of heterochromatic genes of normal embryos and embryos homozygous for a chromosome rearrangement were characterized using chromatin immunoprecipitation and genome tiling arrays. We found that H3-di-methylated-at-lysine 9 (H3K9me2) was depleted at the 5′ ends but enriched throughout transcribed regions of heterochromatic genes. The profile was distinct from that of euchromatic genes and suggests that heterochromatic genes are integrated into, rather than insulated from, the H3K9me2-enriched domain. Moreover, the profile was only subtly affected by a Su(var)3–9 null mutation, implicating a histone methyltransferase other than SU(VAR)3–9 as responsible for most H3K9me2 associated with heterochromatic genes in embryos. On a chromosomal scale, we observed a sharp transition to the H3K9me2 domain, which coincided with increased retrotransposon density in the euchromatin-heterochromatin (eu-het) transition zones on the long chromosome arms. Thus, a certain density of retrotransposons, rather than specific boundary elements, may demarcate Drosophila pericentric heterochromatin. We also demonstrate that a chromosome rearrangement that created a new eu-het junction altered H3K9me2 distribution and induced new euchromatic sites of enrichment as far as several megabases away from the breakpoint. Taken together, the findings argue against simple classification of H3K9me as the definitive signature of silenced genes, and clarify roles of histone modifications and repetitive DNAs in heterochromatin. The results are also relevant for understanding the effects of chromosome aberrations and the megabase scale over which epigenetic position effects can operate in multicellular organisms.

The chromosomal domain “heterochromatin” was first defined at the cytological level by its deeply staining appearance compared to more lightly stained domains called “euchromatin.” Abnormal juxtaposition of these two domains by chromosome rearrangements results in silencing of the nearby euchromatic genes. This effect is mediated by heterochromatin-enriched chromosomal proteins and led to the prevalent view of heterochromatin as incompatible with gene expression. Paradoxically, some expressed genes reside within heterochromatin. In this study, we examined how heterochromatic genes fit into a genomic context known for silencing effects. We found that Drosophila heterochromatic genes are integrated into the domain enriched in the modified histone H3K9me2, suggesting that the effect of this protein on gene expression is context-dependent. We also investigated the molecular nature of euchromatin-heterochromatin transition zones in the normal and rearranged chromosomes. The results provide insights into the functions of repetitive DNAs and H3K9me2 in heterochromatin and document the long distance over which a heterochromatic breakpoint can affect the molecular landscape of a chromosomal region. These findings have implications for understanding the consequences of chromosome abnormalities in organisms, including humans.

Sperm plasma membrane breakdown during Drosophila fertilization requires Sneaky, an acrosomal membrane protein

Fertilization typically involves membrane fusion between sperm and eggs. In Drosophila, however, sperm enter eggs with membranes intact. Consequently, sperm plasma membrane breakdown (PMBD) and subsequent events of sperm activation occur in the egg cytoplasm. We previously proposed that mutations in the sneaky (snky) gene result in male sterility due to failure in PMBD. Here we support this proposal by demonstrating persistence of a plasma membrane protein around the head of snkysperm after entry into the egg. We further show that snky is expressed in testes and encodes a predicted integral membrane protein with multiple transmembrane domains, a DC-STAMP-like domain, and a variant RING finger. Using a transgene that expresses an active Snky-Green fluorescent protein fusion (Snky-GFP), we show that the protein is localized to the acrosome, a membrane-bound vesicle located at the apical tip of sperm. Snky-GFP also allowed us to follow the fate of the protein and the acrosome during fertilization. In many animals, the acrosome is a secretory vesicle with exocytosis essential for sperm penetration through the egg coats. Surprisingly, we find that the Drosophila acrosome is a paternally inherited structure. We provide evidence that the acrosome induces changes in sperm plasma membrane, exclusive of exocytosis and through the action of the acrosomal membrane protein Snky. Existence of testis-expressed Snky-like genes in many animals, including humans, suggests conserved protein function. We relate the characteristics of Drosophila Snky, acrosome function and sperm PMBD to membrane fusion events that occur in other systems.

Oxymoron no more: the expanding world of heterochromatic genes

Heterochromatin has been oversimplified and even misunderstood. In particular, the existence of heterochromatic genes is often overlooked. Diverse types of genes reside within regions classified as constitutive heterochromatin and activating influences of heterochromatin on gene expression in Drosophila are well documented. These properties are usually considered paradoxical because heterochromatin is commonly portrayed as "silent chromatin". In the past, studies of heterochromatic genes were limited to a few Drosophila genes. However, the recent discovery of several hundred heterochromatic genes in Drosophila, plants and mammals through sequencing projects offers new opportunities to examine the variety of ways in which heterochromatin influences gene expression. Comparative genomics is revealing diverse origins of heterochromatic genes and remarkable evolutionary fluidity between heterochromatic and euchromatic domains. These features justify a broader view of heterochromatin, one that accommodates repressive, permissive and activating effects on gene expression, and recognizes chromosomal and evolutionary transitional states between heterochromatin and euchromatin.

Evolution of heterochromatic genes of Drosophila

Heterochromatin is generally associated with gene silencing, yet in Drosophila melanogaster, heterochromatin harbors hundreds of functional protein-encoding genes, some of which depend on heterochromatin for expression. Here we document a recent evolutionary transition of a gene cluster from euchromatin to heterochromatin, which occurred <20 million years ago in the drosophilid lineage. This finding reveals evolutionary fluidity between these two genomic compartments and provides a powerful approach to identifying differences between euchromatic and heterochromatic genes. Promoter mapping of orthologous gene pairs led to the discovery of the "slippery promoter," characterized by multiple transcriptional start sites predominantly at adenines, as a common promoter type found in both heterochromatic and euchromatic genes of Drosophila. Promoter type is diverse within the gene cluster but largely conserved between heterochromatic and euchromatic genes, eliminating the hypothesis that adaptation to heterochromatin required major alterations in promoter structure. Transition to heterochromatin is consistently associated with gene expansion due to the accumulation of transposable elements and increased A-T content. We conclude that heterochromatin-dependent regulation requires specialized enhancers or higher-order interactions and propose a facilitating role for transposable elements.

Toward a comprehensive genetic analysis of male fertility in Drosophila melanogaster

Drosophila melanogaster is a widely used model organism for genetic dissection of developmental processes. To exploit its full potential for studying the genetic basis of male fertility, we performed a large-scale screen for male-sterile (ms) mutations. From a collection of 12,326 strains carrying ethyl-methanesulfonate-treated, homozygous viable second or third chromosomes, 2216 ms lines were identified, constituting the largest collection of ms mutations described to date for any organism. Over 2000 lines were cytologically characterized and, of these, 81% failed during spermatogenesis while 19% manifested postspermatogenic processes. Of the phenotypic categories used to classify the mutants, the largest groups were those that showed visible defects in meiotic chromosome segregation or cytokinesis and those that failed in sperm individualization. We also identified 62 fertile or subfertile lines that showed high levels of chromosome loss due to abnormal mitotic or meiotic chromosome transmission in the male germ line or due to paternal chromosome loss in the early embryo. We argue that the majority of autosomal genes that function in male fertility in Drosophila are represented by one or more alleles in the ms collection. Given the conservation of molecular mechanisms underlying important cellular processes, analysis of these mutations should provide insight into the genetic networks that control male fertility in Drosophila and other organisms, including humans.

Genetic dissection of meiotic cytokinesis in Drosophila males

We have used Drosophila male meiosis as a model system for genetic dissection of the cytokinesis mechanism. Drosophila mutants defective in meiotic cytokinesis can be easily identified by their multinucleate spermatids. Moreover, the large size of meiotic spindles allows characterization of mutant phenotypes with exquisite cytological resolution. We have screened a collection of 1955 homozygous mutant male sterile lines for those with multinucleate spermatids, and thereby identified mutations in 19 genes required for cytokinesis. These include 16 novel loci and three genes, diaphanous, four wheel drive, and pebble, already known to be involved in Drosophila cytokinesis. To define the primary defects leading to failure of cytokinesis, we analyzed meiotic divisions in male mutants for each of these 19 genes. Examination of preparations stained for tubulin, anillin, KLP3A, and F-actin revealed discrete defects in the components of the cytokinetic apparatus, suggesting that these genes act at four major points in a stepwise pathway for cytokinesis. Our results also indicated that the central spindle and the contractile ring are interdependent structures that interact throughout cytokinesis. Moreover, our genetic and cytological analyses provide further evidence for a cell type-specific control of Drosophila cytokinesis, suggesting that several genes required for meiotic cytokinesis in males are not required for mitotic cytokinesis.

A strategy for mapping the heterochromatin of chromosome 2 of Drosophila melanogaster

The heterochromatin of chromosome 2 of Drosophila melanogaster has been among the best characterized models for functional studies of heterochromatin owing to its abundance of genetic markers. To determine whether it might also provide a favorable system for mapping extended regions of heterochromatin, we undertook a project to molecularly map the heterochromatin of the left arm of chromosome 2 (2Lh). In this paper, we describe a strategy that used clones and sequence information available from the Drosophila Genome Project and chromosome rearrangements to construct a map of the distal most portion of 2Lh. We also describe studies that used fluorescent in situ hybridization (FISH) to examine the resolution of this technique for cytologically resolving heterochromatic sequences on mitotic chromosomes. We discuss how these mapping studies can be extended to more proximal regions of the heterochromatin to determine the structural patterns and physical dimensions of 2Lh and the relationship of structure to function.

Suppression of heterochromatic gene variegation can be used to distinguish and characterize E(var) genes potentially important for chromosome structure in Drosophila melanogaster

Hundreds of genic modifiers of position effect variegation (PEV) have been isolated in Drosophila melanogaster with a view to identifying genes important for chromosome structure. Here we propose a supplementary genetic screen to pinpoint candidate genes that are most likely to function in chromosome organization, within the enhancer of variegation [E(var)] class of modifiers. Our strategy takes advantage of the fact that variegating euchromatic and heterochromatic genes respond oppositely to changes in the dosage of heterochromatin proteins. Consequently, only when enhancement of euchromatic gene variegation results from increased formation of heterochromatin should suppression of heterochromatic gene variegation be observed. Mutations in four E(var) genes were tested for the ability to suppress variegation of multiple alleles of the heterochromatic light ( lt) gene in a variety of tissues and at several developmental stages. Mutations in E(var)3-4, E(var)3-5 and modifier of mdg4 [ mod(mdg4)] suppressed lt variegation. In contrast, a mutation in the Trithorax-like ( Trl) gene, which encodes GAGA factor, enhanced or had no effect on lt variegation, consistent with its known role in promoting transcription. These data show that suppression of lt variegation can be used as an assay to distinguish between members of the E(var) class of modifiers.

Heterochromatic sequences in a Drosophila whole-genome shotgun assembly

BACKGROUND

Most eukaryotic genomes include a substantial repeat-rich fraction termed heterochromatin, which is concentrated in centric and telomeric regions. The repetitive nature of heterochromatic sequence makes it difficult to assemble and analyze. To better understand the heterochromatic component of the Drosophila melanogaster genome, we characterized and annotated portions of a whole-genome shotgun sequence assembly.

RESULTS

WGS3, an improved whole-genome shotgun assembly, includes 20.7 Mb of draft-quality sequence not represented in the Release 3 sequence spanning the euchromatin. We annotated this sequence using the methods employed in the re-annotation of the Release 3 euchromatic sequence. This analysis predicted 297 protein-coding genes and six non-protein-coding genes, including known heterochromatic genes, and regions of similarity to known transposable elements. Bacterial artificial chromosome (BAC)-based fluorescence in situ hybridization analysis was used to correlate the genomic sequence with the cytogenetic map in order to refine the genomic definition of the centric heterochromatin; on the basis of our cytological definition, the annotated Release 3 euchromatic sequence extends into the centric heterochromatin on each chromosome arm.

CONCLUSIONS

Whole-genome shotgun assembly produced a reliable draft-quality sequence of a significant part of the Drosophila heterochromatin. Annotation of this sequence defined the intron-exon structures of 30 known protein-coding genes and 267 protein-coding gene models. The cytogenetic mapping suggests that an additional 150 predicted genes are located in heterochromatin at the base of the Release 3 euchromatic sequence. Our analysis suggests strategies for improving the sequence and annotation of the heterochromatic portions of the Drosophila and other complex genomes.

The teflon gene is required for maintenance of autosomal homolog pairing at meiosis I in male Drosophila melanogaster

In recombination-proficient organisms, chiasmata appear to mediate associations between homologs at metaphase of meiosis I. It is less clear how homolog associations are maintained in organisms that lack recombination, such as male Drosophila. In lieu of chiasmata and synaptonemal complexes, there must be molecules that balance poleward forces exerted across homologous centromeres. Here we describe the genetic and cytological characterization of four EMS-induced mutations in teflon (tef), a gene involved in this process in Drosophila melanogaster. All four alleles are male specific and cause meiosis I-specific nondisjunction of the autosomes. They do not measurably perturb sex chromosome segregation, suggesting that there are differences in the genetic control of autosome and sex chromosome segregation in males. Meiotic transmission of univalent chromosomes is unaffected in tef mutants, implicating the tef product in a pairing-dependent process. The segregation of translocations between sex chromosomes and autosomes is altered in tef mutants in a manner that supports this hypothesis. Consistent with these genetic observations, cytological examination of meiotic chromosomes suggests a role of tef in regulating or mediating pairing of autosomal bivalents at meiosis I. We discuss implications of this finding in regard to the evolution of heteromorphic sex chromosomes and the mechanisms that ensure chromosome disjunction in the absence of recombination.

Chromosome rearrangements induce both variegated and reduced, uniform expression of heterochromatic genes in a development-specific manner

In Drosophila melanogaster, chromosome rearrangements that juxtapose euchromatin and heterochromatin can result in position effect variegation (PEV), the variable expression of heterochromatic and euchromatic genes in the vicinity of the novel breakpoint. We examined PEV of the heterochromatic light (lt) and concertina (cta) genes in order to investigate potential tissue or developmental differences in chromosome structure that might be informative for comparing the mechanisms of PEV of heterochromatic and euchromatic genes. We employed tissue pigmentation and in situ hybridization to RNA to assess expression of lt in individual cells of multiple tissues during development. Variegation of lt was induced in the adult eye, larval salivary glands and larval Malpighian tubules for each of three different chromosome rearrangements. The relative severity of the effect in these tissues was not tissue-specific but rather was characteristic of each rearrangement. Surprisingly, larval imaginal discs did not exhibit variegated lt expression. Instead, a uniform reduction of the lt transcript was observed, which correlated in magnitude with the degree of variegation. The same results were obtained for cta expression. These two distinct effects of rearrangements on heterochromatic gene expression correlated with the developmental stage of the tissue. These results have implications for models of heterochromatin formation and the nuclear organization of chromosomes during development and differentiation.

The paternal effect gene ms(3)sneaky is required for sperm activation and the initiation of embryogenesis in Drosophila melanogaster

Although a large number of maternal factors are known to be essential for fertilization or the earliest stages of embryogenesis in Drosophila melanogaster, the role of paternally supplied products is not clearly understood. Paternal effect mutations provide a means to identify factors specifically required by the sperm after its entry into the egg. Here we describe the third strict paternal effect gene to be identified in Drosophila ms(3)sneaky(snky), which defines the earliest developmental arrest phenotype so far described. Characterization of two independently isolated snky mutations showed that they affected male fertility, but not viability or female fertility. Cytological analyses showed that spermatogenesis proceeded normally in snky males. However, the snky defect was evident after sperm entry into the egg; snky sperm did not undergo nuclear decondensation, form a functional male pronucleus, or initiate mitotic divisions in the egg. Immunolocalization of tubulin and Drosophila Centrosomin, a known centrosomal component, showed that snky-inseminated eggs failed to reconstitute a microtubule-organizing center. In addition, snky sperm chromatin retained the histochemical properties of mature sperm chromatin for several hours after sperm entry, showed reduced staining with membrane-impermeant nuclear dyes, and failed to replicate. We conclude that the snky+ product is required for the initial response of the sperm to cytoplasmic cues in the egg and for the subsequent initiation of embryogenesis in Drosophila. We suggest that all of the snky defects can be explained by the failure of the sperm plasma membrane to break down after entry into the egg.

Paternal effects in Drosophila: implications for mechanisms of early development

The study of paternal effects on development provides a means to identify sperm-supplied products required for fertilization and the initiation of embryogenesis. This review describes paternal effects on animal development and discusses their implications for the role of the sperm in egg activation, centrosome activity, and biparental inheritance in different animal species. Paternal effects observed in Caenorhabditis elegans and in mammals are briefly reviewed. Emphasis is placed on paternal effects in Drosophila melanogaster. Genetic and cytologic evidence for paternal imprinting on chromosome behavior and gene expression in Drosophila are summarized. These effects are compared to chromosome imprinting that leads to paternal chromosome loss in sciarid and coccid insects and mammalian gametic imprinting that results in differential expression of paternal and maternal loci. The phenotypes caused by several early-acting maternal effect mutations identify specific maternal factors that affect the behavior of paternal components during fertilization and the early embryonic mitotic divisions. In addition, maternal effect defects suggest that two types of regulatory mechanisms coordinate parental components and synchronize their progression through mitosis. Some activities are coordinated by independent responses of parental components to shared regulatory factors, while others require communication between paternal and maternal components. Analyses of the paternal effects mutations sneaky, K81, paternal loss, and Horka have identified paternal products that play a role in mediating the initial response of the sperm to the egg cytoplasm, participation of the male pronucleus in the first mitosis, and stable inheritance of the paternal chromosomes in the early embryo.

Heterochromatin and gene expression in Drosophila

Heterochromatin is both necessary for the expression of heterochromatic genes and inhibitory for the expression of euchromatic genes. These two properties of heterochromatin have been elucidated from the study of chromosome rearrangements that induce position effect variegation (PEV) in Drosophila melanogaster. Novel euchromatin-heterochromatin junctions can affect the expression of euchromatic and heterochromatic genes located several megabases away, distinguishing higher order chromatin structure from most other regulatory mechanisms. Studies of PEV promise insights into the basis for heterochromatin formation and the role of higher order chromatin and chromosome structure in gene regulation. We evaluate the models and experimental data that address the mechanisms of PEV in different cell types, the potential functions of modifiers of PEV, and the relationship of PEV to other phenomena associated with variegated gene expression in Drosophila.

Cis-effects of heterochromatin on heterochromatic and euchromatic gene activity in Drosophila melanogaster

Chromosomal rearrangements that juxtapose heterochromatin and euchromatin can result in mosaic inactivation of heterochromatic and euchromatic genes. This phenomenon, position effect variegation (PEV), suggests that heterochromatic and euchromatic genes differ in their regulatory requirements. This report describes a novel method for mapping regions required for heterochromatic genes, and those that induce PEV of a euchromatic gene. P transposase mutagenesis was used to generate derivatives of a translocation that variegated for the light+ (lt+) gene and carried the euchromatic white+ (w+) gene on a transposon near the heterochromatin-euchromatin junction. Cytogenetic and genetic analyses of the derivatives showed that P mutagenesis resulted in deletions of several megabases of heterochromatin. Genetic and molecular studies showed that the derivatives shared a euchromatic breakpoint but differed in their heterochromatic breakpoint and their effects on seven heterochromatic genes and the w+ gene. Heterochromatic genes differed in their response to deletions. The lt+ gene was sensitive to the amount of heterochromatin at the breakpoint but the heterochromatic 40Fa gene was not. The severity of variegated w+ phenotype did not depend on the amount of heterochromatin in cis, but varied with local heterochromatic environment. These data are relevant for considering mechanisms of PEV of both heterochromatic and euchromatic genes.

Developmental genetical analysis and molecular cloning of the abnormal oocyte gene of Drosophila melanogaster

Studies of the abnormal oocyte (abo) gene of Drosophila melanogaster have previously been limited to the analysis of a single mutant allele, abnormal oocyte1 (abo1). The abo1 mutation causes a maternal-effect lethality that can be partially rescued zygotically by the abo+ allele and by increasing the dosage of specific regions of heterochromatin denoted ABO. This report describes the properties of abo2, a new P-element-induced allele that allowed us to reexamine the nature of maternal-effect defect. Comparisons of the phenotype of progeny of abo1/abo1 and abo1/abo2 females show that the preblastoderm lethality previously described as a component of the abo mutant maternal effect results from a recessive fertilization defect associated with the abo1 chromosome. We demonstrate here that the abo-induced maternal effect lethality occurs predominately late in embryogenesis after cuticle deposition but before hatching. The phenocritical period for zygotic rescue by heterochromatin coincides with this period of late embryogenesis. We have used the abo2 mutation to map and molecularly clone the gene. We show that the abo gene is located in the 32C cytogenetic interval and identify the putative abo transcript from mRNA isolated from adult females. Using germline transformation, we show that a 9-kb genomic fragment to which the transcript maps, partially fulfills requirement for maternal and zygotic abo+ function.

Genetic characterization of ms (3) K81, a paternal effect gene of Drosophila melanogaster

The vast majority of known male sterile mutants of Drosophila melanogaster fail to produce mature sperm or mate properly. The ms(3) K81(1) mutation is one of a rare class of male sterile mutations in which sterility is caused by developmental arrest after sperm entry into the egg. Previous studies showed that males homozygous for the K81(1) mutation produce progeny that arrest at either of two developmental stages. Most embryos arrest during early nuclear cycles, whereas the remainder are haploid embryos that arrest at a later stage. This description of the mutant phenotype was based on the analysis of a single allele isolated from a natural population. It was therefore unclear whether this unique paternal effect phenotype reflected the normal function of the gene. The genetic analysis and initial molecular characterization of five new K81 mutations are described here. Hemizygous conditions and heteroallelic combinations of the alleles were associated with male sterility caused by defects in embryogenesis. No other mutant phenotypes were observed. Thus, the K81 gene acted as a strict paternal effect gene. Moreover, the biphasic pattern of developmental arrest was common to all the alleles. These findings strongly suggested that the unusual embryonic phenotype caused by all five new alleles was due to loss of function of the K81+ gene. The K81 gene is therefore the first clear example of a strict paternal effect gene in Drosophila. Based on the embryonic lethal phenotypes, we suggest that the K81+ gene encodes a sperm-specific product that is essential for the male pronucleus to participate in the first few embryonic nuclear divisions.

Heterochromatin and gene expression in Drosophila

Heterochromatin is both necessary for the expression of heterochromatic genes and inhibitory for the expression of euchromatic genes. These two properties of heterochromatin have been elucidated from the study of chromosome rearrangements that induce position effect variegation (PEV) in Drosophila melanogaster. Novel euchromatin-heterochromatin junctions can affect the expression of euchromatic and heterochromatic genes located several megabases away, distinguishing higher order chromatin structure from most other regulatory mechanisms. Studies of PEV promise insights into the basis for heterochromatin formation and the role of higher order chromatin and chromosome structure in gene regulation. We evaluate the models and experimental data that address the mechanisms of PEV in different cell types, the potential functions of modifiers of PEV, and the relationship of PEV to other phenomena associated with variegated gene expression in Drosophila.

The effect of modifiers of position-effect variegation on the variegation of heterochromatic genes of Drosophila melanogaster

Dominant modifiers of position-effect variegation of Drosophila melanogaster were tested for their effects on the variegation of genes normally located in heterochromatin. These modifiers were previously isolated as strong suppressors of the variegation of euchromatic genes and have been postulated to encode structural components of heterochromatin or other products that influence chromosome condensation. While eight of the modifiers had weak or no detectable effects, six acted as enhancers of light (lt) variegation. The two modifiers with the strongest effects on lt were shown to also enhance the variegation of neighboring heterochromatic genes. These results suggest that the wild-type gene products of some modifiers of position-effect variegation are required for proper expression of genes normally located within or near the heterochromatin of chromosome 2. We conclude that these heterochromatic genes have fundamentally different regulatory requirements compared to those typical of euchromatic genes.

The effects of chromosome rearrangements on the expression of heterochromatic genes in chromosome 2L of Drosophila melanogaster

The light (lt) gene of Drosophila melanogaster is located at the base of the left arm of chromosome 2, within or very near centromeric heterochromatin (2Lh). Chromosome rearrangements that move the lt+ gene from its normal proximal position and place the gene in distal euchromatin result in mosaic or variegated expression of the gene. The cytogenetic and genetic properties of 17 lt-variegated rearrangements are described in this report. We show that five of the heterochromatic genes adjacent to lt are subject to inactivation by these rearrangements and that the euchromatic loci in proximal 2L are not detectably affected. The properties of the rearrangements suggest that proximity to heterochromatin is an important regulatory requirement for at least six 2Lh genes. We discuss how the properties of the position effects on heterochromatic genes relate to other proximity-dependent phenomena such as transvection.

The organization and expression of the light gene, a heterochromatic gene of Drosophila melanogaster

The light (lt) gene is located in the centromeric heterochromatin of chromosome 2 of Drosophila melanogaster. This gene is necessary for normal levels of pigmentation in a number of adult and larval tissues and is required for viability. Hybrid dysgenic and X-ray induced mutations have been used to identify the gene and compare its organization to that of euchromatic genes. Molecular mapping of lt mutations and its major transcripts has shown that the lt gene is at least 17 kb. By injecting cosmid clones that include this region into lt mutant embryos, we have defined a 30-kb region that can transiently rescue the pigmentation defect in the Malpighian tubules. The major transcription unit of this gene is comprised of exons that are single copy. It is unusual in its organization in having a heterogeneous array of middle repetitive DNA sequences within its intronic and flanking regions.

Characterization and functional expression in mammalian cells of genomic and cDNA clones encoding a Drosophila muscarinic acetylcholine receptor

Genomic and cDNA clones encoding a muscarinic acetylcholine receptor from Drosophila melanogaster have been isolated. Sequence analysis demonstrates that this gene encodes a receptor with a high degree of amino acid identity to the mammalian muscarinic acetylcholine receptors and has three introns in the portion of the gene encoding the third putative cytoplasmic loop. A full-length cDNA clone has been placed under the control of the mouse metallothionein promotor and transfected into mouse Y1 adrenal cells. The receptor expressed in these cells exhibits the high-affinity binding for the antagonists quinuclidinyl benzilate and atropine expected of a muscarinic receptor. The Drosophila muscarinic receptor, when expressed in Y1 cells, is physiologically active, as measured by agonist-dependent stimulation of phosphatidylinositol metabolism.

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.

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.

Defects in embryogenesis in mutants associated with the antennapedia gene complex of Drosophila melanogaster

Embryogenesis in individuals with mutations or deficiencies of the genes in the polytene interval 84A-84B1,2 of Drosophila melanogaster was examined using scanning electron microscopy (SEM). The developmental function of this region of chromosome 3 is of particular interest since it contains the Antennapedia Gene Complex (ANT-C), a gene cluster that includes the homoeotic proboscipedia (pb), Sex combs reduced (Scr), and Antennapedia (Antp) loci. The results of SEM studies, clonal analyses, and temperature-shift experiments show that the fushi tarazu (ftz) and zerknullt (zen) genes, which map between pb and Scr, are involved in processes initiated during embryogenesis. The activity of ftz+ appears to be required within the first 4 hr of development for the establishment of the proper number of segments in the embryonic germ band. Individuals with ftz mutations or deficiencies produce only half the normal number of segments. Each of the segments is twice the normal width and is apparently comprised of cells that would normally form two separate metameres. The zen allele is required from about 2-4 hr of embryogenesis. Mutations of this gene result in disturbances of morphogenetic movements during gastrulation. The mutant phenotype is characterized by the absence of the optic lobe, defects in involution of the head segments, and in some cases, failure of germ band elongation. A requirement during embryogenesis for the activities of other genes residing in the 84A-84B1,2 polytene interval is suggested by the phenotypes of individuals heterozygous or homozygous for chromosomal deficiencies. Using the deficiencies Df(3R)AntpNs+R17, Df(3R)Scr, and Df(3R)ScxW+RX2, we examined the effects of deleting the distal portions or all of the 84A-84B1,2 interval. The defects in deletion heterozygotes suggest that the wild-type activity of some gene(s) other than zen, within or just adjacent to the 84B1,2 doublet, is required to complete normal head involution. The deletion of all the loci in the 84A5-84B1,2 interval results in grossly abnormal morphology and morphogenesis of the gnathocephalic appendages of the embryo. From these studies we conclude that mutations and deficiencies of genes associated with the ANT-C have profound effects on embryogenesis. The mutant phenotypes suggest, in addition to ensuring proper segment identity, the wild-type alleles of the 84A-84B1,2 genes are necessary for normal segmentation and elongation of the germ band and normal head involution.

GENETIC ANALYSIS OF THE ANTENNAPEDIA GENE COMPLEX (ANT-C) AND ADJACENT CHROMOSOMAL REGIONS OF DROSOPHILA MELANOGASTER. II. POLYTENE CHROMOSOME SEGMENTS 84A–84B1,2

The existence of a gene complex in the proximal right arm of chromosome 3 of Drosophila melanogaster involved in the development of the head and thorax was originally suggested by the phenotypes of several dominant homoeotic mutations and their revertants. A screen for mutations utilizing Df(3R) AntpNS+R17 (proximally broken in salivary region 84B1,2) yielded, among 102 recovered mutations, 17 localized by deficiency mapping to the putative homoeotic cluster. These fell into four complementation groups, two of which were characterized by homoeotic phenotypes. To explore the limits of the Antennapedia gene complex (ANT-C) more proximally, a second screen has been undertaken utilizing Df(3R)Scr, a deficiency of 84A1-B1,2.——Of 2832 chromosomes screened, 21 bearing alterations localized to polytene interval 84A-84B1,2 have been recovered. Sixteen are recessive lethals, and five showing reduced viability display a visible phenotype in surviving individuals. Complementation and phenotypic analyses revealed four complementation groups proximal to those identified in the previous screen, including two new alleles of the recessive homoeotic mutation, proboscipedia (pb). Ten of the new mutations correspond to complementation groups defined previously in the Df(3R)AntpNS+R17 screen, four to the EbR11 group, two to the Scr group and four to the Antp group.——On the basis of the phenotypes of the 39 mutations localized to this region, plus their interactions with extant homoeotic mutations, we postulate that there are at least five functional sites comprising the ANT-C. Three have been demonstrated to he homoeotic in nature. The specific homoeotic transformations thus far observed suggest that these loci are critical for normal development of adult labial, maxillary and thoracic structures.

DNA synthesis after polyspermic fertilization in the axolotl

Cytological and autoradiographic studies were done to investigate the cytoplasmic control of DNA synthesis under conditions of physiological polyspermy. The DNA synthetic phases of the egg, principal sperm and accessory sperm nucleic were determined and correlated with nuclear morphology and develpmental fate. Results show that accessory sperm nuclei undergo morphological transition to pronuclei. Their DNA synthetic phase is the same as that of the principal sperm nucleus. Hence accessory sperm nuclei are capable of initiating and completing DNA replication before any cytological evidence of their degeneration is observed.