The role of the ovarian tumor locus in Drosophila melanogaster germ line sex determination

OVO Positively Regulates Essential Maternal Pathways by Binding Near the Transcriptional Start Sites in the Drosophila Female Germline

Differentiation of female germline stem cells into a mature oocyte includes the expression of RNAs and proteins that drive early embryonic development in Drosophila. We have little insight into what activates the expression of these maternal factors. One candidate is the zinc-finger protein OVO. OVO is required for female germline viability and has been shown to positively regulate its own expression, as well as a downstream target, ovarian tumor, by binding to the transcriptional start site (TSS). To find additional OVO targets in the female germline and further elucidate OVO's role in oocyte development, we performed ChIP-seq to determine genome-wide OVO occupancy, as well as RNA-seq comparing hypomorphic and wild type rescue ovo alleles. OVO preferentially binds in close proximity to target TSSs genome-wide, is associated with open chromatin, transcriptionally active histone marks, and OVO-dependent expression. Motif enrichment analysis on OVO ChIP peaks identified a 5'-TAACNGT-3' OVO DNA binding motif spatially enriched near TSSs. However, the OVO DNA binding motif does not exhibit precise motif spacing relative to the TSS characteristic of RNA Polymerase II complex binding core promoter elements. Integrated genomics analysis showed that 525 genes that are bound and increase in expression downstream of OVO are known to be essential maternally expressed genes. These include genes involved in anterior/posterior/germ plasm specification (bcd, exu, swa, osk, nos, aub, pgc, gcl), egg activation (png, plu, gnu, wisp, C(3)g, mtrm), translational regulation (cup, orb, bru1, me31B), and vitelline membrane formation (fs(1)N, fs(1)M3, clos). This suggests that OVO is a master transcriptional regulator of oocyte development and is responsible for the expression of structural components of the egg as well as maternally provided RNAs that are required for early embryonic development.

Female germline expression of OVO transcription factor bridges Drosophila generations

OVO is required for karyotypically female germ cell viability but has no known function in the male germline in Drosophila. ovo is autoregulated by two antagonistic isoforms, OVO-A and OVO-B. All ovo- alleles were created as partial revertants of the antimorphic ovoD1 allele. Creation of new targeted alleles in an ovo+ background indicated that disrupting the germline-specific exon extension of ovo-B leads to an arrested egg chamber phenotype, rather than germ cell death. RNA-seq analysis, including >1K full length cDNAs, indicates that ovo utilizes a number of unannotated splice variations in the extended exon and a minor population of ovo-B transcripts utilizes an alternative splice. This indicates that classical ovo alleles such as ovoD1rv23, are not truly null for ovo, and are likely to be weak antimorphs. To generate bonafide nulls, we deleted the ovo-A and ovo-B promoters showing that only ovo-B is required for female germ cell viability and there is an early and polyphasic developmental requirement for ovo-B in the female germline. To visualize OVO expression and localization, we endogenously tagged ovo and found nuclear OVO in all differentiating female germ cells throughout oogenesis in adults. We also found that OVO is maternally deposited into the embryo, where it showed nuclear localization in newly formed pole cells. Maternal OVO persisted in embryonic germ cells until zygotic OVO expression was detectable, suggesting that there is continuous nuclear OVO expression in the female germline in the transition from one generation to the next.

Identification of novel candidate genes leading to sex differentiation in primordial germ cells of Drosophila

Germline sex determination and differentiation are pivotal processes in reproduction. In Drosophila, sex determination of the germline occurs in primordial germ cells (PGCs), and the sex differentiation of these cells is initiated during embryogenesis. However, the molecular mechanism initiating sex differentiation remains elusive. To address this issue, we identified sex-biased genes using RNA-sequencing data of male and female PGCs. Our research revealed 497 genes that were differentially expressed more than twofold between sexes and expressed at high or moderate levels in either male or female PGCs. Among these genes, we used microarray data of PGCs and whole embryos to select 33 genes, which are predominantly expressed in PGCs compared to the soma, as candidate genes contributing to sex differentiation. Of 497 genes, 13 genes that were differentially expressed more than fourfold between sexes were also selected as candidates. Among the 46 (33 + 13) candidates, we confirmed the sex-biased expression of 15 genes by in situ hybridization and quantitative reverse transcription-polymerase chain reaction (qPCR) analysis. Six and nine genes were predominantly expressed in male and female PGCs, respectively. These results represent a first step toward elucidating the mechanisms that initiate sex differentiation in the germline.

Trajectory mapping of the early Drosophila germline reveals controls of zygotic activation and sex differentiation

Germ cells in Drosophila melanogaster are specified maternally shortly after fertilization and are transcriptionally quiescent until their zygotic genome is activated to sustain further development. To understand the molecular basis of this process, we analyzed the progressing transcriptomes of early male and female germ cells at the single-cell level between germline specification and coalescence with somatic gonadal cells. Our data comprehensively cover zygotic activation in the germline genome, and analyses on genes that exhibit germline-restricted expression reveal that polymerase pausing and differential RNA stability are important mechanisms that establish gene expression differences between the germline and soma. In addition, we observe an immediate bifurcation between the male and female germ cells as zygotic transcription begins. The main difference between the two sexes is an elevation in X Chromosome expression in females relative to males, signifying incomplete dosage compensation, with a few select genes exhibiting even higher expression increases. These indicate that the male program is the default mode in the germline that is driven to female development with a second X Chromosome.

Absence of X-chromosome dosage compensation in the primordial germ cells of Drosophila embryos

Dosage compensation is a mechanism that equalizes sex chromosome gene expression between the sexes. In Drosophila, individuals with two X chromosomes (XX) become female, whereas males have one X chromosome (XY). In males, dosage compensation of the X chromosome in the soma is achieved by five proteins and two non-coding RNAs, which assemble into the male-specific lethal (MSL) complex to upregulate X-linked genes twofold. By contrast, it remains unclear whether dosage compensation occurs in the germline. To address this issue, we performed transcriptome analysis of male and female primordial germ cells (PGCs). We found that the expression levels of X-linked genes were approximately twofold higher in female PGCs than in male PGCs. Acetylation of lysine residue 16 on histone H4 (H4K16ac), which is catalyzed by the MSL complex, was undetectable in these cells. In male PGCs, hyperactivation of X-linked genes and H4K16ac were induced by overexpression of the essential components of the MSL complex, which were expressed at very low levels in PGCs. Together, these findings indicate that failure of MSL complex formation results in the absence of X-chromosome dosage compensation in male PGCs.

An assessment of the reproductive toxicity of GONPs exposure to Bombyx mori

This study aimed to explore the toxicity of environmental residues of graphene oxide nanoparticles (GONPs) to reproduction of Lepidopteron insects using both ovary cell line (BmN) and individual female Bombyx mori as the research subjects. The results showed that GONPs dose dependently affect BmN cells. At higher concentrations (>25 mg/L), GONPs led to oxidative stress, ROS accumulation and DNA damage in BmN cells and significantly reduced their survival rate (p ≤ 0.05). Moreover, feeding female B. mori larvae with mulberry leaves treated with 25 mg/L GONPs significantly decreased their gonadosomatic index (GSI) by 40.84%, and increased oxidation levels and antioxidant enzyme activity in silkworm ovary tissues. Pathological analysis found that exposure to GONPs decreased the numbers of both oogonia and oocytes in ovarian tissues, increased the formation of peroxisome and vacuoles in follicle cells, reduced the transcription of genes (Vg, Ovo, Sxl-s, Sxl-l, and Otu) related to ovarian development in B. mori by 0.61, 0.65, 0.75, 0.72, and 0.42-fold, respectively, and lowered the amount of spawning by 52.25%. Overall, these results revealed that GONPs exposure is toxic to the reproduction of B. mori. The underlying mechanism is that oxidative stress due to GONPs causes oxidative damage to DNA, damages ovarian tissues, as well as hinders B. mori development and spawning. Thus, this study provides important experimental data for safety evaluation of reproductive toxicity due to GONPs exposure.

Mechanism of trace acetamiprid-caused reproductive disorders in silkworm, Bombyx mori

BACKGROUND

Neonicotinoid insecticides are widely used in the prevention and control of pests in agriculture and forestry, but they can also affect the development of nontarget economic insects. In order to determine the effects of trace acetamiprid on the development of reproductive system of silkworm, we studied the gonad development and endogenous hormone metabolism in silkworms exposed to trace acetamiprid.

RESULT

The silkworm showed mild poisoning symptoms after being exposed to trace acetamiprid (0.01 mg L^-1 ). After 96 h of acetamiprid exposure, the body weight of silkworm was decreased by 7.67% and the ovary and fallopian tube were abnormally developed. The egg production in adults was decreased by 197 ± 29, and the egg weight was decreased by 0.52 ± 0.01 mg. Quantitative RT-PCR analysis showed that the relative expression levels of ovarian development-related genes Vg, Ovo, Otu, Sxl-S and Sxl-L were downregulated by 0.71, 0.77, 0.47, 0.67 and 0.88-fold, respectively. The transcriptional expression of ecdysone metabolism-related gene (EcR) in the ovary was downregulated by 0.46-fold while the expression of juvenile hormone-related gene (JHBP2) was upregulated by 1.36-fold. In hemolymph of acetamiprid exposed larvae, the content of ecdysone was reduced whereas the content of juvenile hormone was increased.

CONCLUSION

Trace acetamiprid may cause reproductive dysfunction in silkworms through regulating the metabolism of endogenous hormones. Our study provides a reference for elucidating the mechanism of acetamiprid-induced reproductive disorders in insects. © 2019 Society of Chemical Industry.

Identification of General Patterns of Sex-Biased Expression in Daphnia, a Genus with Environmental Sex Determination

Daphnia reproduce by cyclic-parthenogenesis, where phases of asexual reproduction are intermitted by sexual production of diapause stages. This life cycle, together with environmental sex determination, allow the comparison of gene expression between genetically identical males and females. We investigated gene expression differences between males and females in four genotypes of Daphnia magna and compared the results with published data on sex-biased gene expression in two other Daphnia species, each representing one of the major phylogenetic clades within the genus. We found that 42% of all annotated genes showed sex-biased expression in D. magna This proportion is similar both to estimates from other Daphnia species as well as from species with genetic sex determination, suggesting that sex-biased expression is not reduced under environmental sex determination. Among 7453 single copy, one-to-one orthologs in the three Daphnia species, 707 consistently showed sex-biased expression and 675 were biased in the same direction in all three species. Hence these genes represent a core-set of genes with consistent sex-differential expression in the genus. A functional analysis identified that several of them are involved in known sex determination pathways. Moreover, 75% were overexpressed in females rather than males, a pattern that appears to be a general feature of sex-biased gene expression in Daphnia.

Effect of TiO2 nanoparticles on the reproduction of silkworm

Silkworm (Bombyx mori) is an important economic insect and the model insect of Lepidoptera. Because of its high fecundity and short reproduction cycle, it has been widely used in reproduction and development research. The high concentrations of titanium dioxide nanoparticles (TiO2 NPs) show reproductive toxicity, while low concentrations of TiO2 NPs have been used as feed additive and demonstrated significant biological activities. However, whether the low concentrations of TiO2 NPs affect the reproduction of B. mori has not been reported. In this study, the growth and development of gonad of B. mori fed with a low concentration of TiO2 NPs (5 mg/L) were investigated by assessing egg production and expression of reproduction-related genes. The results showed that the low concentration of TiO2 NPs resulted in faster development of the ovaries and testes and more gamete differentiation and formation, with an average increase of 51 eggs per insect and 0.34 × 10(-4) g per egg after the feeding. The expressions of several reproduction-related genes were upregulated, such as the yolk-development-related genes Ovo-781 and vitellogenin (Vg) were increased by 5.33- and 6.77-folds, respectively. This study shows that TiO2 NPs feeding at low concentration can enhance the reproduction of B. mori, and these results are useful in developing new methods to improve fecundity in B. mori and providing new clues for its broad biological applications.

Genetics of germ cell development

The germ line represents a continuous cellular link between generations and between species, but the germ cells themselves develop in a specialized, organism-specific context. The model organisms Caenorhabditis elegans, Drosophila melanogaster and the mouse display striking similarities, as well as major differences, in the means by which they control germ cell development. Recent developments in genetic technologies allow a more detailed comparison of the germ cells of these three organisms than has previously been possible, shedding light not only on universal aspects of germline regulation, but also on the control of the pluripotent state in vivo and on the earliest steps of embryogenesis. Here, we highlight themes from the comparison of these three alternative strategies for navigating the fundamental cycle of sexual reproduction.

Phf7 controls male sex determination in the Drosophila germline

Establishment of germline sexual identity is critical for production of male and female germline stem cells, as well as sperm versus eggs. Here we identify PHD Finger Protein 7 (PHF7) as an important factor for male germline sexual identity in Drosophila. PHF7 exhibits male-specific expression in early germ cells, germline stem cells, and spermatogonia. It is important for germline stem cell maintenance and gametogenesis in males, whereas ectopic expression in female germ cells ablates the germline. Strikingly, expression of PHF7 promotes spermatogenesis in XX germ cells when they are present in a male soma. PHF7 homologs are also specifically expressed in the mammalian testis, and human PHF7 rescues Drosophila Phf7 mutants. PHF7 associates with chromatin, and both the human and fly proteins bind histone H3 N-terminal tails with a preference for dimethyl lysine 4 (H3K4me2). We propose that PHF7 acts as a conserved epigenetic "reader" that activates the male germline sexual program.

The sex-biased brain: sexual dimorphism in gene expression in two species of songbirds

Background

Despite virtually identical DNA sequences between the sexes, sexual dimorphism is a widespread phenomenon in nature. To a large extent the systematic differences between the sexes must therefore arise from processes involving gene regulation. In accordance, sexual dimorphism in gene expression is common and extensive. Genes with sexually dimorphic regulation are known to evolve rapidly, both in DNA sequence and in gene expression profile. Studies of gene expression in related species can shed light on the flexibility, or degree of conservation, of the gene expression profiles underlying sexual dimorphism.

Results

We have studied the extent of sexual dimorphism in gene expression in the brain of two species of songbirds, the zebra finch (Taeniopygia guttata) and the common whitethroat (Sylvia communis), using large-scale microarray technology. Sexual dimorphism in gene expression was extensive in both species, and predominantly sex-linked: most genes identified were male-biased and Z-linked. Interestingly, approximately 50% of the male-biased Z-linked genes were sex-biased only in one of the study species.

Conclusion

Our results corroborate the results of recent studies in chicken and zebra finch which have been interpreted as caused by a low degree of dosage compensation in female birds (i.e. the heterogametic sex). Moreover, they suggest that zebra finches and common whitethroats dosage compensate partly different sets of genes on the Z chromosome. It is possible that this pattern reflects differences in either the essentiality or the level of sexual antagonism of these genes in the respective species. Such differences might correspond to genes with different rates of evolution related to sexual dimorphism in the avian brain, and might therefore be correlated with differences between the species in sex-specific behaviours.

Drosophila ataxin 2-binding protein 1 marks an intermediate step in the molecular differentiation of female germline cysts

In the Drosophila ovary, extrinsic signaling from the niche and intrinsic translational control machinery regulate the balance between germline stem cell maintenance and the differentiation of their daughters. However, the molecules that promote the continued stepwise development of ovarian germ cells after their exit from the niche remain largely unknown. Here, we report that the early development of germline cysts depends on the Drosophila homolog of the human ataxin 2-binding protein 1 (A2BP1) gene. Drosophila A2BP1 protein expression is first observed in the cytoplasm of 4-, 8- and 16-cell cysts, bridging the expression of the early differentiation factor Bam with late markers such as Orb, Rbp9 and Bruno encoded by arrest. The expression of A2BP1 is lost in bam, sans-fille (snf) and mei-P26 mutants, but is still present in other mutants such as rbp9 and arrest. A2BP1 alleles of varying strength produce mutant phenotypes that include germline counting defects and cystic tumors. Phenotypic analysis reveals that strong A2BP1 alleles disrupt the transition from mitosis to meiosis. These mutant cells continue to express high levels of mitotic cyclins and fail to express markers of terminal differentiation. Biochemical analysis reveals that A2BP1 isoforms bind to each other and associate with Bruno, a known translational repressor protein. These data show that A2BP1 promotes the molecular differentiation of ovarian germline cysts.

Drosophila germline sex determination: integration of germline autonomous cues and somatic signals

The Drosophila testis and ovary are major genetically tractable systems for studying stem cells and their regulation. This has resulted in a deep understanding of germline stem cell regulation by the microenvironment, or niche. The male and female germline niches differ. Since sex is determined through different mechanisms in the soma than in the germline, genetic or physical manipulations can be used to experimentally mismatch somatic and germline sexual identities. The phenotypic consequences of these mismatches have striking similarities to those resulting from manipulations of signals within the niche. A critical role of the germline sex determination pathway may therefore be to ensure the proper receipt and processing of signals from the niche.

Sex determination of germ cells in Drosophila

Many lines of evidence indicate that in Drosophila the mechanism for establishing the sex of the female germline is different from that acting in somatic cells. In the soma Sxl has an embryonic determinative role and is required throughout the life of female flies; in germ cells its expression begins only in the larval ovary. Both the mechanism for activating Sxl and the genes controlled by Sxl are different in the germline. A number of genes have been identified that are essential either for survival (e.g. ovo, otu) or differentiation (snf, Sxl, fl(2)d, bgcn) of female germ cells. ovo is required during embryogenesis for survival of pole cells. Genetic interactions with dominant alleles of ovo and/or Sxl indicate that otu, Sxl, snf and fl(2)d act in the same pathway as does ovo. bgcn differs in that neither ovo nor SxlD mutations affect the bgcn phenotype even though XX bgcn germ cells enter the male pathway. bgcn causes sterility in both sexes. Although the germline defect is cell autonomous in mosaic gonads, bgcn is also required in the somatic tissue for maintaining oogenesis of wild-type germ cells. Several dominant suppressors of bgcn have been identified and some have properties similar to Suppressors of variegation, suggesting that chromatin structure is critical for proper germ cell sex determination.

fused regulates germline cyst mitosis and differentiation during Drosophila oogenesis

The fused gene encodes a serine-threonine kinase that functions as a positive regulator of Hedgehog signal transduction in Drosophila embryogenesis, wing morphogenesis, and somatic cell development during oogenesis. Here, we have characterized the germline ovarian tumors present in adult ovaries of fused mutant females, a phenotype not observed upon deregulation of any other component of Hedgehog signaling. In the strongest fused mutant contexts, we found that tumorous ovarian follicles accumulate early spectrosome-containing germ cells corresponding to germline stem cells and/or early cystoblasts as evidenced by activated Dpp signal transduction and transcriptional repression of bag-of-marbles, encoding the cystoblast determination factor. These early germ cells are maintained far from their usual position in a specialized niche of somatic cells in the apical part of the germarium, which appears normal in size in fused mutant ovarioles. Therefore, these results indicate a novel function for fused in downregulation of Dpp signaling which is necessary for de-repression of bag-of-marbles and consequent cystoblast determination. The abnormal accumulation of these early germ cells seems to be due primarily to defects in differentiation since we show that germline stem cell proliferation in the germarium is not affected. A later block in germline development, at the 16-cell cyst stage before significant nurse cell and oocyte differentiation, was also observed in tumorous follicles when fused function was only partially lowered. Finally, fused mutant ovaries exhibit some germline cysts having undergone a supernumerary fifth mitotic division. Through clonal analysis, we provide evidence that fused regulates these cystocyte divisions cell autonomously, while the tumorous phenotype probably reflects both a somatic and germline requirement for fused for cyst and follicle development.

Core promoter sequences contribute to ovo-B regulation in the Drosophila melanogaster germline

Utilization of tightly linked ovo-A vs. ovo-B germline promoters results in the expression of OVO-A and OVO-B, C(2)H(2) transcription factors with different N -termini, and different effects on target gene transcription and on female germline development. We show that two sex-determination signals, the X chromosome number within the germ cells and a female soma, differentially regulate ovo-B and ovo-A. We have previously shown that OVO regulates ovarian tumor transcription by binding the transcription start site. We have explored the regulation of the ovo-B promoter using an extensive series of transgenic reporter gene constructs to delimit cis-regulatory sequences as assayed in wild-type and sex-transformed flies and flies with altered ovo dose. Minimum regulated expression of ovo-B requires a short region flanking the transcription start site, suggesting that the ovo-B core promoter bears regulatory information in addition to a "basal" activity. In support of this idea, the core promoter region binds distinct factors in ovary and testis extracts, but not in soma extracts, suggesting that regulatory complexes form at the start site. This idea is further supported by the evolutionarily conserved organization of OVO binding sites at or near the start sites of ovo loci in other flies.

RNA binding protein sex-lethal (Sxl) and control of Drosophila sex determination and dosage compensation

In the past two decades, scientists have elucidated the molecular mechanisms behind Drosophila sex determination and dosage compensation. These two processes are controlled essentially by two different sets of genes, which have in common a master regulatory gene, Sex-lethal (Sxl). Sxl encodes one of the best-characterized members of the family of RNA binding proteins. The analysis of different mechanisms involved in the regulation of the three identified Sxl target genes (Sex-lethal itself, transformer, and male specific lethal-2) has contributed to a better understanding of translation repression, as well as constitutive and alternative splicing. Studies using the Drosophila system have identified the features of the protein that contribute to its target specificity and regulatory functions. In this article, we review the existing data concerning Sxl protein, its biological functions, and the regulation of its target genes.

Genetic control of germline sexual dimorphism in Drosophila

Females produce eggs and males produce sperm. Work in Drosophila is helping to elucidate how this sex-specific germline differentiation is genetically encoded. While important details remain somewhat controversial, it is clear that signals generated by somatic cells, probably in the embryonic gonads, are required as extrinsic factors for germline sex determination. It is equally clear that the sex chromosome karyotype of the germ cell is an intrinsic factor for germline sex determination. There is also extensive somatic signaling required for differentiation of germline cells in the adult gonads. Mismatched germline and somatic line sexual identities place germ cells in an inappropriate signaling milieu, which results in either failed maintenance of germline stems cells when female germ cells are in a male soma or overproliferation of germline cells when male germ cells are in a female soma. The well-studied somatic sex determination genes including transformer, transformer-2, and doublesex are clearly involved in the nonautonomous signaling from somatic cells, while the autonomous functions of genes including ovo, ovarian tumor, and Sex-lethal are involved in the germline. The integration of these two pathways is not yet clear.

A host parasite interaction rescues Drosophila oogenesis defects

The cytoplasmically inherited bacterium Wolbachia pipientis is a widespread parasite of arthropods that manipulates the reproductive biology of its hosts, often to their detriment, in order to foster its own transmission through egg cytoplasm. Here we report that infection by Wolbachia restores fertility to Drosophila melanogaster mutant females prevented from making eggs by protein-coding lesions in Sex-lethal (Sxl), the master regulator of sex determination. Suppression of sterility by Wolbachia discriminates markedly among similar germline-specific Sxl alleles, and is not observed for mutations in other genes that produce similar 'tumorous ovary' phenotypes, including one that blocks Sxl germline expression. This allele and gene specificity indicates that suppression probably results from a specific interaction with Sxl protein, rather than from a bypass of the normal germline requirement for this developmental regulator or from an effect on Sxl expression. The Sxl-Wolbachia interaction provides a rare opportunity to explore host-parasite relationships at the molecular level in a model insect. Furthermore, demonstration that a parasite infection can counteract the deleterious effects of mutations in host genes illustrates how hosts might become dependent on parasites.

A germline-specific splicing generates an extended ovo protein isoform required for Drosophila oogenesis

Most regulatory genes are employed multiple times to control different processes during development. The Drosophila Ovo/Shavenbaby (Svb) transcription factor is required both for germline and epidermal differentiation, two roles also found for its ortholog m-ovo1 in mice. In Drosophila, these two distinct functions are contributed by separate control regions directing the expression of Ovo/Svb in the germline (ovo) and soma (svb), respectively. We report here that alternative splicing represents an additional level of the regulation of Ovo/Svb functional specificity. Characterization of the ovo(D1rv23) mutation revealed that the intragenic insertion of a novel retrotransposon, romano, inactivates ovo without altering svb. We provide evidence that this insertion disrupts a germline-specific alternative exon, exon 2b, which encodes a 178-amino-acid internal extension (2B). While both isoforms, Ovo+2B and Ovo-2B, accumulate during oogenesis, only Ovo+2B is able to fulfill germinal ovo functions. Ovo-2B is unable, even when overexpressed, to fully rescue oogenic defects resulting from the absence of wild type ovo product. By contrast, either Ovo+2B or Ovo-2B germline protein can substitute for Svb in the epidermis. Our results emphasize the specific features of splicing in the germline, and reveal its functional importance for the control of ovo/svb-dependent ovarian and epidermal differentiation.

The involvement of ovarian tumour in the intracellular localization of Sex-lethal protein

The Drosophila ovarian tumour gene is required at multiple times in the germline for oogenesis. A second gene, Sex-lethal, controls sex determination in the soma and also has a separate germline function affecting similar oogenic stages as ovarian tumour. We demonstrate that ovarian tumour is not required for early Sex-lethal gene expression in the female germline, as had been previously reported. Instead, we provide evidence that ovarian tumour has a specific role in the developmentally regulated accumulation of SEX-LETHAL protein within the cytoplasm and nucleus. Furthermore, the examination of nurse cell polytene chromosomes produced by certain ovarian tumour mutations showed that SEX-LETHAL protein can associate with discrete chromosomal sites in the germline and that this pattern appears to change as the egg chamber matures. This is the first indication that SEX-LETHAL is capable of direct physical interactions with chromosomes (albeit in a mutant background) and is consistent with the developmentally regulated nuclear localization of SEX-LETHAL being important for oogenesis.

Genetic analysis of Drosophila melanogaster polytene chromosome region 44D-45F: loci required for viability and fertility

A genetic screen to identify mutations in genes in the 45A region on the right arm of chromosome 2 that are involved in oogenesis in Drosophila was undertaken. Several lethal but no female sterile mutations in the region had previously been identified in screens for P-element insertion or utilizing X rays or EMS as a mutagen. Here we report the identification of EMS-induced mutations in 21 essential loci in the 45D-45F region, including 13 previously unidentified loci. In addition, we isolated three mutant alleles of a newly identified locus required for fertility, sine prole. Mutations in sine prole disrupt spermatogenesis at or before individualization of spermatozoa and cause multiple defects in oogenesis, including inappropriate division of the germline cyst and arrest of oogenesis at stage 4.

Half pint regulates alternative splice site selection in Drosophila

Alternative splicing is used by metazoans to increase protein diversity and to alter gene expression during development. However, few factors that control splice site choice in vivo have been identified. Here we describe a factor, Half pint (Hfp), that regulates RNA splicing in Drosophila. Females harboring hypomorphic mutations in hfp lay short eggs and show defects in germline mitosis, nuclear morphology, and RNA localization during oogenesis. We find that hfp encodes the Drosophila ortholog of human PUF60 and functions in both constitutive and alternative splicing in vivo. In particular, hfp mutants display striking defects in the developmentally regulated splicing of ovarian tumor (otu). Furthermore, transgenic expression of the missing otu splice form can rescue the ovarian phenotypes of hfp.

Sex determination signals control ovo-B transcription in Drosophila melanogaster germ cells

Nonautonomous inductive signals from the soma and autonomous signals due to a 2X karyotype determine the sex of Drosophila melanogaster germ cells. These two signals have partially overlapping influences on downstream sex determination genes. The upstream OVO-B transcription factor is required for the viability of 2X germ cells, regardless of sexual identity, and for female germline sexual identity. The influence of inductive and autonomous signals on ovo expression has been controversial. We show that ovo-B is strongly expressed in the 2X germ cells in either a male or a female soma. This indicates that a 2X karyotype controls ovo-B expression in the absence of inductive signals from the female soma. However, we also show that female inductive signals positively regulate ovo-B transcription in the 1X germ cells that do not require ovo-B function. Genetic analysis clearly indicates that inductive signals from the soma are not required for ovo-B function in 2X germ cells. Thus, while somatic inductive signals and chromosome karyotype have overlapping regulatory influences, a 2X karyotype is a critical germline autonomous determinant of ovo-B function in the germline.

Drosophila OVO regulates ovarian tumor transcription by binding unusually near the transcription start site

Evolutionarily conserved ovo loci encode developmentally regulated, sequence-specific, DNA-binding, C(2)H(2)-zinc-finger proteins required in the germline and epidermal cells of flies and mice. The direct targets of OVO activity are not known. Genetic experiments suggest that ovo acts in the same regulatory network as ovarian tumor (otu), but the relative position of these genes in the pathway is controversial. Three OVO-binding sites exist in a compact regulatory region that controls germline expression of the otu gene. Interestingly, the strongest OVO-binding site is very near the otu transcription start, where basal transcriptional complexes must function. Loss-of-function, gain-of-function and promoter swapping constructs demonstrate that OVO binding near the transcription start site is required for OVO-dependent otu transcription in vivo. These data unambiguously identify otu as a direct OVO target gene and raise the tantalizing possibility that an OVO site, at the location normally occupied by basal components, functions as part of a specialized core promoter.

Distinct domains mediate the early and late functions of the Drosophila ovarian tumor proteins

The Drosophila melanogaster ovarian tumor (otu) gene encodes two novel protein isoforms that are required at multiple stages of oogenesis. We have examined the activity of a set of C-terminal truncation Otu proteins as well as a GFP-tagged Otu (Otu-GFP). These experiments have shown that a putative Tudor domain in the central region of the large Otu isoform and a separate domain in the C-terminal region are required for regulation of cyst formation and oocyte maturation, respectively. We also present evidence that a portion of Otu co-fractionates with mRNA/protein complexes (mRNPs) and show that Otu-GFP associates with cytoplasmic aggregates at periphery of the nucleus at an intermediate stage of oogenesis. This study substantially clarifies the relationship between Otu structure and function and reveals new clues about interacting components.

Asymmetric germ cell division and oocyte determination during Drosophila oogenesis

Early oogenesis in Drosophila, with a stereotypic pattern of germ cell division and differentiation, provides an attractive model for studying cell lineage and patterning. Drosophila oogenesis is initiated when a germline stem cell divides asymmetrically to produce a daughter stem cell and a cytoblast. The cystoblast then undergoes four rounds of incomplete mitoses to form a 16-cell cyst, accompanied by the formation of the fusome. Within the cyst, one of the two cells with four intercellular bridges differentiates into an oocyte while the rest become nurse cells. The oocyte then translocates within the cyst to a posterior position, which defines the anterio-posterior axis of the future embryo. Recent studies have shown that the asymmetric germline stem cell division is controlled by somatic signaling involving piwi, fs(1)Yb, and the dpp pathway as well as by intrinsic mechanisms involving pumilio, nanos, arrest, bag-of-marbles, and the spectrosome-the fusome precursor in the stem cells and the cystoblast. The spectrosome in the cystoblast appears to play an important role in polarized fusome growth during cyst formation. The fusome may guide the formation of a polarized microtubule network for the intracyst transport of certain RNAs and proteins to the cystocyte destined to become the oocyte. Genes such as egalitarian, Bicaudal D, stonewall, and encore are important for oocyte determination, while differential adhesion between the oocyte and its surrounding prefollicle cells, as mediated by armadillo, alpha-catenin, shotgun, and the spindle genes, is crucial for oocyte translocation. Early oogenesis shares many parallel features to early spermatogenesis, although distinct differences are also observed at both the phenomenological and mechanistic levels. The study of oogenesis, progressing at an exciting rate, contributes significantly to our understanding of the mechanisms underlying proliferation, differentiation, and patterning.

Cell-autonomous and somatic signals control sex-specific gene expression in XY germ cells of Drosophila

When XX germ cells develop in a testis they become spermatogenic. Thus, somatic signals determine the sex of genetically female germ cells. In contrast, XY germ cells experimentally transferred to an ovary do not differentiate oogenic cells. Because such cells show some male characteristics when analyzed in adults, it was assumed that XY germ cells autonomously become spermatogenic. Recently, however, evidence showing that a female soma feminizes XY germ cells was reported. The conclusion was drawn that the sex determination of XY germ cells is dictated by the sex of the soma. We monitored the fate of XY germ cells placed in a female environment throughout development. Here we report that such germ cells respond to both cell-autonomous and somatic sex-determining signals, depending on the developmental stage. Analyzing the expression of sex-specific molecular markers, we first detected autonomous male-specific gene expression in XY germ cells embedded in female embryos and larvae. At later stages, however, we found that sex-specific regulation of gene expression within XY germ cells is influenced by somatic gonadal cells. After metamorphosis, XY germ cells developing in a female soma start expressing female-specific and male-specific markers. Transcription of female-specific genes is maintained, while that of male-specific genes is later repressed. We show that in such XY germ cells, the female-specific gene Sex-lethal (Sxl) is activated. Within the germline, Sxl expression is required for the activation of a further female-specific gene and the repression of male-specific genes. We thus report for the first time the existence of downstream targets of the gene Sxl in the germline.

Sex determination in the Drosophila germline is dictated by the sexual identity of the surrounding soma

It has been suggested that sexual identity in the germline depends upon the combination of a nonautonomous somatic signaling pathway and an autonomous X chromosome counting system. In the studies reported here, we have examined the role of the sexual differentiation genes transformer (tra) and doublesex (dsx) in regulating the activity of the somatic signaling pathway. We asked whether ectopic somatic expression of the female products of the tra and dsx genes could feminize the germline of XY animals. We find that Tra(F) is sufficient to feminize XY germ cells, shutting off the expression of male-specific markers and activating the expression of female-specific markers. Feminization of the germline depends upon the constitutively expressed transformer-2 (tra-2) gene, but does not seem to require a functional dsx gene. However, feminization of XY germ cells by Tra(F) can be blocked by the male form of the Dsx protein (Dsx(M)). Expression of the female form of dsx, Dsx(F), in XY animals also induced germline expression of female markers. Taken together with a previous analysis of the effects of mutations in tra, tra-2, and dsx on the feminization of XX germ cells in XX animals, our findings indicate that the somatic signaling pathway is redundant at the level tra and dsx. Finally, our studies call into question the idea that a cell-autonomous X chromosome counting system plays a central role in germline sex determination.

Structure, function and evolution of sex-determining systems in Dipteran insects

Nature has evolved an astonishing variety of genetic and epigenetic sex-determining systems which all achieve the same result, the generation of two sexes. Genetic and molecular analyses, mainly performed during the last 20 years, have gradually revealed the mechanisms that govern sexual differentiation in a few model organisms. In this review, we will introduce the sex-determining system of Drosophila and compare the fruitfly to the housefly Musca domestica and other Dipteran insects. Despite the ostensible variety, all these insects use the same basic strategy: a primary genetic signal that is different in males and females, a key gene that responds to the primary signal, and a double-switch gene that eventually selects between two alternative sexual programmes. These parallels, however, do not extend to the molecular level. Except for the double-switch gene doublesex at the end of the cascade, no functional homologies were found between more distantly related insects. In particular, Sex-lethal, the key gene that controls sexual differentiation in Drosophila, does not have a sex-determining function in any other genus studied so far. These results show that sex-determining cascades, in comparison to other regulatory pathways, evolve much more rapidly.

OVO transcription factors function antagonistically in the Drosophila female germline

OVO controls germline and epidermis differentiation in flies and mice. In the Drosophila germline, alternative OVO-B and OVO-A isoforms have a common DNA-binding domain, but different N-termini. We show that these isoforms are transcription factors with opposite regulatory activities. Using yeast one-hybrid assays, we identified a strong activation domain within a common region and a counteracting repression domain within the OVO-A-specific region. In flies, OVO-B positively regulated the ovarian tumor promoter, while OVO-A was a negative regulator of the ovarian tumor and ovo promoters. OVO-B isoforms supplied ovo(+) function in the female germline and epidermis, while OVO-A isoforms had dominant-negative activity in both tissues. Moreover, elevated expression of OVO-A resulted in maternal-effect lethality while the absence of OVO-A resulted in maternal-effect sterility. Our data indicate that tight regulation of antagonistic OVO-B and OVO-A isoforms is critical for germline formation and differentiation.

Requirement of Rbp9 in the maintenance of Drosophila germline sexual identity

Drosophila germline sex determination is controlled by a group of genes expressed at early stages of oogenesis (ovo, otu, bam, and Sxl, etc.). Mutations in these genes cause not only sex transformation of female germ cells, but also ovarian tumors. Although mutations at the Rbp9 locus also cause an ovarian tumor phenotype, Rbp9 has been shown to function during later developmental stages than do other ovarian tumor-causing genes. To test whether Rbp9 is also required for germline sex determination, we examined the sex transformation process of female germ cells in Rbp9 mutant flies. The detection of Sxl male transcripts and other male germline markers in Rbp9 mutant ovaries revealed that the Rbp9 mutation caused a partial germline sex transformation. Therefore, sex determination signals that persist throughout oogenesis appear to be required for proper maintenance of germline sexual identity.

flex, an X-linked female-lethal mutation in Drosophila melanogaster controls the expression of Sex-lethal

The Sex-lethal (Sxl) gene is required in Drosophila females for sexual differentiation of the soma, for gem cell differentiation and dosage compensation. We have isolated three new alleles of female-lethal-on-X (flex), an X-linked female-lethal mutation and have characterized its function in sex determination. SXL protein is missing in flex/flex embryos, however transcription from both Sxl(Pe), the early Sxl promoter and Sxl(Pm), the late maintenance promoter, is normal in flex homozygotes. In flex/flex embryos, Sxl mRNA is spliced in the male mode. Analysis of flex germline clones shows that it also functions in oogenesis, but in contrast to Sxl mutants that show an early arrest tumorous phenotype, flex mutant egg chambers develop to stage 10. In flex ovarian clones, Sxl RNA is also spliced in the male form. Hence, flex is a sex-specific regulator of Sxl functioning in both the soma and the germline. Genetic interaction studies show that flex does not enhance female lethality of Sxl loss-of-function alleles but it rescues the male-specific lethality of both of the gain-of-function Sxl mutations, Sxl(M1)and Sxl(M4.) In contrast to mutations in splicing regulators of Sxl, the female lethality of flex is not rescued by either Sxl(M1)or Sxl(M4). Based on these observations, we propose that flex regulates Sxl at a post-splicing stage and regulates either its translation or the stability of the SXL protein.

Regulatory and functional interactions between ovarian tumor and ovo during Drosophila oogenesis

The ovo and ovarian tumor genes are required during early and late stages of Drosophila oogenesis. The ovo product, a zinc-finger transcription factor, can bind to sites and influence the level of expression of the ovarian tumor promoter. Our examination of ovo null mutant organelles demonstrate that it is required for the differentiation of XX germ cells during larval gonial stages, in addition to its known role in maintaining germ cell numbers. In contrast, ovarian tumor is required during pupal and adult stages for the cystocyte divisions that give rise to the egg chamber. Studies on sexually transformed flies indicate that both the ovo and ovarian tumor null mutant phenotypes are distinctive from and more severe than the germline defects produced when male germ cells develop in female soma. This suggests that ovo and ovarian tumor have oogenic functions other than their putative role in germline sex determination. We also demonstrate that the regulation of ovarian tumor by ovo is stage-specific, as ovarian tumor promoter activity does not require ovo during larval stages but becomes ovo-dependent in the adult ovary. This coincides with when the ovarian tumor promoter becomes responsive to sex-specific signals from the soma suggesting a convergence of somatic and germline regulatory pathways on ovarian tumor during oogenesis.

The Drosophila gene stand still encodes a germline chromatin-associated protein that controls the transcription of the ovarian tumor gene

The Drosophila gene stand still (stil) encodes a novel protein required for survival, sexual identity and differentiation of female germ cells. Using specific antibodies, we show that the Stil protein accumulates in the nucleus of all female germ cells throughout development, and is transiently expressed during early stages of male germline differentiation. Changes of Stil subnuclear localization during oogenesis suggest an association with chromatin. Several mutant alleles, which are point mutations in the Stil N-terminal domain, encode proteins that no longer co-localized with chromatin. We find that Stil binds to many sites on polytene chromosomes with strong preference for decondensed chromatin. This localization is very similar to that of RNA polymerase II. We show that Stil is required for high levels of transcription of the ovarian tumor gene in germ cells. Expression of ovarian tumor in somatic cells can be induced by ectopic expression of Stil. Finally, we find that transient ubiquitous somatic expression of Stil results in lethality of the fly at all stages of development.

The ovo gene required for cuticle formation and oogenesis in flies is involved in hair formation and spermatogenesis in mice

The Drosophila svb/ovo gene gives rise to differentially expressed transcripts encoding a zinc finger protein. svb/ovo has two distinct genetic functions: shavenbaby (svb) is required for proper formation of extracellular projections that are produced by certain epidermal cells in late-stage differentiation; ovo is required for survival and differentiation of female germ cells. We cloned a mouse gene, movo1 encoding a nuclear transcription factor that is highly similar to its fly counterpart in its zinc-finger sequences. In mice, the gene is expressed in skin, where it localizes to the differentiating cells of epidermis and hair follicles, and in testes, where it is present in spermatocytes and spermatids. Using gene targeting, we show that movo1 is required for proper development of both hair and sperm. movo1(-/-) mice are small, produce aberrant hairs, and display hypogenitalism, with a reduced ability to reproduce. These mice also develop abnormalities in kidney, where movo1 is also expressed. Our findings reveal remarkable parallels between mice and flies in epidermal appendage formation and in germ-cell maturation. Furthermore, they uncover a phenotype similar to that of Bardet-Biedl syndrome, a human disorder that maps to the same locus as human ovo1.

Juvenile hormone effect on DNA synthesis and apoptosis in caste-specific differentiation of the larval honey bee (Apis mellifera L.) ovary

Caste-specific differentiation of the honey bee ovary commences in the last larval instar. In this process, formation of germ cell clusters by synchronous and incomplete mitoses occurs in the queen ovary, whereas in the worker ovary programmed cell death is the dominant feature. BrdU and TUNEL labeling were used to study dynamics of cell proliferation and apoptosis-dependent DNA degradation in ovaries of naturally developing queens and workers, as well as in juvenile hormone-treated worker larvae. Cell proliferation in ovaries of last-instar queen larvae generally exceeded that in workers, except for the late feeding phase. This inversion in cell proliferation patterns coincided with the onset of apoptosis in worker ovaries, as evidenced by TUNEL labeling. Juvenile hormone application to early-fifth-instar worker larvae had two noticeable effects. First, it diminished the number of S-phase nuclei in ovaries of late feeding-phase workers, bringing them to queen-like levels. Second, it prevented the induction of apoptotic DNA degradation. Caste-specific regulation of cell division in connection with programmed cell death can thus be attributed to the previously described differences in juvenile hormone titer in queen and worker larvae, adding a new facet to this hormone's multiple functions.

virilizer regulates Sex-lethal in the germline of Drosophila melanogaster

In Drosophila, the gene Sex-lethal (Sxl) is required for female development. It controls sexual differentiation in the soma, dosage compensation and oogenesis. The continuous production of SXL proteins in XX animals is maintained by autoregulation and depends on virilizer (vir). This gene is required in somatic cells for the female-specific splicing of Sxl primary transcripts and for an unknown vital process in both sexes. In the soma, clones of XX cells lacking Sxl or vir are sexually transformed and form male structures; in the germline, XX cells mutant for Sxl extensively proliferate, but are unable to differentiate. We now studied the role of vir in the germline by generating germline chimeras. We found that XX germ cells mutant for vir, in contrast to cells mutant for Sxl, perform oogenesis. We show that the early production of SXL in undifferentiated germ cells is independent of vir while, later in oogenesis, expression of Sxl becomes dependent on vir. We conclude that the early SXL proteins are sufficient for the production of eggs whereas the later SXL proteins are dispensable for this process. However, vir must be active in the female germline to allow normal embryonic development because maternal products of vir are required for the early post-transcriptional regulation of Sxl in XX embryos and for a vital process in embryos of both sexes.

Induction of female Sex-lethal RNA splicing in male germ cells: implications for Drosophila germline sex determination

With a focus on Sex-lethal (Sxl), the master regulator of Drosophila somatic sex determination, we compare the sex determination mechanism that operates in the germline with that in the soma. In both cell types, Sxl is functional in females (2X2A) and nonfunctional in males (1X2A). Somatic cell sex is determined initially by a dose effect of X:A numerator genes on Sxl transcription. Once initiated, the active state of SXL is maintained by a positive autoregulatory feedback loop in which Sxl protein insures its continued synthesis by binding to Sxl pre-mRNA and thereby imposing the productive (female) splicing mode. The gene splicing-necessary factor (snf), which encodes a component of U1 and U2 snRNPs, participates in this RNA splicing control. Here we show that an increase in the dose of snf+ can trigger the female Sxl RNA splicing mode in male germ cells and can feminize triploid intersex (2X3A) germ cells. These snf+ dose effects are as dramatic as those of X:A numerator genes on Sxl in the soma and qualify snf as a numerator element of the X:A signal for Sxl in the germline. We also show that female-specific regulation of Sxl in the germline involves a positive autoregulatory feedback loop on RNA splicing, as it does in the soma. Neither a phenotypically female gonadal soma nor a female dose of X chromosomes in the germline is essential for the operation of this feedback loop, although a female X-chromosome dose in the germline may facilitate it. Engagement of the Sxl splicing feedback loop in somatic cells invariably imposes female development. In contrast, engagement of the Sxl feedback loop in male germ cells does not invariably disrupt spermatogenesis; nevertheless, it is premature to conclude that Sxl is not a switch gene in germ cells for at least some sex-specific aspects of their differentiation. Ironically, the testis may be an excellent organ in which to study the interactions among regulatory genes such as Sxl, snf, ovo and otu which control female-specific processes in the ovary.

The Drosophila ovarian tumor gene is required for the organization of actin filaments during multiple stages in oogenesis

The ovarian tumor gene is required during both early and late stages of oogenesis. Mutations produce a range of phenotypes, including agametic ovarioles, tumorous egg chambers, and late stage oogenic arrest. We demonstrate that each of these phenotypes is associated with specific aberrations in actin distribution. In the earliest case, ovarian tumor mutations cause actin filaments to accumulate ectopically in the fusome. This correlates with abnormal fusome morphology and arrested germ cell development in the germaria. Similarly, ovarian tumor function is required for the localization of actin that is essential for the maturation of ring canals. This defect gives rise to tumorous egg chambers in which germ cell numbers and morphology are profoundly aberrant. We also confirm that ovarian tumor is required for the formation of the nurse cell cytoplasmic actin array that is essential for the nonspecific transport of cytoplasmic contents to the oocyte during late oogenesis. Our data suggest that at this stage ovarian tumor controls the site where actin filaments initiate. Taken together, these studies suggest that the diverse ovarian tumor mutant phenotypes derive from the mislocalization of actin filaments, indicating a role for this gene in organizing the female germline cytoskeleton, and that the misregulation of actin can have profound effects on germ cell division and differentiation.

The Drosophila gene fs(2)cup interacts with otu to define a cytoplasmic pathway required for the structure and function of germ-line chromosomes

The Drosophila ovarian tumor gene (otu) encodes cytoplasmic proteins that are required in germ-line cells for cyst formation, nurse cell chromosome structure and egg maturation. We have analyzed a gene, fs(2)cup, that participates in many of the same processes and interacts with otu genetically. Both nurse cell and oocyte chromosomes require cup to attain a normal morphology. In addition, the gene is needed for the oocyte to grow normally by taking up materials transported from the nurse cells. The gene encodes a 1132-amino-acid protein containing a putative membrane-spanning domain. Cup protein (but not cup RNA) is transported selectively into the oocyte in germarial cysts, like the p104 Otu protein. It is strongly associated with large structures in the cytoplasm and perinuclear region of nurse cells and, like Otu, moves to the periphery of these cells in stages 9–10. Moreover, cup mutations dominantly disrupt meiotic chromosome segregation. We propose that cup, otu and another interacting gene, fs(2)B, take part in a common cytoplasmic pathway with multiple functions during oogenesis.

Vive la différence: males vs females in flies vs worms

For 600 million years, the two best-understood metazoan species, the nematode Caenorhabditis elegans and fruit fly Drosophila melanogaster, have developed independent strategies for solving a biological problem faced by essentially all metazoans: how to generate two sexes in the proper proportions. The genetic program for sexual dimorphism has been a major focus of research in these two organisms almost from the moment they were chosen for study, and it may now be the best-understood general aspect of their development. In this review, we compare and contrast the strategies used for sex determination (including dosage compensation) between "the fly" and "the worm" and the way this understanding has come about. Although no overlap has been found among the molecules used by flies and worms to achieve sex determination, striking similarities have been found in the genetic strategies used by these two species to differentiate their sexes.

Somatic sex-determining signals act on XX germ cells in Drosophila embryos

In Drosophila, the enhancer-trap line mgm1 is already specifically expressed in male germ cells. Staining is first detected in 10-hour-old embryos and it is found in later stem cells. This line, which reveals the earliest sex-specific gene expression in the germline known so far, is a useful molecular marker to assess the sexual pathway that germ cells have entered before any overt sexual dimorphism is apparent. XY germ cells that develop in feminized animals express mgm1, which shows that this marker is autonomously expressed in XY germ cells. However, XX germ cells that develop in masculinized animals also express mgm1. Therefore, somatic sex-determining signals have already acted on XX germ cells in 10-hour-old embryos.

Female germ cells of Drosophila require zygotic ovo and otu product for survival in larvae and pupae respectively

Mutations in the genes ovo or otu can cause abnormal proliferation of XX germ cells, which leads to so-called ovarian tumors, or they can lead to the elimination of XX germ cells, such that adult females possess empty ovaries. Males carrying ovo or otu mutations are unaffected. To find out when this sexual dimorphism affects germ cells, we analyzed the requirement of embryos and larvae for zygotic ovo and otu products. We found that ovo is required for the survival of XX germ cells during larval stages, while XX germ cells lacking otu survive until metamorphosis. Furthermore, we found no sex-transformed mutant larval germ cells and no evidence for an early sex-specific vital process acting in germ cells of the embryo, contrary to what had been suggested earlier.

Multiple signaling pathways establish both the individuation and the polarity of the oocyte follicle in Drosophila

The development of the Drosophila oocyte depends upon a sequential series of interactions between the germline cells and the somatically derived follicle cells to produce individual follicles with appropriate polarities. In the germarium the control of germline cell division depends upon a proper interaction with somatic cells adjacent to the germline stem cells. Both gurken and brainiac are required in the germline, and the Egfr, daughterless, Notch, and Delta genes are required in the somatic cells to produce individual egg chambers with a continuous follicular epithelium. After a follicle forms, components in these same signaling pathways, plus additional genes, are then required for the establishment of the anterior-posterior polarity, followed by the dorsal-ventral polarity of the developing follicle. Initially, gurken mRNA is localized to the posterior edge of the oocyte, where it signals the posterior polar follicle cells to differentiate as posterior. The anterior-posterior assymmetry of the oocyte is then established by a reorganization of the microtubule network, which require a Notch-Delta-dependent signal sent from the posterior polar follicle cells to the oocyte and the activity of protein kinase A in the germ line. This reorganization leads to the localization of the maternal anterior-posterior determinants bicoid and oskar to opposite poles of the oocyte and the repositioning of the oocyte nucleus to the anterior-dorsal surface of the oocyte, gurken mRNA and protein are now concentrated between the oocyte nucleus and the adjacent anterior-dorsal follicle cells, where, in combination with Rhomboid, it locally activates the EGF receptor and its downstream cascade to direct the adjoining cells to adopt a dorsal fate. This process is thought to restrict the action of three follicle cell gene functions, encoded by windbeutel, nudal, and, pipe, to the ventral follicle cells, where they lead to the localized activation of a serine protease cascade required to produce the active Spätzle ligand to activate the Toll receptor. Finally, the termini of the embryo are dependent upon the activation of the Torso receptor, and this requires the localized expression of torso-like in a subset of follicle cells at the anterior and posterior poles of the follicle, which leads to the activation of Trunk, the putative ligand for Torso. In summary, the normal development of the oocyte requires a continuous sequence of germline-follicle cell interactions to provide the polarities responsible for normal development.

Polytene chromosomes show normal gene activity but some mRNAs are abnormally accumulated in the pseudonurse cell nuclei of Drosophila melanogaster otu mutants

Certain mutant alleles of the ovarian tumor (otu) locus give rise to polytene chromosomes in the pseudonurse cells (PNCs). We have previously shown that the banding pattern of these germ line-derived chromosomes is similar to that in the larval salivary gland chromosomes. In this study, we have examined the gene activity of these chromosomes. General gene expression from these chromosomes was studied by uridine autoradiography. The expression of specific genes was monitored by in situ hybridisation to mRNA and also by combining enhancer trap lines with otu mutants. We found that most of the genes studied were expressed in the PNCs as they were in the wild-type nurse cells. Four out of the 12 mRNAs studied accumulated in the nuclei instead of migrating to the cytoplasm. The intensity of accumulation directly correlated with the extent of polytenisation in the PNC nuclei. We suggest that the otu mRNA remains partly attached to the polytene chromosome template after transcription and discuss the effects of this phenomenon on polytenisation of the PNC chromosomes.

A role for the Drosophila bag-of-marbles protein in the differentiation of cystoblasts from germline stem cells

Cell differentiation commonly dictates a change in the cell cycle of mitotic daughters. Previous investigations have suggested that the Drosophila bag of marbles (bam) gene is required for the differentiation of germline stem cell daughters (cystoblasts) from the mother stem cells, perhaps by altering the cell cycle. In this paper, we report the preparation of antibodies to the Bam protein and the use of those reagents to investigate how Bam is required for germ cell development. We find that Bam exists as both a fusome component and as cytoplasmic protein and that cytoplasmic and fusome Bam might have separable activities. We also show that bam mutant germ cells are blocked in differentiation and are trapped as mitotically active cells like stem cells. A model for how Bam might regulate cystocyte differentiation is presented.

Two otu transcripts are selectively localised in Drosophila oogenesis by a mechanism that requires a function of the otu protein

The ovarian tumour gene (otu) is required for several processes during Drosophila oogenesis. The locus encodes two protein isoforms that have been proposed to act during different stages of oogenesis. Here we show that the corresponding otu mRNAs display a dynamic pattern of expression during oogenesis. The 4.1 kb mRNA encoding the 104 kDa isoform is expressed throughout adult oogenesis, but is mainly restricted to nurse cells. The 3.2 kb mRNA encoding the 98 kDa protein isoform is selectively localised in the oocyte up to stage 9. Both mRNAs are expressed abundantly in nurse cells at stages 10-11. We propose that the oocyte-specific function of otu is realised by the 98 kDa isoform. We show that the export of the 3.2 kb mRNA from the nurse cell nuclei requires a functional otu protein. The otu protein is also required for the correct distribution of the pumilio and oskar mRNAs, while the Bic-D, K10 and staufen mRNAs are localised in wild type fashion in otu mutants. Furthermore, we have observed a region of homology between the carboxy-terminal part of the otu protein and the mammalian microtubule associated proteins. The more severe the mutation in this region of homology, the more disturbed mRNA distribution is observed in otu mutants.