How do germ cells choose their sex? Drosophila as a paradigm

CRISPR/Cas9 mediated disruption of the white gene leads to pigmentation deficiency and copulation failure in Drosophila suzukii

The Spotted-wing Drosophila (Drosophila suzukii) is a devastating invasive pest of fruit crops. In D. melanogaster, the white (w) gene was associated with pigmentation and mating behavior, which are also important aspects to understand the invasion biology as well as to develop control strategies for D. suzukii. Here, we show that the generation of D. suzukii white-eyed mutants by CRISPR/Cas9 mutagenesis of the w gene resulted in the complete failure of copulation when w^- males were individually paired with w^- females in small circular arenas (diameter 0.7 cm) for 24 h. Further analysis showed that the mating defect was associated with w^- males and could not be rectified by two years of inbreeding by crossing sibling w^- females with w⁺ males, dim red illumination, male-female sexual training, changing to large arenas (diameter 3.5 cm), or different sex ratios. Profound pigmentation deficiency was detected in the compound eyes, ocelli, Malpighian tubules and testis sheaths in the w^- flies. Specifically, testis imaging showed that w^- males failed to deposit any pigments into pigment cells of the testis sheath, and produced smaller sperms and less seminal fluid compared to those from wildtype males. Together these observations suggest that the w gene plays an essential role in the regulation of sexual behavior and reproduction in D. suzukii. The similarities and differences in w gene function between D. suzukii and D. melanogaster in the context of pigmentation and mating behavior are discussed.

A Novel Saturation Mutagenesis Approach: Single Step Characterization of Regulatory Protein Binding Sites in RNA Using Phosphorothioates

Gene regulation plays an important role in development. Numerous DNA- and RNA-binding proteins bind their target sequences with high specificity to control gene expression. These regulatory proteins control gene expression either at the level of DNA (transcription) or at the level of RNA (pre-mRNA splicing, polyadenylation, mRNA transport, decay, and translation). Identification of regulatory sequences helps understand not only how a gene is switched on or off, but also which downstream genes are regulated by a particular regulatory protein. Here, we describe a one-step approach that allows saturation mutagenesis of a protein binding site in RNA. It involves doping DNA template with non-wild-type nucleotides within the binding site, synthesis of separate RNAs with each phosphorothioate nucleotide, and isolation of the bound fraction following incubation with protein. Interference from non-wild-type nucleotides results in their preferential exclusion from the protein-bound fraction. This is monitored by gel electrophoresis following selective chemical cleavage with iodine of phosphodiester bonds containing phosphorothioates (phosphorothioate mutagenesis or PTM). This single-step saturation mutagenesis approach is applicable to the characterization of any protein binding site in RNA.

Bioinformatics Analysis to Identify RNA-Protein Interactions in Oogenesis

Hundreds of RNA-binding proteins are known, but the biological functions are known for only a few of them. They regulate various aspects of RNA processing or biogenesis such as splicing, polyadenylation, and translation. Here I describe a bioinformatics approach that we developed to identify potential new mRNA target(s) of the Drosophila master sex-switch protein Sex-lethal (SXL) by combining computational analysis with genetic and biochemical investigation. This approach could be used to identify new RNA-protein interactions during oogenesis in the female germline and should be applicable to numerous other posttranscriptional regulatory events.

Sex determination: why so many ways of doing it?

Sexual reproduction is an ancient feature of life on earth, and the familiar X and Y chromosomes in humans and other model species have led to the impression that sex determination mechanisms are old and conserved. In fact, males and females are determined by diverse mechanisms that evolve rapidly in many taxa. Yet this diversity in primary sex-determining signals is coupled with conserved molecular pathways that trigger male or female development. Conflicting selection on different parts of the genome and on the two sexes may drive many of these transitions, but few systems with rapid turnover of sex determination mechanisms have been rigorously studied. Here we survey our current understanding of how and why sex determination evolves in animals and plants and identify important gaps in our knowledge that present exciting research opportunities to characterize the evolutionary forces and molecular pathways underlying the evolution of sex determination.

Molecular genetics of the early stages of germ cell differentiation during Drosophila oogenesis

Germ cells frequently develop in syncytial clusters. We are using molecular genetic approaches to the formation of these clusters in Drosophila as a paradigm for cellular differentiation. The genes described in this paper act during an initial step of cluster formation (bag-of-marbles gene [bam]) and near the end of syncytial divisions (orb gene). The results presented suggest that the bam gene product is required for the four incomplete cytokineses that characterize the initial stages of cluster formation. The orb gene, previously identified as an ovarian-specific cDNA which predicts a new member of the RNA-recognition motif family of RNA-binding proteins, is required for both early and late stages of oogenesis. Strong orb alleles arrest egg development at the time of nurse cell-oocyte cyst formation; weak alleles disrupt formation of the anteroposterior and dorsoventral axes within the oocyte during late oogenesis. We postulate that Orb is a constituent of cytoplasmic multiprotein complexes which deliver RNA molecules to specific addresses within the oocyte.

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.

Drosophila Sex-lethal protein mediates polyadenylation switching in the female germline

The Drosophila master sex-switch protein Sex-lethal (SXL) regulates the splicing and/or translation of three known targets to mediate somatic sexual differentiation. Genetic studies suggest that additional target(s) of SXL exist, particularly in the female germline. Surprisingly, our detailed molecular characterization of a new potential target of SXL, enhancer of rudimentary (e(r)), reveals that SXL regulates e(r) by a novel mechanism--polyadenylation switching--specifically in the female germline. SXL binds to multiple SXL-binding sites, which include the GU-rich poly(A) enhancer, and competes for the binding of CstF64 in vitro. The SXL-binding sites are able to confer sex-specific poly(A) switching onto an otherwise nonresponsive polyadenylation signal in vivo. The sex-specific poly(A) switching of e(r) provides a means for translational regulation in germ cells. We present a model for the SXL-dependent poly(A) site choice in the female germline.

Drosophila polypyrimidine-tract binding protein (PTB) functions specifically in the male germline

The mammalian polypyrimidine-tract binding protein (PTB), which is a heterogeneous ribonucleoprotein, is ubiquitously expressed. Unexpectedly, we found that, in Drosophila melanogaster, the abundant PTB transcript is present only in males (third instar larval, pupal and adult stages) and in adult flies is restricted to the germline. Most importantly, a signal from the somatic sex-determination pathway that is dependent on the male-specific isoform of the doublesex protein (DSX(M)) regulates PTB, providing evidence for the necessity of soma-germline communication in the differentiation of the male germline. Analysis of a P-element insertion directly links PTB function with male fertility. Specifically, loss of dmPTB affects spermatid differentiation, resulting in the accumulation of cysts with elongated spermatids without producing fully separated motile sperms. This male-specific expression of PTB is conserved in D.virilis. Thus, PTB appears to be a particularly potent downstream target of the sex-determination pathway in the male germline, since it can regulate multiple mRNAs.

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.

Recombination and disjunction in female germ cells of Drosophila depend on the germline activity of the gene sex-lethal

Gametogenesis in males and females differs in many ways. An important difference in Drosophila is that recombination between homologous chromosomes occurs only in female meiosis. Here, we report that this process relies on the correct functioning of Sex-lethal (Sxl) which is primarily known as the master gene in somatic sex determination. Certain alleles of this gene (Sxl(fs)) disrupt the germline, but not the somatic function of Sxl and cause an arrest of germ cell development during cystocyte proliferation. Using dominant suppressor mutations that relieve this early block in Sxl(fs) mutant females, we discovered additional requirements of Sxl for normal meiotic differentiation of the oocyte. Females mutant for Sxl(fs) and carrying a suppressor become fertile, but pairing of homologous chromosomes and formation of chiasmata is severely perturbed, resulting in an almost complete lack of recombinants and a high incidence of non-disjunction events. Similar results were obtained when germline expression of wild-type Sxl was compromised by mutations in virilizer (vir), a positive regulator of Sxl. Ectopic expression of a Sxl transgene in premeiotic stages of male germline development, on the other hand, is not sufficient to allow recombination to take place, which suggests that Sxl does not have a discriminatory role in this female-specific process. We propose that Sxl performs at least two tasks in oogenesis: an ‘early’ function in formation of the egg chamber, and a ‘late’ function in progression of the meiotic cell cycle, suggesting that both events are coordinated by a common mechanism.

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.

From behavior to development: genes for sexual behavior define the neuronal sexual switch in Drosophila

The isolation and analysis of Drosophila mutants with altered sexual orientation lead to the identification of novel branches in the sex-determination cascade which govern the sexually dimorphic development of the nervous system. One such example is the fruitless (fru) gene, the mutation of which induces male-to-male courtship and malformation of a male-specific muscle, the muscle of Lawrence (MOL). Since the MOL is formed in wild-type flies when the innervating nerve is male, regardless of the sex of the MOL itself, the primary site of Fru function is likely to be the motoneurons controlling the MOL. The fru gene produces multiple transcripts including sex-specific ones. A female-specific mRNA from the fru locus has a putative Transformer (Tra) binding site in its 5' untranslated region, suggesting that fru is a direct target of Tra. The fru transcripts encode a set of proteins similar to the BTB (Bric à brac, Tramtrack and Broad-complex)-Zn finger family of transcription factors. Mutations in the dissatisfaction (dsf) gene result in male-to-male courtship and reduced sexual receptivity of females. The dsf mutations also give rise to poor curling of the abdomen in males during copulation and failure of egg-laying by females. The latter phenotypes are ascribable to aberrant innervation of the relevant muscles. A genetic analysis reveals that expression of the dsf phenotypes depends on Tra but not on Doublesex (Dsx) or Fru, suggesting that dsf represents another target of Tra. Taken together, these findings suggest that the sex-determination protein Tra has at least three different targets, dsx, fru and dsf, each of which represents the first gene in a branch of the sex-determination hierarchy functioning in a mutually-exclusive set of neuronal cells in the Drosophila central nervous system.

In Drosophila, female gonadal cells repress male-specific gene expression in XX germ cells

In Drosophila, the sex of XX germ cells is determined by somatic signals. Whether sex-specific genes, however, are activated or repressed by somatic signals is not known. We have used mgm1, a germline-marker which is specifically expressed in male germ cells to analyze sex-specific gene expression of embryonic germ cells. We found that XX and XY germ cells that do not contact gonadal tissue can express mgm1. In contrast, XX germ cells that were associated with female somatic gonadal cells never expressed mgm1. Our results suggest that XX germ cells express male-specific genes, unless these genes are repressed by feminizing short range signals produced by the somatic cells of the prospective ovary.

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.

3-Phosphoinositide-dependent protein kinase-1 (PDK1): structural and functional homology with the Drosophila DSTPK61 kinase

BACKGROUND

The activation of protein kinase B (PKB, also known as c-Akt) is stimulated by insulin or growth factors and results from its phosphorylation at Thr308 and Ser473. We recently identified a protein kinase, termed PDK1, that phosphorylates PKB at Thr308 only in the presence of lipid vesicles containing phosphatidylinositol 3,4,5-trisphosphate (Ptdlns(3,4,5)P3) or phosphatidylinositol 3,4-bisphosphate (Ptdlns(3,4)P2).

RESULTS

We have cloned and sequenced human PDK1. The 556-residue monomeric enzyme comprises a catalytic domain that is most similar to the PKA, PKB and PKC subfamily of protein kinases and a carboxy-terminal pleckstrin homology (PH) domain. The PDK1 gene is located on human chromosome 16p13.3 and is expressed ubiquitously in human tissues. Human PDK1 is homologous to the Drosophila protein kinase DSTPK61, which has been implicated in the regulation of sex differentiation, oogenesis and spermatogenesis. Expressed PDK1 and DSTPK61 phosphorylated Thr308 of PKB alpha only in the presence of Ptdlns(3,4,5)P3 or Ptdlns(3,4)P2. Overexpression of PDK1 in 293 cells activated PKB alpha and potentiated the IGF1-induced phosphorylation of PKB alpha at Thr308. Experiments in which the PH domains of either PDK1 or PKB alpha were deleted indicated that the binding of Ptdlns(3,4,5)P3 or Ptdlns(3,4)P2 to PKB alpha is required for phosphorylation and activation by PDK1. IGF1 stimulation of 293 cells did not affect the activity or phosphorylation of PDK1.

CONCLUSIONS

PDK1 is likely to mediate the activation of PKB by insulin or growth factors. DSTPK61 is a Drosophila homologue of PDK1. The effect of Ptdlns(3,4,5)P3/Ptdlns(3,4)P2 in the activation of PKB alpha is at least partly substrate directed.

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.

Encore, a gene required for the regulation of germ line mitosis and oocyte differentiation during Drosophila oogenesis

During Drosophila oogenesis, a stem cell daughter undergoes precisely four rounds of mitosis to generate a cyst of 16 cells interconnected by cytoplasmic bridges. One of the cells becomes the oocyte while the remaining 15 cells differentiate as nurse cells. We hve identified a gene, encore, that is involved both in regulating the number of germline mitoses and in the process of oocyte differentation. Mutations in encore result in exactly one extra round of mitosis in the germline. Genetic and molecular studies indicate that this mitotic defect may be mediated through the gene bag-of-marbles. The isolation and characterization of multiple alleles of encore revealed that they were all temperature sensitive for this phenotype. Mutations in encore also affect the process of oocyte differentiation. Egg chambers are produced in which the oocyte nucleus has undergone endoreplication often resulting in the formation of 16 nurse cells. We argue that these two phenotypes produced by mutations in encore represent two independent requirements for encore during oogenesis.

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.

The gene virilizer is required for female-specific splicing controlled by Sxl, the master gene for sexual development in Drosophila

The gene virilizer (vir) is needed for dosage compensation and sex determination in females and for an unknown vital function in both sexes. In genetic mosaics, XX somatic cells mutant for vir differentiate male structures. One allele, vir2f, is lethal for XX, but not for XY animals. This female-specific lethality can be rescued by constitutive expression of Sxl or by mutations in msl (male-specific lethal) genes. Rescued animals develop as strongly masculinized intersexes or pseudomales. They have male-specifically spliced mRNA of tra, and when rescued by msl, also of Sxl. Our data indicate that vir is a positive regulator of female-specific splicing of Sxl and of tra pre-mRNA.

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.

A sex-specific number of germ cells in embryonic gonads of Drosophila

Male first instar larvae possess more germ cells in their gonads than female larvae of the same stage. To determine the earliest time point of sexual dimorphism in germ cell number, we have counted the germ cells of sexed embryos at different developmental stages. We found no difference in germ cell number of male and female embryos at the blastoderm and early gastrulation stage, or when germ cells are about to exit the midgut pocket. We find, however, that males have significantly more germ cells than females as soon as the germ cells are near the places where the gonads are formed and in all later stages. Our results show that germ cells are subject to a sex-specific control mechanism that regulates the number of germ cells already in embryos.

The somatic sex determines the requirement for ovarian tumor gene activity in the proliferation of the Drosophila germline

Gametogenesis in Drosophila requires sex-specific interactions between the soma and germline to control germ cell viability, proliferation, and differentiation. To determine what genetic components are involved in this interaction, we examined whether changes in the sexual identity of the soma affected the function of the ovarian tumor (otu) and ovo genes. These genes are required cell autonomously in the female germline for germ cell proliferation and differentiation. Mutations in otu and ovo cause a range of ovarian defects, including agametic ovaries and tumorous egg cysts, but do not affect spermatogenesis. We demonstrate that XY germ cells do not require otu when developing in testes, but become dependent on otu function for proliferation when placed in an ovary. This soma-induced requirement can be satisfied by the induced expression of the 98 × 10(3) M(r) OTU product, one of two isoforms produced by differential RNA splicing. These results indicate that the female somatic gonad can induce XY germ cells to become ‘female-like’ because they require an oogenesis-specific gene. In contrast, the requirement for ovo is dependent on a cell autonomous signal derived from the X:A ratio. We propose that differential regulation of the otu and ovo genes provides a mechanism for the female germline to incorporate both somatic and cell autonomous inputs required for oogenesis.

The dual role of hermaphrodite in the Drosophila sex determination regulatory hierarchy

The hermaphrodite (her) locus has both maternal and zygotic functions required for normal female development in Drosophila. Maternal her function is needed for the viability of female offspring, while zygotic her function is needed for female sexual differentiation. Here we focus on understanding how her fits into the sex determination regulatory hierarchy. Maternal her function is needed early in the hierarchy: genetic interactions of her with the sisterless genes (sis-a and sis-b), with function-specific Sex-lethal (Sxl) alleles and with the constitutive allele SxlM#1 suggest that maternal her function is needed for Sxl initiation. When mothers are defective for her function, their daughters fail to activate a reporter gene for the Sxl early promoter and are deficient in Sxl protein expression. Dosage compensation is misregulated in the moribund daughters: some salivary gland cells show binding of the maleless (mle) dosage compensation regulatory protein to the X chromosome, a binding pattern normally seen only in males. Thus maternal her function is needed early in the hierarchy as a positive regulator of Sxl, and the maternal effects of her on female viability probably reflect Sxl's role in regulating dosage compensation. In contrast to her's maternal function, her's zygotic function in sex determination acts at the end of the hierarchy. This zygotic effect is not rescued by constitutive Sxl expression, nor by constitutive transformer (tra) expression. Moreover, the expression of doublesex (dsx) transcripts appears normal in her mutant females. We conclude that the maternal and zygotic functions of her are needed at two distinctly different levels of the sex determination regulatory hierarchy.

Moving up the hierarchy: a hypothesis on the evolution of a genetic sex determination pathway

A hypothesis on the evolutionary origin of the genetic pathway of sex determination in the nematode Caenorhabditis elegans is presented here. It is suggested that the pathway arose in steps, driven by frequency-dependent selection for the minority sex at each step, and involving the sequential acquisition of dominant negative, neomorphic genetic switches, each one reversing the action of the previous one. A central implication is that the genetic pathway evolved in reverse order from the final step in the hierarchy up to the first. The possible applicability of the model to the other well-characterized sex determination pathway, that of Drosophila melanogaster, and to sex determination in mammals, is discussed, along with some potential implications for pathway evolution in general. Finally, the specific molecular and population genetic questions that the model raises are described and some tests are proposed.

Multiple products from the shavenbaby-ovo gene region of Drosophila melanogaster: relationship to genetic complexity

The Drosophila melanogaster shavenbaby (svb)-ovo gene region is a complex locus, containing two distinct but comutable genetic functions. ovo is required for survival and differentiation of female germ line cells and plays a role in germ line sex determination. In contrast, svb is required in both male and female embryos for the production of epidermal locomotor and sensory structures. Sequences required for the two genetic functions are partially overlapping. ovo corresponds to a previously described germ line-dependent 5.0-kb poly(A)+ mRNA that first appears in the germarium and accumulates in nurse cells during oogenesis. The 5.0-kb mRNA is stored in the egg, but it is rapidly lost in the embryos except for its continued presence in the germ line precursor pole cells. The ovo mRNA predicts a 1,028-amino-acid 110.6-kDa protein homologous with transcription factors. We have identified an embryonic mRNA, 7.1 kb in length, that contains exons partially overlapping those of the 5.0-kb poly(A)+ mRNA. The spatial distribution of this newly discovered transcript during midembryogenesis suggests that it corresponds to the svb function. The arrangement of exons common to the 5.0- and 7.1-kb mRNAs suggests that the Ovo and Svb proteins share DNA-binding specificity conferred by four Cys2-His2 zinc finger motifs but differ functionally in their capacity to interact with other components of the transcription machinery.

Multiple products from the shavenbaby-ovo gene region of Drosophila melanogaster: relationship to genetic complexity

The Drosophila melanogaster shavenbaby (svb)-ovo gene region is a complex locus, containing two distinct but comutable genetic functions. ovo is required for survival and differentiation of female germ line cells and plays a role in germ line sex determination. In contrast, svb is required in both male and female embryos for the production of epidermal locomotor and sensory structures. Sequences required for the two genetic functions are partially overlapping. ovo corresponds to a previously described germ line-dependent 5.0-kb poly(A)+ mRNA that first appears in the germarium and accumulates in nurse cells during oogenesis. The 5.0-kb mRNA is stored in the egg, but it is rapidly lost in the embryos except for its continued presence in the germ line precursor pole cells. The ovo mRNA predicts a 1,028-amino-acid 110.6-kDa protein homologous with transcription factors. We have identified an embryonic mRNA, 7.1 kb in length, that contains exons partially overlapping those of the 5.0-kb poly(A)+ mRNA. The spatial distribution of this newly discovered transcript during midembryogenesis suggests that it corresponds to the svb function. The arrangement of exons common to the 5.0- and 7.1-kb mRNAs suggests that the Ovo and Svb proteins share DNA-binding specificity conferred by four Cys2-His2 zinc finger motifs but differ functionally in their capacity to interact with other components of the transcription machinery.

Molecular characterization of ovarian tumors in Drosophila

Certain female-sterile mutations in Drosophila result in the uncontrolled proliferation of X/X germ cells. It has been proposed that this ovarian tumor phenotype results from the sexual transformation of X/X germ cells to a male identity. We present findings inconsistent with this model. We demonstrate that the tumorous cells produced by mutations in the ovarian tumor (otu), Sex-lethal (Sxl) and sans fille (snf) genes are capable of female-specific transcription and RNA processing. This indicates that these ovarian tumor cells still retain some female identity. Therefore, we propose that mutations in these genes do not cause a male transformation of the X/X germ line but instead either cause an ambiguous sexual identity or block specific stages of oogenesis. Our findings indicate that while Sxl is the master sex determination gene in somatic cells, it appears to play a more subsidiary role in the germ line. Finally, we demonstrate that the germ line function of Sxl depends on the activity of a specific OTU isoform.

Sex determination and dosage compensation: lessons from flies and worms

In both Drosophila melanogaster and Caenorhabditis elegans somatic sex determination, germline sex determination, and dosage compensation are controlled by means of a chromosomal signal known as the X:A ratio. A variety of mechanisms are used for establishing and implementing the chromosomal signal, and these do not appear to be similar in the two species. Instead, the study of sex determination and dosage compensation is providing more general lessons about different types of signaling pathways used to control alternative developmental states of cells and organisms.

Sex determination of the Drosophila germ line: tra and dsx control somatic inductive signals

In Drosophila, the sex of germ cells is determined by cell-autonomous and inductive signals. XY germ cells autonomously enter spermatogenesis when developing in a female host. In contrast, XX germ cells non-autonomously become spermatogenic when developing in a male host. In first instar larvae with two X chromosomes, XX germ cells enter the female or the male pathway depending on the presence or absence of transformer (tra) activity in the surrounding soma. In somatic cells, the product of tra regulates the expression of the gene double sex (dsx) which can form a male-specific or a female-specific product. In dsx mutant larvae, XX and XY germ cells develop abnormally, with a seemingly intersexual phenotype. This indicates that female-specific somatic dsx products feminize XX germ cells, and male-specific somatic dsx products masculinize XX and XY germ cells. The results show that tra and dsx control early inductive signals that determine the sex of XX germ cells and that somatic signals also affect the development of XY germ cells. XX germ cells that develop in pseudomales lacking the sex-determining function of Sxl are spermatogenic. If, however, female-specific tra functions are expressed in these animals, XX germ cells become oogenic. Furthermore, transplanted XX germ cells can become oogenic and form eggs in XY animals that express the female-specific function of tra. Therefore, TRA product present in somatic cells of XY animals or in animals lacking the sex-determining function of Sxl, is sufficient to support developing XX germ cells through oogenesis.

The daughterless gene functions together with Notch and Delta in the control of ovarian follicle development in Drosophila

The daughterless (da) gene in Drosophila encodes a broadly expressed transcriptional regulator whose specific functions in the control of sex determination and neurogenesis have been extensively examined. We describe here a third major developmental role for this regulatory gene: follicle formation during oogenesis. A survey of da RNA and protein distribution during oogenesis reveals a multiphasic expression pattern that includes both germline and soma. Whereas the germline expression reflects da's role in progeny sex determination, the somatic ovary expression of da correlates with the gene's role during egg chamber morphogenesis. Severe, but viable, hypomorphic da mutant genotypes exhibit dramatic defects during oogenesis, including aberrantly defined follicles and loss of interfollicular stalks. The follicular defects observed in da mutant ovaries are qualitatively very similar to those described in Notch (N) or Delta (Dl) mutant ovaries. Moreover, in the ovary da- alleles exhibit dominant synergistic interactions with N or Dl mutations. We propose that all three of these genes function in the same regulatory pathway to control follicle formation.

Sxl in the germline of Drosophila: a target for somatic late induction

In Drosophila, the sex of germ cells is determined by autonomous and inductive signals. Somatic inductive signals can drive XX germ cells into oogenesis or into spermatogenesis. An autonomous signal makes XY germ cells male and unresponsive to sex determination by induction. The elements forming the X:A ratio in the soma and the genes tra, tra2, dsx, and ix that determine the sex of somatic cells have no similar role in the germline. The gene Sxl, however, is required for female differentiation of somatic and germ cells. Inductive signals that are dependent on somatic tra and dsx expression already affect the sex-specific development of germ cells of first instar larvae. At this early stage, however, germline expression of Sxl does not appear to affect the sexual characteristics of germ cells. Since inductive signals dependent on tra and dsx nevertheless influence the choice of sex-specific splicing of Sxl, it can be concluded that Sxl is a target of the inductive signal, but that its product is required late for oogenesis. Other genes must therefore control the early sexual dimorphism of larval germ cells.

Primordial germ cell migration in Drosophila melanogaster is controlled by somatic tissue

In Drosophila, as in many other organisms, primordial germ cells show invasive and migratory behavior moving from their site of origin to the somatic component of the gonad. At a characteristic time in development, the primordial germ cells pass across the primordium of the gut and migrate on its outer surface toward the mesoderm, where they eventually associate with the somatic tissues of the gonad. Here we demonstrate that the exit and migration are specific behaviors of the primordial germ cells and that they are controlled by the somatic tissue of the embryo rather than by a germ cell autonomous clock. Using mutations, we show that these controlling somatic events probably occur in the tissue of the gut primordium itself.

The regulation of sex determination and sexually dimorphic differentiation in Drosophila

Sex determination and sexually dimorphic differentiation in Drosophila involve multiple regulatory mechanisms, including alternative splicing, transcriptional control, subcellular compartmentalization, and intercellular signal transduction. Regulatory interactions occur throughout the development of the fly, some requiring the continuous function of the genes involved, and others being temporally limited, but having permanent consequences. The control of sexual differentiation in Drosophila is, for the most part, subject to the continuous active control of numerous regulatory proteins operating at many levels.

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

The locus ovarian tumor (otu) is involved in several aspects of oogenesis in Drosophila melanogaster. The possible role of otu in the determination of the sexual identity of germ cells has not been extensively explored. Some otu alleles produce a phenotype known as ovarian tumors: ovarioles are filled with numerous poorly differentiated germ cells. We show that these mutant germ cells have a morphology similar to primary spermatocytes and that they express male germ line-specific reporter genes. This indicates that they are engaged along the male pathway of germ line differentiation. Consistent with this conclusion, we found that the splicing of Sex-lethal (Sxl) pre-mRNAs occurs in the male-specific mode in otu-transformed germ cells. The position of the otu locus in the regulatory cascade of germ line sex determination has been studied by using mutations that constitutively express the feminizing activity of the Sxl gene. The sexual transformation of the germ cells observed with several combinations of otu alleles can be reversed by constitutive expression of Sxl. This shows that otu acts upstream of Sxl in the process of germ line sex determination. Other phenotypes of otu mutations were not rescued by constitutive expression of Sxl, suggesting that several functions of otu are likely to be independent of sex determination. Finally, we show that the gene dosage of otu modifies the phenotype of ovaries heterozygous for the dominant alleles of ovo, another gene involved in germ line sex determination. One dose of otu+ enhances the ovoD ovarian phenotypes, while three doses partially suppress these phenotypes. Synergistic interaction between ovoD1 and otu alleles leads to the occasional transformation of chromosomally female germ cells into early spermatocytes. These interactions are similar to those observed between ovoD and one allele of the sans fille (snf) locus. Altogether, our results imply that the otu locus acts, along with ovo, snf, and Sxl, in a pathway (or parallel pathways) required for proper sex determination of the female germ line.

Expression of the Sex-lethal gene is controlled at multiple levels during Drosophila oogenesis

In addition to controlling somatic sexual development in Drosophila melanogaster, the Sex-lethal (Sxl) gene is required for proper differentiation of female germ cells. To investigate its role in germ-line development, we have examined the expression of Sxl in wild-type ovaries and ovaries that are defective in early steps of germ cell differentiation. As in the soma, the basic mechanism for on/off regulation of Sxl relies on sex-specific processing of its transcripts in germ cells. One class of female-sterile mutations, which includes fs(1)1621 and the tumorous-ovary-producing allele of the ovarian tumor gene, otu1, is defective in the splicing process. These mutants have germ lines with high amounts of Sxl RNA spliced in the male mode and a severe reduction of protein levels in the germ cells. Another class of female-sterile mutations produces a phenotype similar to that seen in fs(1)1621 and otu1 but appears to express normal levels of Sxl protein in the germ cells. However, this second class does not show the changes in protein distribution normally observed in wild-type germ cells. In the wild-type germarium, the non-differentiated germ cells show a strong cytoplasmic accumulation of Sxl protein followed, as the germ cells differentiate, by a dramatic reduction and redistribution of the protein into nuclear foci. Interestingly, two female-sterile alleles of Sxl, Sxlf4 and Sxlf5 belong to the second class, which shows persistent cytoplasmic accumulation of Sxl protein. These Sxl female-sterile mutants encode an altered protein indicating that Sxl regulates processes that eventually lead to the changes in Sxl protein distribution. Lastly, we demonstrate that during the final stages of oogenesis several mechanisms must operate to prevent the progeny from inheriting Sxl protein. Conceivably, this regulation safeguards the inadvertent activation of the Sxl autoregulatory feedback loop in the male zygote.