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

Maternal Piwi Regulates Primordial Germ Cell Development to Ensure the Fertility of Female Progeny in Drosophila

In many animals, germline development is initiated by proteins and RNAs that are expressed maternally. PIWI proteins and their associated small noncoding PIWI-interacting RNAs (piRNAs), which guide PIWI to target RNAs by base-pairing, are among the maternal components deposited into the germline of the Drosophila early embryo. Piwi has been extensively studied in the adult ovary and testis, where it is required for transposon suppression, germline stem cell self-renewal, and fertility. Consequently, loss of Piwi in the adult ovary using piwi-null alleles or knockdown from early oogenesis results in complete sterility, limiting investigation into possible embryonic functions of maternal Piwi. In this study, we show that the maternal Piwi protein persists in the embryonic germline through gonad coalescence, suggesting that maternal Piwi can regulate germline development beyond early embryogenesis. Using a maternal knockdown strategy, we find that maternal Piwi is required for the fertility and normal gonad morphology of female, but not male, progeny. Following maternal piwi knockdown, transposons were mildly derepressed in the early embryo but were fully repressed in the ovaries of adult progeny. Furthermore, the maternal piRNA pool was diminished, reducing the capacity of the PIWI/piRNA complex to target zygotic genes during embryogenesis. Examination of embryonic germ cell proliferation and ovarian gene expression showed that the germline of female progeny was partially masculinized by maternal piwi knockdown. Our study reveals a novel role for maternal Piwi in the germline development of female progeny and suggests that the PIWI/piRNA pathway is involved in germline sex determination in Drosophila.

CRISPR Disruption of BmOvo Resulted in the Failure of Emergence and Affected the Wing and Gonad Development in the Silkworm Bombyx mori

The domesticated silkworm is an economically important insect that is widely used as a lepidopteran insect model. Although somatic sex determination in the silkworm is well characterized, germline sex determination is not. Here, we used the transgenic-based CRISPR/Cas9 genome editing system to study the function of the Ovo gene in Bombyx mori. BmOvo is the homolog of a factor important in germline sex determination in Drosophila melanogaster. BmOvo mutants had abnormally shaped eggs that were disordered in the ovarioles, and gonad development was abnormal. Interestingly, wing discs and wings did not develop properly, and most of the mutants failed to eclose. Gene expression analyses by qRT-PCR showed that BmOvo gene was highly expressed in the wing disc and epidermis. Genes involved in the WNT signaling pathway and wing development genes BmWCP10 and BmE74 were downregulated in the BmOvo mutants when compared with wild-type animals. These results demonstrate that the BmOvo gene product plays an important role in wing metamorphosis. Thus, this study provides new insights into the multiple functions of BmOvo beyond germline sex determination.

Functional characterization of BmOVOs in silkworm, Bombyx mori

Background

In our previous study, we identified four isoforms of the Bmovo gene, Bmovo-1, Bmovo-2, Bmovo-3 and Bmovo-4 from the silkworm ovary and verified that ovarian development was regulated by the BmOVO proteins. Results: To understand the regulatory mechanisms of ovarian development, the regulation of four BmOVO isoforms on the B. mori ovarian tumor (Bmotu) promoter activity was investigated with luciferase reporter assays. The results showed the Bmotu promoter activity was positively regulated by BmOVO-1, BmOVO-2, BmOVO-3 and BmOVO-4 in a dose-dependent manner, of which BmOVO-2 had the highest transcriptional activation. However, the first (A1) and third acidic domains (A3) at the N-terminus of BmOVO-1 are transcriptional repression domains, while the fourth (A4) and fifth acidic domains (A5) are transcriptional activation domains. A recombinant BmOVO zinc-finger domain was found to bind to the GTACCGTTGTA sequence located at the Bmotu promoter. Furthermore, the Bmotu promoter activity was negatively regulated by ‘Tal-like’ peptide, which can trigger BmOVO-1 degradation at the N-terminus.

Conclusions

These results will help us to further understand the regulatory mechanisms of BmOVO isoforms on Bmotu promoter activity and ovarian development in the silkworm.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5697-y) contains supplementary material, which is available to authorized users.

Transcripts immunoprecipitated with Sxl protein in primordial germ cells of Drosophila embryos

In Drosophila, Sex lethal (Sxl), an RNA binding protein, is required for induction of female sexual identity in both somatic and germline cells. Although the Sxl-dependent feminizing pathway in the soma was previously elucidated, the downstream targets for Sxl in the germline remained elusive. To identify these target genes, we selected transcripts associated with Sxl in primordial germ cells (PGCs) of embryos using RNA immunoprecipitation coupled to sequencing (RIP-seq) analysis. A total of 308 transcripts encoded by 282 genes were obtained. Seven of these genes, expressed at higher levels in PGCs as determined by microarray and in situ hybridization analyses, were subjected to RNAi-mediated functional analyses. Knockdown of Neos, Kap-alpha3, and CG32075 throughout germline development caused gonadal dysgenesis in a sex-dependent manner, and Su(var)2-10 knockdown caused gonadal dysgenesis in both sexes. Moreover, as with knockdown of Sxl, knockdown of Su(var)2-10 in PGCs gave rise to a tumorous phenotype of germline cells in ovaries. Because this phenotype indicates loss of female identity of germline cells, we consider Su(var)2-10 to be a strong candidate target of Sxl in PGCs. Our results represent a first step toward elucidating the Sxl-dependent feminizing pathway in the germline.

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.

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.

Ovarian tumor expression is dependent on the functions of the somatic sex regulatory genes transformer-2 and doublesex

The doublesex-dependent sex regulatory pathway in Drosophila controls major aspects of somatic sexual differentiation, but its expression is not required in the X/X germline. Nevertheless, mutations in doublesex and in the genes that directly regulate its expression, transformer and transformer-2, disrupt early stages of oogenic differentiation to produce gonads containing immature germ cells. This indicates a critical, but uncharacterized, set of soma-germline interactions essential for oogenesis. In this paper, we examined the effects of mutations in transformer-2 on the expression and function of the germline-specific ovarian tumor gene. We demonstrated that in transformer-2 mutants, there was a marked reduction in the activity of the ovarian tumor promoter in the mutant germline. In addition, the phenotypic effects on the arrested germline could be partially suppressed by the simultaneous over-expression of both ovarian tumor and a second germline gene, Sex-lethal. This differs from transformer mutations, in which the over-expression of ovarian tumor alone is sufficient for a similar improvement in germline differentiation. In contrast to transformer-2, doublesex activity was not required for ovarian tumor promoter activity and we found indirect evidence that the doublesex male-specific function might have a negative regulatory effect. These data indicate that the components of the genetic pathway regulating somatic sexual differentiation have specific and differential effects on germline gene activity.

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.

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.

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.

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.

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.

Regulatory and functional interactions between the somatic sex regulatory gene transformer and the germline genes ovo and ovarian tumor

In Drosophila, compatibility between the sexually differentiated state of the soma and the sex chromosome constitution of the germline is required for normal gametogenesis. In this study, we defined important aspects of the soma-germline interactions controlling early oogenesis. In particular, the sex-specific germline activity of the ovarian tumor promoter was found to be dependent upon somatic factors controlled by the somatic sex differentiation gene transformer. This regulation defines whether there is sufficient ovarian tumor expression in adult XX germ cells to support oogenesis. In addition, the ovarian tumor function required for female germline differentiation is dependent on the activity of another germline gene, ovo, whose regulation is transformer-independent. These and other data indicate that ovarian tumor plays a central role in coordinating regulatory inputs from the soma (as regulated by transformer) with those from the germline (involving ovo). We also demonstrate that transformer-dependent interactions influence whether XX germ cells require ovarian tumor or ovo functions to undergo early gametogenic differentiation. These results are incorporated into a model hypothesizing that the functions of ovarian tumor and ovo are dependent on an early sex determination decision in the XX germline that is at least partially controlled by somatic transformer activity.

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.

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.

Germ cell development in Drosophila

More than 100 years have passed since Weismann first recognized the role of germ cells in the continuity of a species. Today, it remains unclear how a germ cell is initially set aside from somatic cells and how it chooses its unique developmental path. In this review, we address various aspects of germ cell development in Drosophila, such as germ cell determination, germ cell migration, gonad formation, sex determination, and gametogenesis. Many aspects of germ cell development, including the morphology of germ cells, their migratory behavior, as well as the processes of gonad formation and gametogenesis, show striking similarities among organisms. Considering the conservation of factors that regulate somatic development, it is likely that some aspects of germ cell development are shared not only on a morphological but also on the molecular level between Drosophila and other organisms.

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.

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.