Suppression of distinct ovo phenotypes in the Drosophila female germline by maleless- and Sex-lethal

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.

The Drosophila Helicase MLE Targets Hairpin Structures in Genomic Transcripts

RNA hairpins are a common type of secondary structures that play a role in every aspect of RNA biochemistry including RNA editing, mRNA stability, localization and translation of transcripts, and in the activation of the RNA interference (RNAi) and microRNA (miRNA) pathways. Participation in these functions often requires restructuring the RNA molecules by the association of single-strand (ss) RNA-binding proteins or by the action of helicases. The Drosophila MLE helicase has long been identified as a member of the MSL complex responsible for dosage compensation. The complex includes one of two long non-coding RNAs and MLE was shown to remodel the roX RNA hairpin structures in order to initiate assembly of the complex. Here we report that this function of MLE may apply to the hairpins present in the primary RNA transcripts that generate the small molecules responsible for RNA interference. Using stocks from the Transgenic RNAi Project and the Vienna Drosophila Research Center, we show that MLE specifically targets hairpin RNAs at their site of transcription. The association of MLE at these sites is independent of sequence and chromosome location. We use two functional assays to test the biological relevance of this association and determine that MLE participates in the RNAi pathway.

The translation initiation factor eIF4E regulates the sex-specific expression of the master switch gene Sxl in Drosophila melanogaster

In female fruit flies, Sex-lethal (Sxl) turns off the X chromosome dosage compensation system by a mechanism involving a combination of alternative splicing and translational repression of the male specific lethal-2 (msl-2) mRNA. A genetic screen identified the translation initiation factor eif4e as a gene that acts together with Sxl to repress expression of the Msl-2 protein. However, eif4e is not required for Sxl mediated repression of msl-2 mRNA translation. Instead, eif4e functions as a co-factor in Sxl-dependent female-specific alternative splicing of msl-2 and also Sxl pre-mRNAs. Like other factors required for Sxl regulation of splicing, eif4e shows maternal-effect female-lethal interactions with Sxl. This female lethality can be enhanced by mutations in other co-factors that promote female-specific splicing and is caused by a failure to properly activate the Sxl-positive autoregulatory feedback loop in early embryos. In this feedback loop Sxl proteins promote their own synthesis by directing the female-specific alternative splicing of Sxl-Pm pre-mRNAs. Analysis of pre-mRNA splicing when eif4e activity is compromised demonstrates that Sxl-dependent female-specific splicing of both Sxl-Pm and msl-2 pre-mRNAs requires eif4e activity. Consistent with a direct involvement in Sxl-dependent alternative splicing, eIF4E is associated with unspliced Sxl-Pm pre-mRNAs and is found in complexes that contain early acting splicing factors—the U1/U2 snRNP protein Sans-fils (Snf), the U1 snRNP protein U1-70k, U2AF38, U2AF50, and the Wilms' Tumor 1 Associated Protein Fl(2)d—that have been directly implicated in Sxl splicing regulation.

Gene expression in eukaryotes is a complex process that occurs in several discrete steps. Some of those steps are separated into different sub-cellular compartments and thus might be expected to occur independently of one another and involve entirely distinct factors. For example pre-mRNA splicing takes place in the nucleus where it is coupled with transcription, while mRNA translation requires export to the cytoplasm and ribosome loading. We describe studies on the fruit fly Drosophila which indicate that a cytoplasmic translation initiation factor, the cap binding protein eIF4E, plays a key role in alternative splicing in the nucleus. When eIF4E activity is compromised, we observe defects in sex-specific splicing of pre-mRNAs that are regulated by the sex determination master switch gene Sex-lethal. Our data argue that eIF4E likely plays a direct role in the regulation of alternative splicing by Sex-lethal.

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.

The control of sexual identity in the Drosophila germline

Whether to be male or female is a critical decision in development. Nowhere is this more important than in the germ cells, which must produce either the sperm or eggs necessary for the perpetuation of the species. How does a germ cell make this decision and how is it executed? One thing that is clear is that this process is very different in germ cells compared with other cells of the embryo. Here, we explore how sexual identity is established in the Drosophila germline, how this affects other aspects of germ cell development and what studies in Drosophila can teach us about mammalian germ cells.

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.

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.

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.

Sequence conservation and expression of the sex-lethal homologue in the fly Megaselia scalaris

Sex-lethal (Sxl) is Drosophila melanogaster's key regulating gene in the sex-determining cascade. Its homologue in Megaselia scalaris, the chromosome 3 gene Megsxl, codes for a protein with an overall similarity of 77% with the corresponding D. melanogaster sequence. Expression in M. scalaris, however, is very unlike that in D. melanogaster. Megsxl transcripts with a long ORF occur in both sexes. Differential splicing is conserved but not sex-specific. There are several splice variants, among them one is common to gonads and somatic tissues of all developmental stages investigated, one is specific for ovaries and embryos, and a third one is not found in ovaries. In the ovary, Megsxl is heavily transcribed in nurse cells and transported into eggs. These results suggest a non-sex-determining function during early embryogenesis; the presence of Megsxl RNA in testes and somatic tissues calls for other (or more) functions.

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.