The interaction between daughterless and sex-lethal in triploids: a lethal sex-transforming maternal effect linking sex determination and dosage compensation in Drosophila melanogaster

The Drosophila CPEB Protein Orb Specifies Oocyte Fate by a 3'UTR-Dependent Autoregulatory Loop

orb encodes one of the two fly CPEB proteins. These widely conserved proteins bind to the 3'UTRs of target messenger RNAs (mRNAs) and activate or repress their translation. We show here that a positive autoregulatory loop driven by the orb gene propels the specification of oocyte identity in Drosophila egg chambers. Oocyte fate specification is mediated by a 3'UTR-dependent mechanism that concentrates orb mRNAs and proteins in one of the two pro-oocytes in the 16-cell germline cyst. When the orb 3'UTR is deleted, orb mRNA and protein fail to localize and all 16 cells become nurse cells. In wild type, the oocyte is specified when orb and other gene products concentrate in a single cell in region 2b of the germarium. A partially functional orb 3'UTR replacement delays oocyte specification until the egg chambers reach stage 2 of oogenesis. Before this point, orb mRNA and protein are unlocalized, as are other markers of oocyte identity, and the oocyte is not specified. After stage 2, ∼50% of the chambers successfully localize orb in a single cell, and this cell assumes oocyte identity. In the remaining chambers, the orb autoregulatory loop is not activated and no oocyte is formed. Finally, maintenance of oocyte identity requires continuous orb activity.

Sex and the Single Fly: A Perspective on the Career of Bruce S. Baker

Bruce Baker, a preeminent Drosophila geneticist who made fundamental contributions to our understanding of the molecular genetic basis of sex differences, passed away July 1, 2018 at the age of 72. Members of Bruce's laboratory remember him as an intensely dedicated, rigorous, creative, deep-thinking, and fearless scientist. His trainees also remember his strong commitment to teaching students at every level. Bruce's career studying sex differences had three major epochs, where the laboratory was focused on: (1) sex determination and dosage compensation, (2) the development of sex-specific structures, and (3) the molecular genetic basis for sex differences in behavior. Several members of the Baker laboratory have come together to honor Bruce by highlighting some of the laboratory's major scientific contributions in these areas.

Sex Differences in Drosophila melanogaster Heterochromatin Are Regulated by Non-Sex Specific Factors

The eukaryotic genome is assembled into distinct types of chromatin. Gene-rich euchromatin has active chromatin marks, while heterochromatin is gene-poor and enriched for silencing marks. In spite of this, genes native to heterochromatic regions are dependent on their normal environment for full expression. Expression of genes in autosomal heterochromatin is reduced in male flies mutated for the noncoding roX RNAs, but not in females. roX mutations also disrupt silencing of reporter genes in male, but not female, heterochromatin, revealing a sex difference in heterochromatin. We adopted a genetic approach to determine how this difference is regulated, and found no evidence that known X chromosome counting elements, or the sex determination pathway that these control, are involved. This suggested that the sex chromosome karyotype regulates autosomal heterochromatin by a different mechanism. To address this, candidate genes that regulate chromosome organization were examined. In XX flies mutation of Topoisomerase II (Top2), a gene involved in chromatin organization and homolog pairing, made heterochromatic silencing dependent on roX, and thus male-like. Interestingly, Top2 also binds to a large block of pericentromeric satellite repeats (359 bp repeats) that are unique to the X chromosome. Deletion of X heterochromatin also makes autosomal heterochromatin in XX flies dependent on roX and enhances the effect of Top2 mutations, suggesting a combinatorial action. We postulate that Top2 and X heterochromatin in Drosophila comprise a novel karyotype-sensing pathway that determines the sensitivity of autosomal heterochromatin to loss of roX RNA.

Novel female-specific splice form of dsx in the silkworm, Bombyx mori

The Bombyx mori doublesex (Bmdsx), a homologue of doublesex of Drosophila, is the bottom most gene of the sex determination cascade. Bmdsx plays a very crucial role in somatic sexual development. Its pre-mRNA sex-specifically splices to generate two splice variants; one encodes female-specific and the other encodes male-specific polypeptides which differ only at their C-termini. The open reading frame of Bmdsx consists of 5 exons, of which exons 3 and 4 are female-specific and are skipped in males. In the present study, we have identified a third splice form of the Bmdsx which is specific only to females and differs from the previously reported Bmdsxf isoform by the presence of 15 bp sequence. This new female splice form is generated as a result of alternative 5' splice site selection in the third exon adding additional 15 bp sequence in exon 3 which results in alteration of the reading frame leading to incorporation of an early stop codon. Thus the protein encoded by this splice form is 20 aa shorter than the known BmDsxF. Initial results obtained from the study of dsx homologues in Saturniid silkmoths suggest that both the female-specific Dsx proteins are essential for female sexual differentiation. It remains to be seen whether female-specific multiple splice forms of dsx are characteristic feature of only silkmoths or widespread among lepidopterans. The findings that sex determination mechanism is unique in lepidopterans offer an opportunity to develop genetic sexing methods in beneficial as well as economically destructive lepidopteran pests.

Cellular autonomy of hyperactivity in segmentalXchromosomal aneuploids ofDrosophilaand dosage compensation

Transcription in 9A–11A aneuploid mosaic female larvae ofDrosophila melanogasterhas been assessed autoradiographically. Eleven larvae were found to exhibit mosaicism out of sixty-six larvae scanned and the percentage ofXOandXXnuclei varied from approximately 9 to 100. Irrespective of the number ofXXnuclei present theXOnuclei (duplicated for 9A–11A) invariably showed hyperactivity for both the segments. TheXXnucleus exhibited a dosage effect for all the three segments of 9A–11A. Results support the transcriptional constancy of the entireXchromosome, as proposed by Maroni and Lucchesi. Cellular autonomy of hyperactivity of the singleXchromosome even at the level of segments of theXis thus evident from the present results.

Sexual back talk with evolutionary implications: stimulation of the Drosophila sex-determination gene sex-lethal by its target transformer

We describe a surprising new regulatory relationship between two key genes of the Drosophila sex-determination gene hierarchy, Sex-lethal (Sxl) and transformer (tra). A positive autoregulatory feedback loop for Sxl was known to maintain somatic cell female identity by producing SXL-F protein to continually instruct the target gene transformer (tra) to make its feminizing product, TRA-F. We discovered the reciprocal regulatory effect by studying genetically sensitized females: TRA-F from either maternal or zygotic tra expression stimulates Sxl-positive autoregulation. We found female-specific tra mRNA in eggs as predicted by this tra maternal effect, but not predicted by the prevailing view that tra has no germline function. TRA-F stimulation of Sxl seems to be direct at some point, since Sxl harbors highly conserved predicted TRA-F binding sites. Nevertheless, TRA-F stimulation of Sxl autoregulation in the gonadal soma also appears to have a cell-nonautonomous aspect, unprecedented for somatic Sxl regulation. This tra-Sxl retrograde regulatory circuit has evolutionary implications. In some Diptera, tra occupies Sxl's position as the gene that epigenetically maintains female identity through direct positive feedback on pre-mRNA splicing. The tra-mediated Sxl feedback in Drosophila may be a vestige of regulatory redundancy that facilitated the evolutionary transition from tra to Sxl as the master sex switch.

Indirect effects of ploidy suggest X chromosome dose, not the X:A ratio, signals sex in Drosophila

In the textbook view, the ratio of X chromosomes to autosome sets, X:A, is the primary signal specifying sexual fate in Drosophila. An alternative idea is that X chromosome number signals sex through the direct actions of several X-encoded signal element (XSE) proteins. In this alternative, the influence of autosome dose on X chromosome counting is largely indirect. Haploids (1X;1A), which possess the male number of X chromosomes but the female X:A of 1.0, and triploid intersexes (XX;AAA), which possess a female dose of two X chromosomes and the ambiguous X:A ratio of 0.67, represent critical tests of these hypotheses. To directly address the effects of ploidy in primary sex determination, we compared the responses of the signal target, the female-specific SxlPe promoter of the switch gene Sex-lethal, in haploid, diploid, and triploid embryos. We found that haploids activate SxlPe because an extra precellular nuclear division elevates total X chromosome numbers and XSE levels beyond those in diploid males. Conversely, triploid embryos cellularize one cycle earlier than diploids, causing premature cessation of SxlPe expression. This prevents XX;AAA embryos from fully engaging the autoregulatory mechanism that maintains subsequent Sxl expression, causing them to develop as sexual mosaics. We conclude that the X:A ratio predicts sexual fate, but does not actively specify it. Instead, the instructive X chromosome signal is more appropriately seen as collective XSE dose in the early embryo. Our findings reiterate that correlations between X:A ratios and cell fates in other organisms need not implicate the value of the ratio as an active signal.

A Theoretical Model for the Regulation of Sex-lethal, a Gene That Controls Sex Determination and Dosage Compensation in Drosophila melanogaster

Cell fate commitment relies upon making a choice between different developmental pathways and subsequently remembering that choice. Experimental studies have thoroughly investigated this central theme in biology for sex determination. In the somatic cells of Drosophila melanogaster, Sex-lethal (Sxl) is the master regulatory gene that specifies sexual identity. We have developed a theoretical model for the initial sex-specific regulation of Sxl expression. The model is based on the well-documented molecular details of the system and uses a stochastic formulation of transcription. Numerical simulations allow quantitative assessment of the role of different regulatory mechanisms in achieving a robust switch. We establish on a formal basis that the autoregulatory loop involved in the alternative splicing of Sxl primary transcripts generates an all-or-none bistable behavior and constitutes an efficient stabilization and memorization device. The model indicates that production of a small amount of early Sxl proteins leaves the autoregulatory loop in its off state. Numerical simulations of mutant genotypes enable us to reproduce and explain the phenotypic effects of perturbations induced in the dosage of genes whose products participate in the early Sxl promoter activation.

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.

The helix-loop-helix proteins dAP-4 and daughterless bind both in vitro and in vivo to SEBP3 sites required for transcriptional activation of the Drosophila gene Sgs-4

The expression of Sgs genes in the salivary gland of the third instar larva of Drosophila is a spatially restricted response to signalling by the steroid hormone 20-hydroxyecdysone. For Sgs-4, we have previously demonstrated that its strictly tissue and stage-specific expression is the result of combined action of the ecdysone receptor and secretion enhancer binding proteins (SEBPs). One of these SEBPs, SEBP2, was shown to be the product of the homeotic gene fork head. Together with SEBP3, SEBP2 appears to be responsible for the spatial restriction of the hormone response of Sgs-4. Here, we show that SEBP3 is a heterogeneous binding activity that consists of different helix-loop-helix (HLH) proteins. We cloned the Drosophila homologue of human transcription factor AP-4 (dAP-4) and identified it as one of these HLH proteins. The dAP-4 protein shows great similarity to its human and Caenorhabditis counterparts within the bHLHZip domain, the second leucine zipper dimerization motif, and a third region of unknown function. The expression pattern of dAP-4 indicates that it is a ubiquitously expressed HLH protein in Drosophila. As a second component of SEBP3 we identified the Daughterless (Da) protein, which is also ubiquitously expressed and binds to SEBP3 sites independent of dAP-4. Since both dAP-4 and Da can be detected in situ at transposed Sgs-4 transcriptional control elements in polytene salivary gland chromosomes, we propose that each of the two proteins contributes to the transcriptional control of Sgs-4.

The male-determining activity on the Y chromosome of the housefly (Musca domestica L.) consists of separable elements

In the common housefly, the presence or absence of a male-determining factor, M, is responsible for sex determination. In different strains, M has been found on the Y, on the X, or on any of the five autosomes. By analyzing a Y-autosomal translocation and a ring-shaped, truncated Y chromosome, we could show that M on the Y consists of at least two regions with M activity: One of them can be assigned to the short arm of the Y chromosome (MYS), which is largely C-banding negative, the other region lies on the C-banding positive long arm of the Y, including the centromeric part (MYL). Each region alone behaves as a hypomorphic M factor, causing many carriers to develop as intersexes of the mosaic type instead of as males. When introduced into the female germ line by transplantation of progenitor germ cells (pole cells), the MYS shows an almost complete maternal effect that predetermines 96% of the genotypic female (NoM) animals to develop as males. In contrast, the MYL has largely lost its maternal effect, and most of the NoM animals develop as females. Increasing the amount of product made by either of the two hypomorphic M factors (by combining the MYS and MYL or two MYS) leads to complete male development in almost every case. We thus assume that the Y chromosome carries at least two copies of M, and that these are functionally equivalent.

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.

Sex-specific control of Sex-lethal is a conserved mechanism for sex determination in the genus Drosophila

In D. melanogaster the binary switch gene Sex-lethal (Sxl) plays a pivotal role in somatic sex determination -- when the Sxl gene is on the female pathway is followed, while the male pathway is followed when the gene is off. In the present study we have asked whether the Sxl gene is present in other species of the genus Drosophila and whether it is subject to a similar sex-specific on-off regulation. Sxl proteins were found in all of the drosophilids examined, and they display a sex-specific pattern of expression. Furthermore, characterization of the Sxl gene in the distant drosophilan relative, D. virilis, reveals that the structure and sequence organization of the gene has been well conserved and that, like melanogaster, alternative RNA processing is responsible for its sex-specific expression. Hence, this posttranscriptional on-off regulatory mechanism probably existed before the separation of the drosophilan and sophophoran subgenera and it seems likely that Sxl functions as a sex determination switch gene in most species in the Drosophila genus. Although alternative splicing appears to be responsible for the on-off regulation of the Sxl gene in D. virilis, this species is unusual in that Sxl proteins are present not only in females but also in males. The D. virilis female and male proteins appear to be identical over most of the length except for the amino-terminal approx. 25 aa which are encoded by the differentially spliced exons. In transcriptionally active polytene chromosomes, the male and female proteins bind to the same cytogenetic loci, including the sites corresponding to the D. virilis Sxl and tra genes. Hence, though the male proteins are able to interact with appropriate target pre-mRNAs, they are apparently incapable of altering the splicing pattern of these pre-mRNAs.

The dosage compensation regulators MLE, MSL-1 and MSL-2 are interdependent since early embryogenesis in Drosophila

We have analyzed the expression pattern and localization of MLE, MLS-1, MSL-2, and histone H4Ac16 during embryogenesis to determine when msl-dependent dosage compensation begins. Maternal MSL-1 and MLE are present in both sexes at fertilization. MSL-2 lacks a maternal component, and male-specific zygotic expression is detectable at the end of blastoderm. During germ band extension, MSL-1, MSL-2, MLE, and histone H4Ac16 display coincident sub-nuclear localization in male embryos. In embryos lacking one of the MSL proteins, the sub-nuclear localization of the other MSLs and of histone H4Ac16 is not detected. We conclude that the MSL proteins associate with the X chromosome and are interdependent since early embryogenesis.

Transcriptional regulation of the Sex-lethal gene by helix-loop-helix proteins

Somatic sex determination in Drosophila depends on the expression of Sex-lethal (Sxl), whose level is determined by the relative number of X chromosomes and sets of autosomes (X:A ratio). The first step in regulation of Sxl expression is transcriptional control from its early promoter and several genes encoding transcription factors of the helix-loop-helix (HLH) family such as daughterless (da), sisterless-b (sis-b), deadpan (dpn) and extramacrochaetae (emc) have been implicated. By the use of transfection assays and in vitro binding experiments, here we show that da/sis-b heterodimers bind several sites on the Sxl early promoter with different affinities and consequently tune the level of active transcription from this promoter. Interestingly, our data indicate that repression by the dpn product of da/sis-b dependent activation results from specific binding of dpn protein to a unique site within the promoter. This contrasts with the mode of emc repression, which inhibits the formation of the da/sis-b heterodimers. These results reveal the molecular mechanisms by which Sxl gene transcription is positively or negatively regulated to control somatic sex determination.

scute (sis-b) function in Drosophila sex determination

The primary sex determination signal, the X chromosome-to-autosome (X/A) ratio, controls the choice of sexual identity in the Drosophila melanogaster embryo by regulating the activity of the early promoter of the Sex-lethal gene, Sxl-Pe. This promoter is activated in females (2X/2A), while it remains off in males (1X/2A). Promoter activation in females is dependent upon X-linked numerator genes. One of these genes, sisterless-b (sis-b), corresponds to the scute (sc) locus of the achaete-scute complex, and it encodes a helix-loop-helix transcription factor. In the studies reported here we have used monoclonal antibodies to study the expression and functioning of the sc(sis-b) protein. Sc is first detected at nuclear cycle 6 to 7, well before Sxl-Pe is first active. At this stage, the protein is in the cytoplasm, not the nucleus. Only after the formation of the syncytial blastoderm, at nuclear cycle 10 to 11, does a substantial fraction of the protein enter the nucleus, and this nuclear import closely coincides with the initial activation of Sxl-Pe. Consistent with the idea that the dose of sc(sis-b) is critical for its function as an X-chromosome counting element, wild-type syncytial blastoderm embryos could be grouped into two classes based on the level of protein. Western blot (immunoblot) analysis demonstrates that this difference in protein level correlates directly with the activity state of the Sxl gene. Finally, we provide the first direct evidence that Sc forms heteromeric complexes in vivo in early embryos with the maternally derived helix-loop-helix protein Daughterless. This in vivo complex is likely to be critical for Sc function in Sxl-Pe activation.

Expression of msl-2 causes assembly of dosage compensation regulators on the X chromosomes and female lethality in Drosophila

Male-specific lethal-2 (msl-2) is a RING finger protein that is required for X chromosome dosage compensation in Drosophila males. Consistent with the formation of a dosage compensation protein complex, msl-2 colocalizes with the other MSL proteins on the male X chromosome and coimmunoprecipitates with msl-1 from male larval extracts. Ectopic expression of msl-2 in females results in the appearance of the other MSL dosage compensation regulators on the female X chromosomes and decreased female viability. We suggest that msl-2 RNA is the primary target of SxI regulation in the dosage compensation pathway and present a speculative model for the regulation of two distinct modes of dosage compensation by SxI.

Multiple response elements in the Sex-lethal early promoter ensure its female-specific expression pattern

The choice of sexual identity in somatic tissues of the fruit fly Drosophila melanogaster is determined early in embryogenesis by the X-chromosome-to-autosome (X/A) ratio. The system that signals the X/A ratio selects the sexual development pathway by determining the activity state of the binary switch Sex-lethal (Sxl). In 2X/2A animals, the X/A signalling system turns the Sxl gene on, ultimately activating an RNA-splicing autoregulatory feedback loop which serves to maintain the female state during the remainder of development. In 1X/2A animals, this autoregulatory feedback loop is not activated and the male state is subsequently maintained by the default splicing machinery. In the studies reported here, we have examined how the X/A signalling system controls the initial choice of sexual identity through its action on a special early embryonic Sxl promoter, Sxl-Pe. We show that in the early embryo, the activity of Sxl-Pe is controlled in a highly dose-sensitive fashion by the genes on the X chromosome that function as numerator elements and by genes located on the autosomes that function as denominator elements. Functional dissection of Sxl-Pe indicates that activating the promoter in females requires the cumulative action of multiple numerator genes which appear to exert their effects through reiterated cis-acting target sites in the promoter. Conversely, maintaining the promoter silent in males requires the repressive activities of denominator genes, and at least one of the denominator genes also appears to function through target sequences within the promoter.

Polytene chromosomes of monogenic and amphogenic Chrysomya species (Calliphoridae, Diptera): analysis of banding patterns and in situ hybridization with Drosophila sex determining gene sequences

Standard maps for the five banded polytene chromosomes found in trichogen cell nuclei of the monogenic blowfly Chrysomya rufifacies and the amphogenic Chrysomya pinguis are presented. The chromosomes are highly homologous in the two species; differences in banding patterns are predominantly caused by one pericentric and ten paracentric inversions. In chromosome 5 of the amphogenic Chrysomya phaonis, also analysed in this paper, an additional paracentric inversion was observed. The distribution of species specific inversions indicates that the monogenic C. rufifacies is phylogenetically older than the amphogenic species. The maternal sex realizer locus F'/f on polytene chromosome 5 of C. rufifacies is not associated with a structural heterozygosity. Chromosome pair 6 of C. rufifacies and the sex chromosome pair of C. pinguis are under-replicated in polytene nuclei; they consist of irregular chromatin granules, frequently associated with nucleolus material. Evolution of heteromorphic sex chromosomes in Chrysomya is probably correlated with heterochromatin accumulation. A search for sex determining genes in Chrysomya was initiated using sex determining sequences from Drosophila melanogaster for in situ hybridization. The polytene band 41A1 on chromosome 5 of monogenic and amphogenic Chrysomya species contains sequences homologous to the maternal sex determining gene daughterless (da). Homology to the zygotic gene Sex-lethal (Sxl) of Drosophila is detected in band 39A1 on chromosome 5 of C. rufifacies. The findings reported here are the first evidence for a possible homology between the da gene of Drosophila and the maternal sex realizer F' of C. rufifacies. An hypothesis for the evolution of the maternal effect sex determination of C. rufifacies is proposed.

The Drosophila sex determination signal: how do flies count to two?

Seventy years after the discovery that sex in Drosophila melanogaster is determined by the balance between X chromosomes and autosomes, we can finally identify some of the specific genes whose relative dosage is responsible for the male/female decision in somatic cells and study how they act at the molecular level. Discovery of these sex determination genes was delayed because their mutant phenotypes were unanticipated. It now seems appropriate to consider how the concept of the X/A balance may have limited thinking about the fruit fly sex determination signal.

A bZIP protein, sisterless-a, collaborates with bHLH transcription factors early in Drosophila development to determine sex

Sexual identity in Drosophila is determined by zygotic X-chromosome dose. Two potent indicators of X-chromosome dose are sisterless-a (sis-a) and sisterless-b (sis-b). Genetic analysis has shown that a diplo-X dose of these genes activates their regulatory target, the feminizing switch gene Sex-lethal (Sxl), whereas a haplo-X dose leaves Sxl inactive. sis-b encodes a transcriptional activator of the bHLH family that dimerizes with several other HLH proteins required for the proper assessment of X dose. Here, we report that sis-a encodes a bZIP protein homolog that functions in all somatic nuclei to activate Sxl transcription. In contrast with other elements of the sex-determination signal, the functioning of this transcription factor in somatic cells may be specific to X-chromosome counting. Using in situ hybridization, we determined the time course of sis-a, sis-b, and Sxl transcription during the first few hours after fertilization. The pattern of sis-a RNA accumulation is very similar to that for sis-b, with a peak in nuclear cycle 12 at about the time of onset of Sxl transcription. Considered in the context of other studies, these results suggest that the ability to distinguish one X from two is attributable to combinatorial interactions between bZIP and bHLH proteins and their target, Sxl, as well as to positive and negative interactions with maternally supplied and zygotically produced proteins.

The daughterless gene product in Drosophila is a nuclear protein that is broadly expressed throughout the organism during development

The daughterless (da) gene in Drosophila functions in the regulation of at least three significant developmental pathways: sex determination, neurogenesis and oogenesis. As a member of the helix-loop-helix (HLH) family of DNA binding proteins, the da gene product appears to act as a transcription factor. Based on the genetic and molecular characterization of da, it has been proposed that the da protein (Da) functions as a generic member of this family, serving throughout development as a necessary binding partner for an assortment of other HLH proteins. As a result of temporally and/or spatially restricted expression, these binding partners would provide some regulatory specificity to the functional transcription complex. In order to participate in this way in the regulation of multiple genes, Da must be expressed in numerous times and places during development. Using anti-Da antibodies, we validate two predictions of this scenario of Da function: (1) Da protein is not only nuclear localized, but also associated with chromosomes in vivo; and (2) Da protein is widely distributed, both spatially and temporally, throughout development. With regard to the essential role of maternal da+ in progeny sex determination, little, if any, Da protein is synthesized in the maternal germline. This suggests that the female-specific germline function of da+ is provided to the zygote as maternally synthesized RNA that becomes translated early in embryogenesis.

achaete-scute feminizing activities and Drosophila sex determination

Sex determination in Drosophila depends on X-linked ‘numerator’ genes activating early Sex-lethal (Sxl) transcription in females. One numerator gene, sisterless-b (sis-b), corresponds to the achaete-scute (AS-C) T4 basic-helix-loop-helix (bHLH) gene. Two other closely related AS-C bHLH genes, T3 and T5, appear not to function as numerator elements. We analyzed endogenous AS-C expression and show that T4 is the major AS-C numerator gene because it is expressed earlier and more strongly than are T3 and T5. Only T4 expression is detectable during the early syncytial stages when Sxl state is being determined. Nevertheless, the effects of ectopic AS-C gene expression show that T3 and T5 proteins display weak but significant feminizing activities, enhancing male-lethality, and rescuing the female-lethality of sis mutations. Detailed examination of Sxl expression in rescued embryos suggests that female cells may be viable in the absence of detectable Sxl protein expression.

Regulation of the sex-specific binding of the maleless dosage compensation protein to the male X chromosome in Drosophila

In Drosophila, the single male X chromosome is transcribed at twice the rate of a single female X chromosome. This hypertranscription requires the functions of at least four autosomal male-specific lethal genes (msls) and is under the control of the Sex-lethal (Sxl) gene. One of the msls, the maleless (mle) gene, encodes a protein that is associated with the male X chromosome. To investigate how dosage compensation is regulated, we have determined whether Sxl and the other msls are required for mle X chromosome binding. We have found that in females, Sxl functions to prevent mle from binding to the two X chromosomes. Additionally, we have found that mle X chromosome binding requires wild-type msl1, msl2, and msl3 functions. These data support a model whereby the activity of the mle protein is regulated through its association with one or more of the other msl proteins.

The Drosophila couch potato protein is expressed in nuclei of peripheral neuronal precursors and shows homology to RNA-binding proteins

Through enhancer detection screens we have isolated and cloned an essential gene that is expressed in the neuronal precursors and their daughter cells in the Drosophila embryonic peripheral nervous system (PNS). The gene is named couch potato (cpo), because several partial loss-of-function alleles cause hypoactive behavior in adults. Here, we present evidence that the structure of the cpo locus is unusually complex: It spans > 100 kb, encodes three different messages, is differentially spliced, lacks an AUG initiation codon, and may encode three different proteins. Two putative Cpo proteins contain similar but nonidentical RNA-binding domains that are most homologous to the RNA-binding domains of the Drosophila embryonic lethal abnormal vision (elav) gene and a human brain protein that has been implicated in a paraneoplastic sensory neuropathy. Polyclonal antibodies raised against a fusion protein localize Cpo to the nucleus. Immunocytochemical studies demonstrate that the achaete-scute and daughterless genes are required for proper expression of cpo in the PNS but not in other cells that express cpo. On the basis of our observations, we present a model in which cpo is controlled by genes that determine cells to become PNS cells. Cpo, in turn, may control the processing of RNA molecules required for the proper functioning of the PNS.

deadpan, an essential pan-neural gene encoding an HLH protein, acts as a denominator in Drosophila sex determination

In Drosophila, sex is determined by the X:A ratio. One major numerator element on the X chromosome is sisterless-b (sis-b), also called scute, which encodes an HLH-type transcription factor. We report here that an essential pan-neural gene, the autosomal HLH gene deadpan (dpn), acts as a denominator element. As revealed by dosage-dependent dominant interactions, males die with too high a ratio of sc+ to dpn+, caused by misexpression of Sex lethal (Sxl) in embryos, and females die with too low a ratio of sc+ to dpn+, because of altered embryonic Sxl expression. In addition, we found that the HLH gene extramacrochaetae (emc), like daughterless (da), is needed maternally for proper communication of the X:A ratio, thus supporting the idea that a set of HLH genes comprises a functional cassette that makes a sensitive and stable genetic switch used in both neural determination and sex determination.

The primary sex determination signal of Drosophila acts at the level of transcription

For Drosophila, the choice between male and female development is made by the switch gene, Sxl, in response to the X:A ratio. Once Sxl is turned on in females, it actively maintains the determined state, independent of the X:A signal, by a positive autoregulatory feedback loop in which Sxl proteins direct the female-specific splicing of Sxl transcripts. In this paper we have investigated the mechanism controlling pathway initiation. Our results suggest a two-step model for the initial activation of Sxl in females. In the first step, a special class of Sxl mRNAs is expressed in female embryos from an early promoter that responds to the genes signaling the X:A ratio. The proteins produced from these early mRNAs then initiate the autoregulatory loop by directing the female-specific processing of transcripts from the late Sxl promoter.

The maleless protein associates with the X chromosome to regulate dosage compensation in Drosophila

The maleless (mle) gene is one of four known regulatory loci required for increased transcription (dosage compensation) of X-linked genes in D. melanogaster males. A predicted mle protein (MLE) contains seven short segments that define a superfamily of known and putative RNA and DNA helicases. MLE, while present in the nuclei of both male and female cells, differs in its association with polytene X chromosomes in the two sexes. MLE is associated with hundreds of discrete sites along the length of the X chromosome in males and not in females. The predominant localization of MLE to the X chromosome in males makes it a strong candidate to be a direct regulator of dosage compensation.

The complex set of late transcripts from the Drosophila sex determination gene sex-lethal encodes multiple related polypeptides

Sex-lethal (Sxl), a key sex determination gene in Drosophila melanogaster, is known to express a set of three early transcripts arising during early embryogenesis and a set of seven late transcripts occurring from midembryogenesis through adulthood. Among the late transcripts, male-specific mRNAs were distinguished from their female counterparts by the presence of an extra exon interrupting an otherwise long open reading frame (ORF). We have now analyzed the structures of the late Sxl transcripts by cDNA sequencing, Northern (RNA) blotting, primer extension, and RNase protection. The late transcripts appear to use a common 5' end but differ at their 3' ends by the use of alternative polyadenylation sites. Two of these sites lack canonical AATAAA sequences, and their use correlates in females with the presence of a functional germ line, suggesting possible tissue-specific polyadenylation. Besides the presence of the male-specific exon, no additional sex-specific splicing events were detected, although a number of non-sex-specific splicing variants were observed. In females, the various forms of late Sxl transcript potentially encode up to six slightly different polypeptides. All of the protein-coding differences occur outside the previously defined ribonucleoprotein motifs. One class of Sxl mRNAs also includes a second long ORF in the same frame as the first ORF but separated from it by a single ochre codon. The function of this second ORF is unknown. Significant amounts of apparently partially processed Sxl RNAs were observed, consistent with the hypothesis that the regulated Sxl splices occur relatively slowly.

The complex set of late transcripts from the Drosophila sex determination gene sex-lethal encodes multiple related polypeptides

Sex-lethal (Sxl), a key sex determination gene in Drosophila melanogaster, is known to express a set of three early transcripts arising during early embryogenesis and a set of seven late transcripts occurring from midembryogenesis through adulthood. Among the late transcripts, male-specific mRNAs were distinguished from their female counterparts by the presence of an extra exon interrupting an otherwise long open reading frame (ORF). We have now analyzed the structures of the late Sxl transcripts by cDNA sequencing, Northern (RNA) blotting, primer extension, and RNase protection. The late transcripts appear to use a common 5' end but differ at their 3' ends by the use of alternative polyadenylation sites. Two of these sites lack canonical AATAAA sequences, and their use correlates in females with the presence of a functional germ line, suggesting possible tissue-specific polyadenylation. Besides the presence of the male-specific exon, no additional sex-specific splicing events were detected, although a number of non-sex-specific splicing variants were observed. In females, the various forms of late Sxl transcript potentially encode up to six slightly different polypeptides. All of the protein-coding differences occur outside the previously defined ribonucleoprotein motifs. One class of Sxl mRNAs also includes a second long ORF in the same frame as the first ORF but separated from it by a single ochre codon. The function of this second ORF is unknown. Significant amounts of apparently partially processed Sxl RNAs were observed, consistent with the hypothesis that the regulated Sxl splices occur relatively slowly.

Positive autoregulation of sex-lethal by alternative splicing maintains the female determined state in Drosophila

Sex-lethal is a binary switch gene that controls all aspects of Drosophila sexual dimorphism. It must be active in females and inactive in males. The on/off regulation reflects alternative RNA splicing in which full-length proteins are produced only in females. Here we investigate the role of Sxl in maintaining sexual pathway commitments. By ectopic expression of a female Sxl cDNA in transgenic male flies, we show that Sxl protein induces a rapid switch from male- to female-specific splicing. The ectopically expressed Sxl protein wil trans-activate an endogenous wild-type Sxl gene. This establishes a feedback loop in which Sxl proteins induce their own synthesis by directing the female-specific splicing of Sxl transcripts. We conclude that the female determined state is maintained by Sxl through positive autoregulation, while the male determined state is maintained by default.

Molecular nature of the Drosophila sex determination signal and its link to neurogenesis

In 1921 it was discovered that the sexual fate of Drosophila is determined by the ratio of X chromosomes to sets of autosomes. Only recently has it been found that the X chromosome to autosome (X:A) ratio is communicated in part by the dose of sisterless-b (sis-b), an X-linked genetic element located within the achaete-scute complex of genes involved in neurogenesis. In this report, the molecular nature of the primary sex determination signal and its relation to these proneural genes was determined by analysis of sis-b+ germline transformants. The sis-b+ function is confered by protein T4, a member of the helix-loop-helix family of transcription factors. Although T4 is shared by sis-b and scute-alpha, the regulatory regions of sis-b, which control T4 expression in sex determination, are both separable from and simpler than those of scute-alpha, which control T4 expression in neurogenesis. Dose-sensitive cooperative interactions in the assembly or binding of sis-dependent transcription factors may directly determine the activity of the female-specific promoter of Sex-lethal, the master regulator of sexual development. In this model there is no need to invoke the existence of analogous autosomal negative regulators of Sex-lethal.

Developmental distribution of female-specific Sex-lethal proteins in Drosophila melanogaster

The binary switch gene Sex-lethal (Sxl) must be on in females and off in males to allow the proper elaboration of the appropriate sexual developmental pathway in Drosophila melanogaster. Previous studies suggested a mechanism in which the on/off regulation of Sxl occurs post-transcriptionally at the level of RNA splicing. A critical prediction of this model is that functional Sxl proteins are absent in males but present in females. In this report we show that the expected full-length proteins are only present in female animals. Multiple forms of Sxl protein are found in females, some of which are expressed in a stage- and tissue-specific pattern. Consistent with a role of Sxl proteins in regulating alternate splicing, the proteins are localized in the nucleus where they exhibit a punctate staining pattern. Surprisingly, several minor Sxl proteins appear to be present in specific tissues of both sexes of adults. The possible origin of these species is discussed. We also show that Sxl expression in the early embryo is sex specific and depends on maternal daughterless and zygotic sisterless-b activity in accordance with the established roles of these genes as positive regulators of Sxl. The onset of Sxl expression in the germ line occurs later than that in the soma.

X:A ratio, the primary sex-determining signal in Drosophila, is transduced by helix-loop-helix proteins

Drosophila determines its sex by "counting" X chromosomes. We show that premature expression of the pair-rule segmentation gene hairy interferes with this process, resulting in female-specific lethality by inhibiting initiation of the master control gene Sex-lethal (Sxl). The female-specific lethality can be suppressed by a constitutive Sxl allele or by extra copies of X-linked "counting elements." These results are best explained by competition between hairy and other helix-loop-helix transcription factors that act in chromosome counting. We have confirmed this model by showing that misexpression of the achaete-scute T4 gene induces ectopic Sxl expression and male-specific lethality, confirming that achaete-scute T4 is the sisterless-b counting element. We propose that X chromosomes are counted through heterodimers of helix-loop-helix transcription factors that act synergistically to initiate Sxl expression.

Alternative splicing of the sex determination gene transformer-2 is sex-specific in the germ line but not in the soma

The transformer-2 (tra-2) gene of Drosophila melanogaster plays essential roles in both sexual differentiation in the female soma and spermatogenesis in the male germ line. In the female soma, tra-2 is known to act with other genes in the sex determination regulatory cascade to control the sex-specific alternative splicing of transcripts from the doublesex gene. Here, we determine whether or not any sex-specific tra-2 products are expressed that may account for either of these sex-specific activities. Sequence analysis of the tra-2 gene and 10 tra-2 cDNA clones coupled with nuclease protection analysis reveals a variety of alternatively spliced tra-2 mRNAs that each encode one of four distinct but overlapping polypeptides. Three of the encoded polypeptides contain both a ribonucleoprotein consensus sequence and arginine/serine-rich regions, suggesting a direct role for these products in RNA splicing. We show that although two transcripts are expressed male specifically in the germ line, the tra-2 transcripts expressed in the soma are not sex-specific. The translation of products from a tra-2-lacZ fusion gene in both sexes suggests that the female-specific functioning of tra-2 in somatic tissues is not attributable to a translational mechanism. We suggest that tra-2 activity in somatic tissues is regulated through a post-translational sex-specific interaction with the product of the tra gene rather than through the expression of a female-specific tra-2 polypeptide.

The control of sexual development in Drosophila melanogaster: genetic and molecular analysis of a genetic regulatory hierarchy--a minireview

Genetic analyses of mutants affecting the sexual development of the fruitfly, Drosophila melanogaster, have led to a genetic model that describes how the wild-type genes interact with one another as parts of a single regulatory pathway. More recent molecular studies on some of these sex differentiation regulatory genes have lent support to this genetic model, and have defined the molecular nature of some of these genetic interactions. One surprising feature of the regulatory hierarchy to emerge from these molecular studies is that many of the sex-specific genetic switches in this pathway are not transcriptional regulation events but, rather, are based on sex-specific alternative RNA processing events (pre-mRNA splicing and/or polyadenylation site selection).

Sex in flies: the splice of life

The discovery that the primary transcripts of many genes are processed to produce more than one kind of messenger RNA focuses attention on the control of RNA processing as a developmental regulatory mechanism. In the fruit-fly Drosophila melanogaster, differential splicing of a hierarchy of regulatory genes determines sex, and thus the molecular biology of sex determination in the fruit-fly may lead to insights into the mechanisms by which alternative splicing is regulated.

Does postzygotic isolation result from improper dosage compensation?

The X chromosome invariably has the largest effect on postzygotic isolation between animal species. One explanation of this pattern is that inviability and sterility result from a breakdown in the dosage compensation of X-linked genes in hybrids. In Drosophila, such breakdown could result from divergence of the genes used to assess the X/autosomal (X/A) ratio, and thus the sex, of an individual. I test this hypothesis by introducing mutant alleles of the Sex-lethal locus into Drosophila melanogaster-Drosophila simulans hybrids. These mutants "ignore" any perceived anomalous X/A ratio and thus can be used to ensure proper dosage compensation in hybrids. These mutants do not rescue hybrid viability or fertility, implying that postzygotic isolation in this hybridization does not result from a disruption of dosage compensation caused by divergence of the X/A counting system.

The Drosophila female-specific sex-determination gene, Sex-lethal, has stage-, tissue-, and sex-specific RNAs suggesting multiple modes of regulation

For proper sexual development of females, the Sex-lethal (Sxl) gene must be activated early in development and remain on during the rest of the life cycle. Conversely, in males, Sxl must remain functionally off through development. Here, we show that the Sxl transcription unit spans a DNA segment of greater than 20 kb and encodes at least 10 distinct, but overlapping, RNA species. These RNAs range in size from 4.4 to 1.7 kb and exhibit sex, stage, and tissue specificity. Six RNAs, three female specific and three male specific, are first detected by midembryogenesis and persist through the adult stage: Their expression reflects the on/off regulation of Sxl's activity at the level of sex-specific alternate splicing. Four Sxl RNAs are found in adult females. Two of these RNAs are dependent on the presence of a functional germ line and may be relevant to Sxl's role in adult germ-line development. All four are present in unfertilized eggs. Finally, three Sxl RNAs are found only transiently during very early embryogenesis; we suggest that the expression of these RNAs may reflect an early regulation of Sxl at the level of transcription and that these transcripts are involved in the initial selection of the Sxl activity state in response to the primary sex-determination signal, the X/A ratio.

Molecular genetics of transformer, a genetic switch controlling sexual differentiation in Drosophila

The transformer gene is one of a set of regulatory genes that form the hierarchy controlling all aspects of somatic sexual differentiation in Drosophila melanogaster. The gene transformer occupies an intermediate position in this hierarchy. Analysis of this gene has allowed us to determine the mechanism by which it is regulated in a sex-specific manner and to examine the way in which the regulatory hierarchy is organized. The female-specific expression of the tra gene, previously inferred from genetic observations, is based on sex-specific alternative splicing of tra pre-mRNA and is not the result of sex-specific transcriptional activation. The female-specific RNA produced by this alternative splicing is the functional mediator of tra activity. Multiple genetic, molecular, and transformation experiments show that female-specific activation of genes or gene products occurs in the order Sex lethal greater than transformer greater than transformer-2 greater than doublesex greater than or equal to intersex greater than female differentiation. The results do not distinguish the level at which transformer might regulate the downstream gene transformer-2. Neither transformer nor any of the down-stream genes feedback on, or participate in, alternative splicing of transformer RNA. The mechanism by which Sex lethal regulates transformer splicing appears to be a repression of the use of one of a pair of splice acceptor sites.