Regulation of doublesex pre-mRNA processing occurs by 3'-splice site activation

Population suppression by release of insects carrying a dominant sterile homing gene drive targeting doublesex in Drosophila

CRISPR homing gene drives can suppress pest populations by targeting female fertility genes, converting wild-type alleles into drive alleles in the germline of drive heterozygotes. fsRIDL (female-specific Release of Insects carrying a Dominant Lethal) is a self-limiting population suppression strategy involving continual release of transgenic males carrying female lethal alleles. Here, we propose an improved pest suppression system called "Release of Insects carrying a Dominant-sterile Drive" (RIDD), combining performance characteristics of homing drive and fsRIDL. We construct a split RIDD system in Drosophila melanogaster by creating a 3-gRNA drive disrupting the doublesex female exon. Drive alleles bias their inheritance in males, while drive alleles and resistance alleles formed by end-joining cause dominant female sterility. Weekly releases of RIDD males progressively suppressed and eventually eliminated cage populations. Modeling shows that RIDD is substantially stronger than SIT and fsRIDL. RIDD is also self-limiting, potentially allowing targeted population suppression.

Cellular sex throughout the organism underlies somatic sexual differentiation

Sex chromosomes underlie the development of male or female sex organs across species. While systemic signals derived from sex organs prominently contribute to sex-linked differences, it is unclear whether the intrinsic presence of sex chromosomes in somatic tissues has a specific function. Here, we use genetic tools to show that cellular sex is crucial for sexual differentiation throughout the body in Drosophila melanogaster. We reveal that every somatic cell converts the intrinsic presence of sex chromosomes into the active production of a sex determinant, a female specific serine- and arginine-rich (SR) splicing factor. This discovery dismisses the mosaic model which posits that only a subset of cells has the potential to sexually differentiate. Using cell-specific sex reversals, we show that this prevalence of cellular sex drives sex differences in organ size and body weight and is essential for fecundity. These findings demonstrate that cellular sex drives differentiation programs at an organismal scale and highlight the importance of cellular sex pathways in sex trait evolution.

Female Sex Determination Factors in Ceratitis capitata: Molecular and Structural Basis of TRA and TRA2 Recognition

In the model system for genetics, Drosophila melanogaster, sexual differentiation and male courtship behavior are controlled by sex-specific splicing of doublesex (dsx) and fruitless (fru). In vitro and in vivo studies showed that female-specific Transformer (TRA) and the non-sex-specific Transformer 2 (TRA2) splicing factors interact, forming a complex promoting dsx and fru female-specific splicing. TRA/TRA2 complex binds to 13 nt long sequence repeats in their pre-mRNAs. In the Mediterranean fruitfly Ceratitis capitata (Medfly), a major agricultural pest, which shares with Drosophila a ~120 million years old ancestor, Cctra and Cctra2 genes seem to promote female-specific splicing of Ccdsx and Ccfru, which contain conserved TRA/TRA2 binding repeats. Unlike Drosophila tra, Cctra autoregulates its female-specific splicing through these putative regulatory repeats. Here, a yeast two-hybrid assay shows that CcTRA interacts with CcTRA2, despite its high amino acid divergence compared to Drosophila TRA. Interestingly, CcTRA2 interacts with itself, as also observed for Drosophila TRA2. We also generated a three-dimensional model of the complex formed by CcTRA and CcTRA2 using predictive approaches based on Artificial Intelligence. This structure also identified an evolutionary and highly conserved putative TRA2 recognition motif in the TRA sequence. The Y2H approach, combined with powerful predictive tools of three-dimensional protein structures, could use helpful also in this and other insect species to understand the potential links between different upstream proteins acting as primary sex-determining signals and the conserved TRA and TRA2 transducers.

A history of the genetic and molecular identification of genes and their functions controlling insect sex determination

The genetics of the sex determination regulatory cascade in Drosophila melanogaster has a fascinating history, interlinked with the foundation of the Genetics discipline itself. The discovery that alternative splicing rather than differential transcription is the molecular mechanism underlying the upstream control of sex differences in the Drosophila model system was surprising. This notion is now fully integrated into the scientific canon, appearing in many genetics textbooks and online education resources. In the last three decades, it was a key reference point for starting evolutionary studies in other insect species by using homology-based approaches. This review will introduce a very brief history of Drosophila genetics. It will describe the genetic and molecular approaches applied for the identifying and cloning key genes involved in sex determination in Drosophila and in many other insect species. These comparative analyses led to supporting the idea that sex-determining pathways have evolved mainly by recruiting different upstream signals/genes while maintaining widely conserved intermediate and downstream regulatory genes. The review also provides examples of the link between technological advances and research achievements, to stimulate reflections on how science is produced. It aims to hopefully strengthen the related historical and conceptual knowledge of general readers of other disciplines and of younger geneticists, often focused on the latest technical-molecular approaches.

DMRT Transcription Factors in the Control of Nervous System Sexual Differentiation

Sexual phenotypic differences in the nervous system are one of the most prevalent features across the animal kingdom. The molecular mechanisms responsible for sexual dimorphism throughout metazoan nervous systems are extremely diverse, ranging from intrinsic cell autonomous mechanisms to gonad-dependent endocrine control of sexual traits, or even extrinsic environmental cues. In recent years, the DMRT ancient family of transcription factors has emerged as being central in the development of sex-specific differentiation in all animals in which they have been studied. In this review, we provide an overview of the function of Dmrt genes in nervous system sexual regulation from an evolutionary perspective.

The AalNix3&4 isoform is required and sufficient to convert Aedes albopictus females into males

Aedes albopictus is one of the most invasive insect species in the world and an effective vector for many important arboviruses. We reported previously that Ae. albopictus Nix (AalNix) is the male-determining factor of this species. However, whether AalNix alone is sufficient to initiate male development is unknown. Transgenic lines that express each of the three AalNix isoforms from the native promoter were obtained using piggyBac transformation. We verified the stable expression of AalNix isoforms in the transgenic lines and confirm that one isoform, AalNix3&4, is sufficient to convert females into fertile males (pseudo-males) that are indistinguishable from wild-type males. We also established a stable sex-converted female mosquito strain, AalNix3&4-♂4-pseudo-male. The pseudo-male mosquitoes can fly and mate normally with wild-type female, although their mating competitiveness is lower than wild-type. This work further clarifies the role of AalNix in the sex determination pathway and will facilitate the development of Ae. albopictus control strategies that rely on male-only releases such as SIT and sex-ratio distortion.

A feminizing switch in a hemimetabolous insect

The mechanism of sex determination remains poorly understood in hemimetabolous insects. Here, in the brown planthopper (BPH), Nilaparvata lugens, a hemipteran rice pest, we identified a feminizing switch or a female determiner (Nlfmd) that encodes a serine/arginine-rich protein. Knockdown of Nlfmd in female nymphs resulted in masculinization of both the somatic morphology and doublesex splicing. The female-specific isoform of Nlfmd, Nlfmd-F, is maternally deposited and zygotically transcribed. Depletion of Nlfmd by maternal RNAi or CRISPR-Cas9 resulted in female-specific embryonic lethality. Knockdown of an hnRNP40 family gene named female determiner 2 (Nlfmd2) also conferred masculinization. In vitro experiments showed that an Nlfmd2 isoform, NlFMD2³⁴⁰, bound the RAAGAA repeat motif in the Nldsx pre-mRNA and formed a protein complex with NlFMD-F to modulate Nldsx splicing, suggesting that NlFMD2 may function as an RNA binding partner of the feminizing switch NlFMD. Our results provide novel insights into the diverse mechanisms of insect sex determination.

Molecular Mechanisms of Sexually Dimorphic Nervous System Patterning in Flies and Worms

Male and female brains display anatomical and functional differences. Such differences are observed in species across the animal kingdom, including humans, but have been particularly well-studied in two classic animal model systems, the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans. Here we summarize recent advances in understanding how the worm and fly brain acquire sexually dimorphic features during development. We highlight the advantages of each system, illustrating how the precise anatomical delineation of sexual dimorphisms in worms has enabled recent analysis into how these dimorphisms become specified during development, and how focusing on sexually dimorphic neurons in the fly has enabled an increasingly detailed understanding of sex-specific behaviors.

An Evaluation of Differentially Spliced Genes as Markers of Sex for Forensic Entomology,

Blow flies (Calliphoridae) are important medically and economically and are commonly used in forensics as temporal markers in death investigations. While phenotypic traits in adult flies can be sexually dimorphic, sex identification in immatures is difficult. Consequently, little is known about how sex may result in developmental disparities among sexes even though there are indications that they may be important in some instances. Since genetic mechanisms for sex are well studied in model flies and species of agricultural and medical importance, we exploit the sex-specifically spliced genes transformer (tra) and doublesex (dsx) in the sex determination pathway to optimize a sex identification assay for immatures. Using known primer sets for tra and with a novel one for dsx, we develop PCR assays for identifying sex in four forensically relevant Calliphoridae species: Lucilia sericata (Meigen), Lucilia cuprina (Wiedemann), Cochliomyia macellaria (Fabricius), and Chrysomya rufifacies (Macquart) and evaluated their performance. Band detection rates were found to range from 71 to 100%, call rates ranged from 90 to 100%, and no error was found when bands could be called. Such information is informative for purposes of testimony and in preparation for development studies. The developed assays will assist in further differentiating sexually dimorphic differences in development of the Calliphoridae and aid in more accurately estimating insect age when age predictive markers (size, development time, molecular expression) are sexually dimorphic.

Mutation of doublesex in Hyphantria cunea results in sex-specific sterility

BACKGROUND

The gene doublesex (dsx) plays pivotal roles in sex determination and controls sexually dimorphic development in certain insects. Importantly, it also displays a potential candidate target for pest management due to its sex-specific splicing. Therefore, we used CRISPR/Cas9-mediated gene disruption to investigate the function of dsx in Hyphantria cunea, an invasive forest pest.

RESULT

In the present study, we identified the dsx gene from H. cunea which showed a sex-biased expression pattern that was different from other lepidopteran insects. Referring to sex-specific functional analyses in Bombyx mori, we performed a site-specific knockout of the Hcdsx gene by using a CRISPR/Cas9 system, which induced severe abnormalities in external genitalia and some incomplete sex reversal phenotypes, which in turn led to reduced sex-specific fecundity. An alternative splicing pattern of Hcdsx was altered by CRISPR/Cas9-induced mutation, and alterations in splicing affected expression of downstream genes encoding pheromone binding protein 1, vg1 and vg2 (encoding vitellogenin), which contributed to the sex-specific sterility phenotypes in the Hcdsx mutants.

CONCLUSION

The Hcdsx gene plays important roles in sexual differentiation in H. cunea. Disruption of Hcdsx induced sex-specific sterility, demonstrating a potential application in control of this pest. © 2019 Society of Chemical Industry.

Identification of sex determination genes and their evolution in Phlebotominae sand flies (Diptera, Nematocera)

BACKGROUND

Phlebotomine sand flies (Diptera, Nematocera) are important vectors of several pathogens, including Leishmania parasites, causing serious diseases of humans and dogs. Despite their importance as disease vectors, most aspects of sand fly biology remain unknown including the molecular basis of their reproduction and sex determination, aspects also relevant for the development of novel vector control strategies.

RESULTS

Using comparative genomics/transcriptomics data mining and transcriptional profiling, we identified the sex determining genes in phlebotomine sand flies and proposed the first model for the sex determination cascade of these insects. For all the genes identified, we produced manually curated gene models, developmental gene expression profile and performed evolutionary molecular analysis. We identified and characterized, for the first time in a Nematocera species, the transformer (tra) homolog which exhibits both conserved and novel features. The analysis of the tra locus in sand flies and its expression pattern suggest that this gene is able to autoregulate its own splicing, as observed in the fruit fly Ceratitis capitata and several other insect species.

CONCLUSIONS

Our results permit to fill the gap about sex determination in sand flies, contribute to a better understanding of this developmental pathway in Nematocera and open the way for the identification of sex determining orthologs in other species of this important Diptera sub-order. Furthermore, the sex determination genes identified in our work also provide the opportunity of future biotechnological applications to control natural population of sand flies, reducing their impact on public health.

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.

Conserved Domains in the Transformer Protein Act Complementary to Regulate Sex-Specific Splicing of Its Own Pre-mRNA

The transformer (tra) gene, which is a female-determining master gene in the housefly Musca domestica, acts as a memory device for sex determination via its auto-regulatory function, i.e., through the contribution of the TRA protein to female-specific splicing of its own pre-mRNA. The TRA protein contains 4 small domains that are specifically conserved among TRA proteins (domains 1-4). Domain 2, also named TRA-CAM domain, is the most conserved, but its function remains unknown. To examine whether these domains are involved in the auto-regulatory function, we performed in vitro splicing assays using a tra minigene containing a partial genomic sequence of the M. domestica tra (Mdtra) gene. Co-transfection of the Mdtra minigene and an MdTRA protein expression vector into cultured insect cells strongly induced female-specific splicing of the minigene. A series of deletion mutation analyses demonstrated that these domains act complementarily to induce female-specific splicing. Domain 1 and the TRA-CAM domain were necessary for the female-specific splicing when the MdTRA protein lacked both domains 3 and 4. In this situation, mutation of the well-conserved 3 amino acids (GEG) in the TRA-CAM domain significantly reduced the female-specific splicing activity of MdTRA. GST-pull down analyses demonstrated that the MdTRA protein specifically enriched on the male-specific exonic region (exon 2b), which contains the putative TRA/TRA-2 binding sites, and that the GEG mutation disrupts this enrichment. Since the MdTRA protein interacts with its own pre-mRNA through TRA-2, our findings suggest that the conserved amino acid residues in the TRA-CAM domain may be crucial for the interaction between MdTRA and TRA-2, enhancing MdTRA recruitment on its pre-mRNA to induce female-specific splicing of tra in the housefly.

Sex determination and differentiation genes in a functional hermaphrodite scallop, Nodipecten subnodosus

The lion-paw, Nodipecten subnodosus is one of three scallop species commercially exploited on the west coast of the Peninsula of Baja California. Because nothing is known about sex determination and sexual differentiation in hermaphrodite scallops, in the present work, a global transcriptomic analysis was performed in two early developmental stages, settling eyed-larvae and spat, as well as in three tissues (undifferentiated gonad, digestive gland, and adductor muscle). Over 27 million Illumina paired-end reads were obtained through the MiSeq platform. After processing the reads a total of 243,774 transcripts were assembled with an N50 of 980 and an average length of 775nt. A total of 43,252 proteins were inferred and 36,103 transcripts had at least one homolog in the SwissProt database according to a blastx search. After differential expression analyses and GO annotations it was possible to identify several sex-related genes in the scallop, including one known to be involved in the sex determination pathway of the hermaphrodite model organism Caenorhabditis elegans, N. subnodosus-sex1 (Ns-sex1). Other interesting sex determination and differentiation genes were Ns-dmrta2, Ns-sox9, Ns-wnt4, Ns-doa, Ns-ovo, Ns-vir, among others. Most of these genes were mainly expressed in the testis region, suggesting their participation in male gonad region sex differentiation. These results represent the first available information on the genetics of sex determination and differentiation in a functional hermaphrodite scallop.

Recent progress and open questions in Drosophila dosage compensation

Sexual dimorphism is observed in many traits across diverse taxa, and often it is quite extreme. Within a species, individuals of opposing sex can appear strikingly different, reflecting differences at the molecular level that may be similarly striking. Among the most extreme cases of such molecular sexual dimorphism is the quantity of sex chromosomes that each sex possesses. Hemizygous sex chromosomes are common to many species, and various mechanisms have evolved to regulate transcriptional activity to ensure appropriate sex chromosome-to-autosome gene expression stoichiometry. Among the most thoroughly investigated of these mechanisms is Drosophila melanogaster's male-specific lethal (MSL) complex-mediated dosage compensation. In Drosophila, the male X chromosome transcription is upregulated approximately two-fold in somatic tissues to counterbalance the effects of sex chromosome hemizygosity on transcript abundance. Despite dramatic advances in our understanding of the Drosophila dosage compensation, many questions remain unanswered, and our understanding of its molecular underpinnings remains incomplete. In this review, we synthesize recent progress in the field as a means to highlight open questions, including how the MSL complex targets the X chromosome, how dosage compensation has shaped evolution of X-linked genes, and the degree to which MSL complex-mediated dosage compensation varies in activity across somatic tissues.

A genome-wide survey of sexually dimorphic expression of Drosophila miRNAs identifies the steroid hormone-induced miRNA let-7 as a regulator of sexual identity

MiRNAs bear an increasing number of functions throughout development and in the aging adult. Here we address their role in establishing sexually dimorphic traits and sexual identity in male and female Drosophila. Our survey of miRNA populations in each sex identifies sets of miRNAs differentially expressed in male and female tissues across various stages of development. The pervasive sex-biased expression of miRNAs generally increases with the complexity and sexual dimorphism of tissues, gonads revealing the most striking biases. We find that the male-specific regulation of the X chromosome is relevant to miRNA expression on two levels. First, in the male gonad, testis-biased miRNAs tend to reside on the X chromosome. Second, in the soma, X-linked miRNAs do not systematically rely on dosage compensation. We set out to address the importance of a sex-biased expression of miRNAs in establishing sexually dimorphic traits. Our study of the conserved let-7-C miRNA cluster controlled by the sex-biased hormone ecdysone places let-7 as a primary modulator of the sex-determination hierarchy. Flies with modified let-7 levels present doublesex-related phenotypes and express sex-determination genes normally restricted to the opposite sex. In testes and ovaries, alterations of the ecdysone-induced let-7 result in aberrant gonadal somatic cell behavior and non-cell-autonomous defects in early germline differentiation. Gonadal defects as well as aberrant expression of sex-determination genes persist in aging adults under hormonal control. Together, our findings place ecdysone and let-7 as modulators of a somatic systemic signal that helps establish and sustain sexual identity in males and females and differentiation in gonads. This work establishes the foundation for a role of miRNAs in sexual dimorphism and demonstrates that similar to vertebrate hormonal control of cellular sexual identity exists in Drosophila.

Alternative splicing in Drosophila neuronal development

Post-transcriptional pre-mRNA splicing has emerged as a critical step in the gene expression cascade greatly influencing diversification and spatiotemporal control of the proteome in many developmental processes. The percentage of genes targeted by alternative splicing (AS) is shown to be over 95% in humans and 60% in Drosophila. Therefore, it is evident that deregulation of this process underlies many genetic diseases. Among all tissues, the brain shows the highest transcriptome diversity, which is not surprising in view of the complex inter- and intracellular networks underlying the development of this organ. Reports of isoforms known to function at different steps during Drosophila nervous system development are rapidly increasing as well as knowledge on their regulation and function, highlighting the role of AS during neuronal development in Drosophila.

Half pint/Puf68 is required for negative regulation of splicing by the SR splicing factor Transformer2

The SR family of proteins plays important regulatory roles in the control of alternative splicing in a wide range of organisms. These factors affect splicing through both positive and negative controls of splice site recognition by pre-spliceosomal factors. Recent studies indicate that the Drosophila SR factor Transformer 2 (Tra2) activates and represses splicing through distinct and separable effector regions of the protein. While the interactions of its Arg-Ser-rich activator region have been well studied, cofactors involved in splicing repression have yet to be found. Here we use a luciferase-based splicing reporter assay to screen for novel proteins necessary for Tra2-dependent repression of splicing. This approach identified Half pint, also known as Puf68, as a co-repressor required for Tra2-mediated autoregulation of the M1 intron. In vivo, Half pint is required for Tra2-dependent repression of M1 splicing but is not necessary for Tra2-dependent activation of doublesex splicing. Further experiments indicate that the effect of Hfp is sequence-specific and that it associates with these target transcripts in cells. Importantly, known M1 splicing regulatory elements are sufficient to sensitize a heterologous intron to Hfp regulation. Two alternative proteins deriving from Hfp transcripts, Hfp68, and Hfp58, were found to be expressed in vivo but differed dramatically in their effect on M1 splicing. Comparison of the cellular localization of these forms in S2 cells revealed that Hfp68 is predominantly localized to the nucleus while Hfp58 is distributed across both the nucleus and cytoplasm. This accords with their observed effects on splicing and suggests that differential compartmentalization may contribute to the specificity of these isoforms. Together, these studies reveal a function for Half pint in splicing repression and demonstrate it to be specifically required for Tra2-dependent intron inclusion.

Sex modifies genetic effects on residual variance in urinary calcium excretion in rat (Rattus norvegicus)

Conventional genetics assumes common variance among alleles or genetic groups. However, evidence from vertebrate and invertebrate models suggests that residual genotypic variance may itself be under partial genetic control. Such a phenomenon would have great significance: high-variability alleles might confound the detection of "classically" acting genes or scatter predicted evolutionary outcomes among unpredicted trajectories. Of the few works on this phenomenon, many implicate sex in some aspect of its control. We found that female genetic hypercalciuric stone-forming (GHS) rats (Rattus norvegicus) had higher coefficients of variation (CVs) for urinary calcium (CV = 0.14) than GHS males (CV = 0.06), and the reverse in normocalciuric Wistar-Kyoto rats (WKY) (CV(♂) = 0.14; CV(♀) = 0.09), suggesting sex-by-genotype interaction on residual variance. We therefore investigated the effect of sex on absolute-transformed residuals in urinary calcium in an F(2) GHS × WKY mapping cohort. Absolute residuals were associated with genotype at two microsatellites, D3Rat46 (RNO3, 33.9 Mb) and D4Mgh1 (RNO4, 84.8 MB) at Bonferroni thresholds across the entire cohort, and with the microsatellites D3Rat46, D9Mgh2 (RNO9, 84.4 Mb), and D12Rat25 (RNO12, 40.4 Mb) in females (P < 0.05) but not males. In GHS chromosome 1 congenic lines bred onto a WKY genomic background, we found that congenic males had significantly (P < 0.0001) higher CVs for urinary calcium (CV = 0.25) than females (CV = 0.15), supporting the hypothesis of the inheritance of sex-by-genotype interaction on this effect. Our findings suggest that genetic effects on residual variance are sex linked; heritable, sex-specific residuals might have great potential implications for evolution, adaptation, and genetic analysis.

Investigating alternative RNA splicing in Xenopus

Alternative splicing, the process by which distinct mature mRNAs can be produced from a single primary transcript, is a key mechanism to increase the organism complexity. The generation of alternative splicing pattern is a means to expand the proteome diversity and also to control gene expression through the regulation of mRNA abundance. Alternative splicing is therefore particularly prevalent during development and accordingly numerous splicing events are regulated in a tissue or temporal manner. To study the roles of alternative splicing during developmental processes and decipher the molecular mechanisms that underlie temporal and spatial regulation, it is important to develop in vivo whole animal studies. In this chapter, we present the advantages of using the amphibian Xenopus as a fully in vivo model to study alternative splicing and we describe the experimental procedures that can be used with Xenopus laevis embryos and oocytes to define the cis-regulatory elements and identify the associated trans-acting factors.

Activation and repression functions of an SR splicing regulator depend on exonic versus intronic-binding position

SR proteins and related factors play widespread roles in alternative pre-mRNA splicing and are known to promote splice site recognition through their Arg–Ser-rich effector domains. However, binding of SR regulators to some targets results in repression of splice sites through a distinct mechanism. Here, we investigate how activated and repressed targets of the Drosophila SR regulator Transformer2 elicit its differing effects on splicing. We find that, like activation, repression affects early steps in the recognition of splice sites and spliceosome assembly. Repositioning of regulatory elements reveals that Tra2 complexes that normally repress splicing from intronic positions activate splicing when located in an exon. Protein tethering experiments demonstrate that this position dependence is an intrinsic property of Tra2 and further show that repression and activation are mediated by separate effector domains of this protein. When other Drosophila SR factors (SF2 and Rbp1) that activate splicing from exonic positions were tethered intronically they failed to either activate or repress splicing. Interestingly, both activities of Tra2 favor the exonic identity of the RNA sequences that encompass its binding sites. This suggests a model in which these two opposite functions act in concert to define both the position and extent of alternatively spliced exons.

The gene transformer-2 of Sciara (Diptera, Nematocera) and its effect on Drosophila sexual development

BACKGROUND

The gene transformer-2, which is involved in sex determination, has been studied in Drosophila, Musca, Ceratitis, Anastrepha and Lucilia. All these members of Diptera belong to the suborder Brachycera. In this work, it is reported the isolation and characterisation of genes transformer-2 of the dipterans Sciara ocellaris and Bradysia coprophila (formerly Sciara coprophila), which belong to the much less extensively analysed Sciaridae Family of the Suborder Nematocera, which is paraphyletic with respect to Suborder Brachycera.

RESULTS

The transformer-2 genes of the studied Sciara species were found to be transcribed in both sexes during development and adult life, in both the soma and germ lines. They produced a single primary transcript, which follows the same alternative splicing in both sexes, giving rise to different mRNAs isoforms. In S. ocellaris the most abundant mRNA isoform encoded a full-length protein of 251 amino acids, while that of B. coprophila encoded a protein of 246 amino acids. Both showed the features of the SR protein family. The less significant mRNA isoforms of both species encoded truncated, presumably non-functional Transformer-2 proteins. The comparison of the functional Sciara Transformer-2 proteins among themselves and those of other insects revealed the greatest degree of conservation in the RRM domain and linker region. In contrast, the RS1 and RS2 domains showed extensive variation with respect to their number of amino acids and their arginine-serine (RS) dipeptide content. The expression of S. ocellaris Transformer-2 protein in Drosophila XX pseudomales lacking the endogenous transformer-2 function caused their partial feminisation.

CONCLUSIONS

The transformer-2 genes of both Sciaridae species encode a single protein in both sexes that shares the characteristics of the Transformer-2 proteins of other insects. These proteins showed conserved sex-determination function in Drosophila; i.e., they were able to form a complex with the endogenous Drosophila Transformer protein that controls the female-specific splicing of the Drosophila doublesex pre-mRNA. However, it appears that the complex formed between the Drosophila Transformer protein and the Sciara Transformer-2 protein is less effective at inducing the female-specific splicing of the endogenous Drosophila doublesex pre-mRNA than the DrosophilaTransformer-Transformer2 complex. This suggests the existence of species-specific co-evolution of the Transformer and Transformer-2 proteins.

Distinct regulatory programs establish widespread sex-specific alternative splicing in Drosophila melanogaster

In Drosophila melanogaster, female-specific expression of Sex-lethal (SXL) and Transformer (TRA) proteins controls sex-specific alternative splicing and/or translation of a handful of regulatory genes responsible for sexual differentiation and behavior. Recent findings in 2009 by Telonis-Scott et al. document widespread sex-biased alternative splicing in fruitflies, including instances of tissue-restricted sex-specific splicing. Here we report results arguing that some of these novel sex-specific splicing events are regulated by mechanisms distinct from those established by female-specific expression of SXL and TRA. Bioinformatic analysis of SXL/TRA binding sites, experimental analysis of sex-specific splicing in S2 and Kc cells lines and of the effects of SXL knockdown in Kc cells indicate that SXL-dependent and SXL-independent regulatory mechanisms coexist within the same cell. Additional determinants of sex-specific splicing can be provided by sex-specific differences in the expression of RNA binding proteins, including Hrp40/Squid. We report that sex-specific alternative splicing of the gene hrp40/squid leads to sex-specific differences in the levels of this hnRNP protein. The significant overlap between sex-regulated alternative splicing changes and those induced by knockdown of hrp40/squid and the presence of related sequence motifs enriched near subsets of Hrp40/Squid-regulated and sex-regulated splice sites indicate that this protein contributes to sex-specific splicing regulation. A significant fraction of sex-specific splicing differences are absent in germline-less tudor mutant flies. Intriguingly, these include alternative splicing events that are differentially spliced in tissues distant from the germline. Collectively, our results reveal that distinct genetic programs control widespread sex-specific splicing in Drosophila melanogaster.

Conservation and sex-specific splicing of the doublesex gene in the economically important pest species Lucilia cuprina

Genetic control of sex determination in insects has been best characterized in Drosophila melanogaster, where the master gene Sxl codes for RNA that is sex specifically spliced to produce a functional protein only in females. SXL regulates the sex-specific splicing of transformer (tra) RNA which, in turn, regulates the splicing of dsx RNA to produce functional male and female proteins. In the Australian sheep blowfly (Lucilia cuprina), the tra gene (Lctra) is required for female development and Lctra transcripts are sex-specifically spliced such that only female Lctra mRNA codes for functional protein. In males, a factor encoded by the Y-linked male determining gene is thought to prevent the female-mode of splicing of Lctra RNA. To further our understanding of the sex determination regulatory hierarchy in L. cuprina, we have isolated the dsx gene (Lcdsx) from this species. We found that the Lcdsx transcripts are sex-specifically spliced in a similar manner as their counterparts in D. melanogaster, housefly and tephritids. The LcDSX proteins are well conserved and the male form of DSX contains a motif encoded by a male-specific exon that is within the female-specific intron. This intron/exon arrangement had previously been found only in the housefly dsx gene, suggesting this may be a unique feature of dsx genes of Calyptratae species.

The gene transformer-2 of Anastrepha fruit flies (Diptera, Tephritidae) and its evolution in insects

BACKGROUND

In the tephritids Ceratitis, Bactrocera and Anastrepha, the gene transformer provides the memory device for sex determination via its auto-regulation; only in females is functional Tra protein produced. To date, the isolation and characterisation of the gene transformer-2 in the tephritids has only been undertaken in Ceratitis, and it has been shown that its function is required for the female-specific splicing of doublesex and transformer pre-mRNA. It therefore participates in transformer auto-regulatory function. In this work, the characterisation of this gene in eleven tephritid species belonging to the less extensively analysed genus Anastrepha was undertaken in order to throw light on the evolution of transformer-2.

RESULTS

The gene transformer-2 produces a protein of 249 amino acids in both sexes, which shows the features of the SR protein family. No significant partially spliced mRNA isoform specific to the male germ line was detected, unlike in Drosophila. It is transcribed in both sexes during development and in adult life, in both the soma and germ line. The injection of Anastrepha transformer-2 dsRNA into Anastrepha embryos caused a change in the splicing pattern of the endogenous transformer and doublesex pre-mRNA of XX females from the female to the male mode. Consequently, these XX females were transformed into pseudomales. The comparison of the eleven Anastrepha Transformer-2 proteins among themselves, and with the Transformer-2 proteins of other insects, suggests the existence of negative selection acting at the protein level to maintain Transformer-2 structural features.

CONCLUSIONS

These results indicate that transformer-2 is required for sex determination in Anastrepha through its participation in the female-specific splicing of transformer and doublesex pre-mRNAs. It is therefore needed for the auto-regulation of the gene transformer. Thus, the transformer/transfomer-2 > doublesex elements at the bottom of the cascade, and their relationships, probably represent the ancestral state (which still exists in the Tephritidae, Calliphoridae and Muscidae lineages) of the extant cascade found in the Drosophilidae lineage (in which tra is just another component of the sex determination gene cascade regulated by Sex-lethal). In the phylogenetic lineage that gave rise to the drosophilids, evolution co-opted for Sex-lethal, modified it, and converted it into the key gene controlling sex determination.

Expression of the Drosophila secreted cuticle protein 73 (dsc73) requires Shavenbaby

Low stringency genomic library screens with genomic fragments from the sex determination gene doublesex identified the Drosophila secreted cuticle protein 73 (dsc73) gene, which encodes an 852-residue protein with an N-terminal signal sequence. In embryos, dsc73 RNA and protein are expressed to high levels in the epidermal cells that secrete the larval cuticle as well as in other cuticle-secreting tissues such as the trachea and salivary duct. Embryonic expression of dsc73 requires Shavenbaby, a transcription factor regulating cuticle formation. Double-labeling experiments with alphaCrb and alphaSAS reveal that, as with chitin and other known cuticle proteins, Dsc73 is secreted apically. Zygotic loss of dsc73 results in larval lethality but loss does not result in overt patterning defects or overt morphological defects in the embryonic tissues in which it is expressed. Thus, dsc73 encodes a novel secreted protein, and it is conserved within the Drosophila group. dsc73 may serve as a useful embryonic marker for cuticular patterning.

Neurogenetics of courtship and mating in Drosophila

The reproductive biology of Drosophila melanogaster is described and critically discussed, primarily with regard to genetic studies of sex-specific behavior and its neural underpinnings. The investigatory history of this system includes, in addition to a host of recent neurobiological analyses of reproductive phenotypes, studies of mating as well as the behaviors leading up to that event. Courtship and mating have been delved into mostly with regard to male-specific behavior and biology, although a small number of studies has also pointed to the neural substrates of female reproduction. Sensory influences on interactions between courting flies have long been studied, partly by application of mutants and partly by surgical experiments. More recently, molecular-genetic approaches to sensations passing between flies in reproductive contexts have aimed to "dissect" further the meaning of separate sensory modalities. Notable among these are olfactory and contact-chemosensory stimuli, which perhaps have received an inordinate amount of attention in terms of the possibility that they could comprise the key cues involved in triggering and sustaining courtship actions. But visual and auditory stimuli are heavily involved as well--appreciated mainly from older experiments, but analyzable further using elementary approaches (single-gene mutations mutants and surgeries), as well as by applying the molecularly defined factors alluded to above. Regarding regulation of reproductive behavior by components of Drosophila's central nervous system (CNS), once again significant invigoration of the relevant inquiries has been stimulated and propelled by identification and application of molecular-genetic materials. A distinct plurality of the tools applied involves transposons inserted in the fly's chromosomes, defining "enhancer-trap" strains that can be used to label various portions of the nervous system and, in parallel, disrupt their structure and function by "driving" companion transgenes predesigned for these experimental purposes. Thus, certain components of interneuronal routes, functioning along pathways whose starting points are sensory reception by the peripheral nervous system (PNS), have been manipulated to enhance appreciation of sexually important sensory modalities, as well as to promote understanding of where such inputs end up within the CNS: Where are reproductively related stimuli processed, such that different kinds of sensation would putatively be integrated to mediate sex-specific behavioral readouts? In line with generic sensory studies that have tended to concentrate on chemical stimuli, PNS-to-CNS pathways focused upon in reproductive experiments relying on genic enhancers have mostly involved smell and taste. Enhancer traps have also been applied to disrupt various regions within the CNS to ask about the various ganglia, and portions thereof, that contribute to male- or female-specific behavior. These manipulations have encompassed structural or functional disruptions of such regions as well as application of molecular-genetic tricks to feminize or masculinize a given component of the CNS. Results of such experiments have, indeed, identified certain discrete subsets of centrally located ganglia that, on the one hand, lead to courtship defects when disrupted or, on the other, must apparently maintain sex-specific identity if the requisite courtship actions are to be performed. As just implied, perturbations of certain neural tissues not based on manipulating "sex factors" might lead to reproductive behavioral abnormalities, even though changing the sexual identity of such structures would not necessarily have analogous consequences. It has been valuable to uncover these sexually significant subsets of the Drosophila nervous system, although it must be said that not all of the transgenically based dissection outcomes are in agreement. Thus, the good news is that not all of the CNS is devoted to courtship control, whereby any and all locales disrupted might have led to sex-specific deficits; but the bad news is that the enhancer-trap approach to these matters has not led to definitive homing-in on some tractable number of mutually agreed-upon "courtship centers" within the brain or within the ventral nerve cord (VNC). The latter neural region, which comprises about half of the fly's CNS, is underanalyzed as to its sex-specific significance: How, for example, are various kinds of sensory inputs to posteriorly located PNS structures processed, such that they eventually end up modulating brain functions underlying courtship? And how are sex-specific motor outputs mediated by discrete collections of neurons within VNC ganglia--so that, for instance, male-specific whole-animal motor actions and appendage usages are evoked? These behaviors can be thought of as fixed action patterns. But it is increasingly appreciated that elements of the fly's reproductive behavior can be modulated by previous experience. In this regard, the neural substrates of conditioned courtship are being more and more analyzed, principally by further usages of various transgenic types. Additionally, a set of molecular neurogenetic experiments devoted to experience-dependent courtship was based on manipulations of a salient "sex gene" in D. melanogaster. This well-defined factor is called fruitless (fru). The gene, its encoded products, along with their behavioral and neurobiological significance, have become objects of frenetic attention in recent years. How normal, mutated, and molecularly manipulated forms of fru seem to be generating a good deal of knowledge and insight about male-specific courtship and mating is worthy of much attention. This previews the fact that fruitless matters are woven throughout this chapter as well as having a conspicuous section allocated to them. Finally, an acknowledgment that the reader is being subjected to lengthy preview of an article about this subject is given. This matter is mentioned because--in conjunction with the contemporary broadening and deepening of this investigatory area--brief summaries of its findings are appearing with increasing frequency. This chapter will, from time to time, present our opinion that a fair fraction of the recent minireviews are replete with too many catch phrases about what is really known. This is one reason why the treatment that follows not only attempts to describe the pertinent primary reports in detail but also pauses often to discuss our views about current understandings of sex-specific behavior in Drosophila and its underlying biology.

The gene transformer of anastrepha fruit flies (Diptera, tephritidae) and its evolution in insects

In the tephritids Ceratitis capitata and Bactrocera oleae, the gene transformer acts as the memory device for sex determination, via an auto-regulatory function; and functional Tra protein is produced only in females. This paper investigates the evolution of the gene tra, which was characterised in twelve tephritid species belonging to the less extensively analysed genus Anastrepha. Our study provided the following major conclusions. Firstly, the memory device mechanism used by this gene in sex determination in tephritids likely existed in the common ancestor of the Ceratitis, Bactrocera and Anastrepha phylogenetic lineages. This mechanism would represent the ancestral state with respect to the extant cascade seen in the more evolved Drosophila lineage. Secondly, Transformer2-specific binding intronic splicing silencer sites were found in the splicing regulatory region of transformer but not in doublesex pre-mRNAs in these tephritids. Thus, these sites probably provide the discriminating feature for the putative dual splicing activity of the Tra-Tra2 complex in tephritids. It acts as a splicing activator in dsx pre-mRNA splicing (its binding to the female-specific exon promotes the inclusion of this exon into the mature mRNA), and as a splicing inhibitor in tra pre-mRNA splicing (its binding to the male-specific exons prevents the inclusion of these exons into the mature mRNA). Further, a highly conserved region was found in the specific amino-terminal region of the tephritid Tra protein that might be involved in Tra auto-regulatory function and hence in its repressive splicing behaviour. Finally, the Tra proteins conserved the SR dipeptides, which are essential for Tra functionality.

Sex in flies: what 'body--mind' dichotomy?

Sexual behavior in Drosophila results from interactions of multiple neural and genetic pathways. Male-specific fruitless (fruM) is a major component inducing male behaviors, but recent work indicates key roles for other sex-specific and sex-non-specific components. Notably, male-like courtship by retained (retn) mutant females reveals an intrinsic pathway for male behavior independent of fruM, while behavioral differences between males and females with equal levels of fruM expression indicate involvement of another sex-specific component. Indeed, sex-specific products of doublesex (dsxF and dsxM), that control sexual differentiation of the body, also contribute to sexual behavior and neural development of both sexes. In addition, the single product of the dissatisfaction (dsf) gene is needed for appropriate behavior in both sexes, implying additional complexities and levels of control. The genetic mechanisms controlling sexual behavior are similar to those controlling body sexual development, suggesting biological advantages of modifying an intermediate intrinsic pathway in generation of two substantially different behavioral or morphological states.

The doublesex splicing enhancer components Tra2 and Rbp1 also repress splicing through an intronic silencer

The activation of sex-specific alternative splice sites in the Drosophila melanogaster doublesex and fruitless pre-mRNAs has been well studied and depends on the serine-arginine-rich (SR) splicing factors Tra, Tra2, and Rbp1. Little is known, however, about how SR factors negatively regulate splice sites in other RNAs. Here we examine how Tra2 blocks splicing of the M1 intron from its own transcript. We identify an intronic splicing silencer (ISS) adjacent to the M1 branch point that is sufficient to confer Tra2-dependent repression on another RNA. The ISS was found to function independently of its position within the intron, arguing against the idea that bound repressors function by simply interfering with branch point accessibility to general splicing factors. Conserved subelements of the silencer include five short repeated sequences that are required for Tra2 binding but differ from repeated binding sites found in Tra2-dependent splicing enhancers. The ISS also contains a consensus binding site for Rbp1, and this protein was found to facilitate repression of M1 splicing both in vitro and in Drosophila larvae. In contrast to the cooperative binding of SR proteins observed on the doublesex splicing enhancer, we found that Rbp1 and Tra2 bind to the ISS independently through distinct sequences. Our results suggest that functionally synergistic interactions of these SR factors can cause either splicing activation or repression.

Control of male sexual behavior in Drosophila by the sex determination pathway

Understanding how genes influence behavior, including sexuality, is one of biology's greatest challenges. Much of the recent progress in understanding how single genes can influence behavior has come from the study of innate behaviors in the fruit fly Drosophila melanogaster. In particular, the elaborate courtship ritual performed by the male fly has provided remarkable insights into how the neural circuitry underlying sexual behavior--which is largely innate in flies--is built into the nervous system during development, and how this circuitry functions in the adult. In this review we will discuss how genes of the sex determination pathway in Drosophila orchestrate the developmental events necessary for sex-specific behaviors and physiology, and the broader lessons this can teach us about the mechanisms underlying the development of sex-specific neural circuitry.

The splicing regulators Tra and Tra2 are unusually potent activators of pre-mRNA splicing

Sexual differentiation in Drosophila is regulated through alternative splicing of doublesex. Female-specific splicing is activated through the activity of splicing enhancer complexes assembled on multiple repeat elements. Each of these repeats serves as a binding platform for the cooperative assembly of a heterotrimeric complex consisting of the SR proteins Tra, Tra2 and 9G8. Using quantitative kinetic analyses, we demonstrate that each component of the enhancer complex is capable of recruiting the spliceosome. Surprisingly, Tra, Tra2 and 9G8 are much stronger splicing activators than other SR protein family members and their activation potential is significantly higher than expected from their serine/arginine content. 9G8 activates splicing not only through its RS domains but also through its RNA-binding domain. The RS domains of Tra and Tra2 are required but not sufficient for efficient complex assembly. Thus, the regulated assembly of the dsx enhancer complexes leads to the generation of an extended activation domain to guarantee the ‘all or none’ splicing switch that is required during Drosophila sexual differentiation.

Systematic identification of factors involved in post-transcriptional processes in wheat grain

Post-transcriptional processing of primary transcripts can significantly affect both the quantity and the structure of mature mRNAs and the corresponding protein products. It is an important mechanism of gene regulation in animals, yeast and plants. Here we have investigated the interactive networks of pre-mRNA processing factors in the developing grain of wheat (Triticum aestivum), one of the world's major food staples. As a first step we isolated a homologue of the plant specific AtRSZ33 splicing factor, which has been shown to be involved in the early stages of embryo development in Arabidopsis. Real-time PCR showed that the wheat gene, designated TaRSZ38, is expressed mainly in young, developing organs (flowers, root, stem), and expression peaks in immature grain. In situ hybridization and immunodetection revealed preferential abundance of TaRSZ38 in mitotically active tissues of the major storage organ of the grain, the endosperm. The protein encoded by TaRSZ38 was subsequently used as a starting bait in a two-hybrid screen to identify additional factors in grain that are involved in pre-mRNA processing. Most of the identified proteins showed high homology to known splicing factors and splicing related proteins, supporting a role for TaRSZ38 in spliceosome formation and 5' site selection. Several clones were selected as baits in further yeast two-hybrid screens. In total, cDNAs for 16 proteins were isolated. Among these proteins, TaRSZ22, TaSRp30, TaU1-70K, and the large and small subunits of TaU2AF, are wheat homologues of known plant splicing factors. Several, additional proteins are novel for plants and show homology to known pre-mRNA splicing, splicing related and mRNA export factors from yeast and mammals.

Isoform-specific control of male neuronal differentiation and behavior in Drosophila by the fruitless gene

BACKGROUND

How the central nervous system (CNS) develops to implement innate behaviors remains largely unknown. Drosophila male sexual behavior has long been used as a model to address this question. The male-specific products of fruitless (fru) are pivotal to the emergence of this behavior. These putative transcription factors, containing one of three alternative DNA binding domains, determine the neuronal substrates for sexual behavior in male CNS.

RESULTS

We isolated the first fru coding mutation, resulting in complete loss of one isoform. At the neuronal level, this isoform alone controls differentiation of a male-specific muscle and its associated motorneuron. Conversely, a combination of isoforms is required for development of serotonergic neurons implicated in male copulatory behavior. Full development of these neurons requires the male-specific product of doublesex, a gene previously thought to act independently of fru. At the behavioral level, missing one isoform leads to diminished courtship behavior and infertility. We achieved the first rescue of a distinct fru behavioral phenotype, expressing a wild-type isoform in a defined subset of its normal expression pattern.

CONCLUSION

This study exemplifies how complex behaviors can be controlled by a single locus through multiple isoforms regulating both developmental and physiological pathways in different neuronal substrates.

Sex-lethal imparts a sex-specific function to UNR by recruiting it to the msl-2 mRNA 3' UTR: translational repression for dosage compensation

MSL-2 (male-specific lethal 2) is the limiting component of the Drosophila dosage compensation complex (DCC) that specifically increases transcription from the male X chromosome. Ectopic expression of MSL-2 protein in females causes DCC assembly on both X chromosomes and lethality. Inhibition of MSL-2 synthesis requires the female-specific protein sex-lethal (SXL), which binds to the msl-2 mRNA 5' and 3' untranslated regions (UTRs) and blocks translation through distinct UTR-specific mechanisms. Here, we purify translationally silenced msl-2 mRNPs and identify UNR (upstream of N-ras) as a protein recruited to the 3' UTR by SXL. We demonstrate that SXL requires UNR as a corepressor for 3'-UTR-mediated regulation, imparting a female-specific function to the ubiquitously expressed UNR protein. Our results reveal a novel functional role for UNR as a translational repressor and indicate that UNR is a key component of a "fail-safe" dosage compensation regulatory system that prevents toxic MSL-2 synthesis in female cells.

Functional conservation of the fruitless male sex-determination gene across 250 Myr of insect evolution

Male sexual behavior in the fruit fly Drosophila melanogaster is regulated by fruitless (fru), a sex-determination gene specifying the synthesis of BTB-Zn finger proteins that likely function as male-specific transcriptional regulators. Expression of fru in the nervous system specifies male sexual behavior and the muscle of Lawrence (MOL), an abdominal muscle that develops in males but not in females. We have isolated the fru ortholog from the malaria mosquito Anopheles gambiae and show the gene's conserved genomic structure. We demonstrate that male-specific mosquito fru protein isoforms arise by conserved mechanisms of sex-specifically activated and alternative exon splicing. A male-determining function of mosquito fru is revealed by ectopic expression of the male mosquito isoform FRUMC in fruit flies; this results in MOL development in both fru-mutant males and fru+ females who otherwise develop no MOL. In parallel, we provide evidence of a unique feature of muscle differentiation within the fifth abdominal segment of male mosquitoes that strongly resembles the fruit fly MOL. Given these conserved features within the context of 250 Myr of evolutionary divergence between Drosophila and Anopheles, we hypothesize that fru is the prototypic gene of male sexual behavior among dipteran insects.

Global regulation of X chromosomal genes by the MSL complex in Drosophila melanogaster

A long-standing model postulates that X-chromosome dosage compensation in Drosophila occurs by twofold up-regulation of the single male X, but previous data cannot exclude an alternative model, in which male autosomes are down-regulated to balance gene expression. To distinguish between the two models, we used RNA interference to deplete Male-Specific Lethal (MSL) complexes from male-like tissue culture cells. We found that expression of many genes from the X chromosome decreased, while expression from the autosomes was largely unchanged. We conclude that the primary role of the MSL complex is to up-regulate the male X chromosome.

Accurate pre-mRNA splicing using a nuclear extract from Bombyx mori fat body

A cell-free pre-mRNA splicing system was developed from the nuclear extract of Bombyx mori fat bodies. The synthetic pre-mRNA from the cytoplasmic actin gene of B. mori (BmA3 ) was used as a substrate for in vitro splicing. The pre-mRNA was accurately spliced in the nuclear extract prepared from the fat body. Sequencing of the amplicon by RT-PCR revealed that the spliced site in vitro was identical to the site in vivo. We found that a part of the input capped pre-mRNA was accurately spliced after 2h of incubation. In addition, the RT-PCR analysis precisely mapped the branch point to an adenine located at 30 nucleotides before the 3' end of the first intron in BmA3. This system was a first example that differentiated tissue in insects yielded active extracts for in vitro splicing.

Isolation and characterization of the Bactrocera oleae genes orthologous to the sex determining Sex-lethal and doublesex genes of Drosophila melanogaster

Here we report the isolation and characterization of the olive fruit fly Bactrocera oleae genes orthologous to the Drosophila melanogaster sex-determining genes Sex-lethal (Sxl) and doublesex (dsx). Fragments of the Sxl and dsx orthologous were isolated with RT-PCR. Genomic and cDNA clones were then obtained by screening a genomic library and separate male and female cDNA adult libraries using the RT-PCR products as probes in both cases. B. oleae Sxl gene (BoSxl) expresses the same pattern of transcripts which encode for a single common polypeptide in both male and female flies. The gene shares a high degree of similarity in sequence and expression to its Ceratitis capitata orthologous and does not appear to play a key regulatory role in the sex-determining cascade. B. oleae dsx gene (Bodsx) expands in a chromosomal region of more than 50 kb, with 6 exons-5 introns, producing different sex-specific mRNAs, according to the Drosophila model. The cDNA sequences are almost identical to the gene orthologous of Bactrocera tryoni. Four repeat elements identical to the D. melanogaster TRA/TRA-2 binding sites have been found in the untranslated region of the female-specific exon 4, predicting a common regulatory splicing mechanism in all studied species of Diptera.

The transformer2 gene in Musca domestica is required for selecting and maintaining the female pathway of development

We present the isolation and functional analysis of a transformer2 homologue Mdtra2 in the housefly Musca domestica. Compromising the activity of this gene by injecting dsRNA into embryos causes complete sex reversal of genotypically female individuals into fertile males, revealing an essential function of Mdtra2 in female development of the housefly. Mdtra2 is required for female-specific splicing of Musca doublesex (Mddsx) which structurally and functionally corresponds to Drosophila dsx, the bottom-most regulator in the sex-determining pathway. Since Mdtra2 is expressed in males and females, we propose that Mdtra2 serves as an essential co-factor of F, the key sex-determining switch upstream of Mddsx. We also provide evidence that Mdtra2 acts upstream as a positive regulator of F supporting genetic data which suggest that F relies on an autocatalytic activity to select and maintain the female path of development. We further show that repression of male courtship behavior by F requires Mdtra2. This function of F and Mdtra2 appears not to be mediated by Mddsx, suggesting that bifurcation of the pathway at this level is a conserved feature in the genetic architecture of Musca and Drosophila.

Exonic splicing enhancers in fission yeast: functional conservation demonstrates an early evolutionary origin

Discrete sequence elements known as exonic splicing enhancers (ESEs) have been shown to influence both the efficiency of splicing and the profile of mature mRNAs in multicellular eukaryotes. While the existence of ESEs has not been demonstrated previously in unicellular eukaryotes, the factors known to recognize these elements and mediate their communication with the core splicing machinery are conserved and essential in the fission yeast Schizosaccharomyces pombe. Here, we provide evidence that ESE function is conserved through evolution by demonstrating that three exonic splicing enhancers derived from vertebrates (chicken ASLV, mouse IgM, and human cTNT) promote splicing of two distinct S. pombe pre-messenger RNAs (pre-mRNAs). Second, as in extracts from mammalian cells, ESE function in S. pombe is compromised by mutations and increased distance from the 3'-splice site. Third, three-hybrid analyses indicate that the essential SR (serine/arginine-rich) protein Srp2p, but not the dispensable Srp1p, binds specifically to both native and heterologous purine-rich elements; thus, Srp2p is the likely mediator of ESE function in fission yeast. Finally, we have identified five natural purine-rich elements from S. pombe that promote splicing of our reporter pre-mRNAs. Taken together, these results provide strong evidence that the genesis of ESE-mediated splicing occurred early in eukaryotic evolution.

Identification of two teleost homologs of the Drosophila sex determination factor, transformer-2 in medaka (Oryzias latipes)

Transformer-2 (Tra2), an RNA-binding protein, is an important regulator in Drosophila sex determination. In vertebrates, however, the role of Tra2 homologues is not known. We identified two teleost homologues of Tra2, which we named Tra2a and Tra2b, in medaka (Oryzias latipes). Furthermore, we demonstrated that both Tra2 mRNAs were predominantly expressed in germ cells of both sexes before the onset of sex differentiation, suggesting that both Tra2 homologues might be involved in the sex differentiation in medaka.

Sex determination in Drosophila melanogaster and Musca domestica converges at the level of the terminal regulator doublesex

Sex-determining cascades are supposed to have evolved in a retrograde manner from bottom to top. Wilkins' 1995 hypothesis finds support from our comparative studies in Drosophila melanogaster and Musca domestica, two dipteran species that separated some 120 million years ago. The sex-determining cascades in these flies differ at the level of the primary sex-determining signal and their targets, Sxl in Drosophila and F in Musca. Here we present evidence that they converge at the level of the terminal regulator, doublesex ( dsx), which conveys the selected sexual fate to the differentiation genes. The dsx homologue in Musca, Md-dsx, encodes male-specific (MdDSX(M)) and female-specific (MdDSX(F)) protein variants which correspond in structure to those in Drosophila. Sex-specific regulation of Md-dsx is controlled by the switch gene F via a splicing mechanism that is similar but in some relevant aspects different from that in Drosophila. MdDSX(F) expression can activate the vitellogenin genes in Drosophila and Musca males, and MdDSX(M) expression in Drosophila females can cause male-like pigmentation of posterior tergites, suggesting that these Musca dsx variants are conserved not only in structure but also in function. Furthermore, downregulation of Md-dsx activity in Musca by injecting dsRNA into embryos leads to intersexual differentiation of the gonads. These results strongly support a role of Md-dsx as the final regulatory gene in the sex-determining hierarchy of the housefly.

Direct repression of splicing by transformer-2

The Drosophila melanogaster sex determination factor Tra2 positively regulates the splicing of both doublesex (dsx) and fruitless (fru) pre-mRNAs but negatively affects the splicing of the M1 intron in tra2 pre-mRNA. Retention of the M1 intron is known to be part of a negative-feedback mechanism wherein the Tra2 protein limits its own synthesis, but the mechanism responsible for accumulation of M1-containing RNA is unknown. Here we show that the recombinant Tra2 protein specifically represses M1 splicing in Drosophila nuclear extracts. We find that the Tra2 protein binds directly to several sites in and near the M1 intron and that, when Tra2 binding is competed with other RNAs, the splicing of M1 is restored. Mapping the RNA sequences functionally required for M1 repression identified both a 34-nucleotide (nt) A/C-rich sequence immediately upstream of the M1 5' splice site and a region within the intron itself. The AC-rich sequence is largely composed of a repeated 4-nt sequence that also forms a subrepeat within the repeated 13-nt splicing enhancer elements of fru and dsx RNAs. Although required for repression, the element also enhances M1 splicing in the absence of Tra2. We propose that Tra2 represses M1 splicing by interacting with multiple sequences in the pre-mRNA and interfering with enhancer function.

Enhancer-dependent 5'-splice site control of fruitless pre-mRNA splicing

The Drosophila fruitless (fru) gene encodes a transcription factor that essentially regulates all aspects of male courtship behavior. The use of alternative 5'-splice sites generates fru isoforms that determine gender-appropriate sexual behaviors. Alternative splicing of fru is regulated by TRA and TRA2 and depends on an exonic splicing enhancer (fruRE) consisting of three 13-nucleotide repeat elements, nearly identical to those that regulate alternative sex-specific 3'-splice site choice in the doublesex (dsx) gene. dsx has provided a useful model system to investigate the mechanisms of enhancer-dependent 3'-splice site choice. However, little is known about enhancer-dependent regulation of alternative 5'-splice sites. The mechanisms of this process were investigated using an in vitro system in which recombinant TRA/TRA2 could activate the female-specific 5'-splice site of fru. Mutational analysis demonstrated that one 13-nucleotide repeat element within the fruRE is required and sufficient to activate the regulated female-specific splice site. As was established for dsx, the fruRE can be replaced by a short element encompassing tandem 13-nucleotide repeat elements, by heterologous splicing enhancers, and by artificially tethering a splicing activator to the pre-mRNA. Complementation experiments showed that Ser/Arg-rich proteins facilitate enhancer-dependent 5'-splice site activation. We conclude that splicing enhancers function similarly in activating regulated 5'- and 3'-splice sites. These results suggest that exonic splicing enhancers recruit multiple spliceosomal components required for the initial recognition of 5'- and 3'-splice sites.

Masters change, slaves remain

Sex determination offers an opportunity to address many classic questions of developmental biology. In addition, because sex determination evolves rapidly, it offers an opportunity to investigate the evolution of genetic hierarchies. Sex determination in Drosophila melanogaster is controlled by the master regulatory gene, Sex lethal (Sxl). DmSxl controls the alternative splicing of a downstream gene, transformer (tra), which acts with tra2 to control alternative splicing of doublesex (dsx). DmSxl also controls its own splicing, creating an autoregulatory feedback loop that ensures expression of Sxl in females, but not males. A recent paper has shown that in the dipteran Ceratitis capitata later (downstream) steps in the regulatory hierarchy are conserved, while earlier (upstream) steps are not. Cctra is regulated by alternative splicing and apparently controls the alternative splicing of Ccdsx. However, Cctra is not regulated by CcSxl. Instead it appears to autoregulate in a manner similar to the autoregulation seen with DmSxl.