The regulation of sex determination and sexually dimorphic differentiation in Drosophila

Integrating lipid metabolism, pheromone production and perception by Fruitless and Hepatocyte Nuclear Factor 4

Sexual attraction and perception are crucial for mating and reproductive success. In Drosophila melanogaster, the male-specific isoform of Fruitless (Fru), FruM, is a known master neuro-regulator of innate courtship behavior to control the perception of sex pheromones in sensory neurons. Here, we show that the non-sex-specific Fru isoform (FruCOM) is necessary for pheromone biosynthesis in hepatocyte-like oenocytes for sexual attraction. Loss of FruCOM in oenocytes resulted in adults with reduced levels of cuticular hydrocarbons (CHCs), including sex pheromones, and show altered sexual attraction and reduced cuticular hydrophobicity. We further identify Hepatocyte nuclear factor 4 (Hnf4) as a key target of FruCOM in directing fatty acid conversion to hydrocarbons. Fru or Hnf4 depletion in oenocytes disrupts lipid homeostasis, resulting in a sex-dimorphic CHC profile that differs from doublesex- and transformer-dependent CHC dimorphism. Thus, Fru couples pheromone perception and production in separate organs to regulate chemosensory communications and ensure efficient mating behavior.

Genomic and cDNA selection-amplification identifies transcriptome-wide binding sites for the Drosophila protein sex-lethal

BACKGROUND

Downstream targets for a large number of RNA-binding proteins remain to be identified. The Drosophila master sex-switch protein Sex-lethal (SXL) is an RNA-binding protein that controls splicing, polyadenylation, or translation of certain mRNAs to mediate female-specific sexual differentiation. Whereas some targets of SXL are known, previous studies indicate that additional targets of SXL have escaped genetic screens.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we have used an alternative molecular approach of GEnomic Selective Enrichment of Ligands by Exponential enrichment (GESELEX) using both the genomic DNA and cDNA pools from several Drosophila developmental stages to identify new potential targets of SXL. Our systematic analysis provides a comprehensive view of the Drosophila transcriptome for potential SXL-binding sites.

CONCLUSION/SIGNIFICANCE

We have successfully identified new SXL-binding sites in the Drosophila transcriptome. We discuss the significance of our analysis and that the newly identified binding sites and sequences could serve as a useful resource for the research community. This approach should also be applicable to other RNA-binding proteins for which downstream targets are unknown.

The biological basis of sexual orientation: How hormonal, genetic, and environmental factors influence to whom we are sexually attracted

Humans develop relatively stable attractions to sexual partners during maturation and present a spectrum of sexual orientation from homosexuality to heterosexuality encompassing varying degrees of bisexuality, with some individuals also displaying asexuality. Sexual orientation represents a basic life phenomenon for humans. However, the molecular mechanisms underlying these diverse traits of sexual orientation remain highly controversial. In this review, we systematically discuss recent advancements in sexual orientation research, including those related to measurements and associated brain regions. Current findings regarding sexual orientation modulation by hormonal, genetic, maternal immune system, and environmental factors are summarized in both human and model systems. We also emphasize that future studies should recognize the differences between males and females and pay more attention to minor traits and the epigenetic regulation of sexual orientation. A comprehensive view of sexual orientation may promote our understanding of the biological basis of sex, and that of human reproduction, and evolution.

Molecular mechanisms underlying sexual differentiation of the nervous system

A long-standing goal in developmental neuroscience is to understand the mechanisms by which steroid sex hormones pattern the mammalian central nervous system along male or female pathways to enable subsequent displays of sexually dimorphic behaviors. In this article, we review recent advances in understanding the epigenetic and transcriptional mechanisms mediating sexual differentiation of the brain in mammals, flies, and worms. These studies suggest a model of sexual differentiation wherein master regulators of sex determination initiate a cascade of sexually dimorphic gene expression that controls development of neural pathways and behavioral displays in a strikingly modular manner. With these advances in molecular genetics, it is now feasible to disassemble different components of sexually dimorphic social behaviors without disrupting other behavioral interactions. Such experimental tractability promises rapid advances in this exciting field.

Silencing the buzz: a new approach to population suppression of mosquitoes by feeding larvae double-stranded RNAs

BACKGROUND

Mosquito-borne diseases threaten over half the world's human population, making the need for environmentally-safe mosquito population control tools critical. The sterile insect technique (SIT) is a biological control method that can reduce pest insect populations by releasing a large number of sterile males to compete with wild males for female mates to reduce the number of progeny produced. Typically, males are sterilized using radiation, but such methods can reduce their mating competitiveness. The method is also most effective if only males are produced, but this requires the development of effective sex-sorting methods. Recent efforts to use transgenic methods to produce sterile male mosquitoes have increased interest in using SIT to control some of our most serious disease vectors, but the release of genetically modified mosquitoes will undoubtedly encounter considerable delays as regulatory agencies deal with safety issues and public concerns.

METHODS

Testis genes in the dengue vector Aedes aegypti were identified using a suppression subtractive hybridization technique. Mosquito larvae were fed double-stranded RNAs (dsRNAs) that targeted both the testis genes and a female sex determination gene (doublesex) to induce RNA interference (RNAi) -mediated sterility and inhibition of female development. Fertility and mating competiveness of the treated males were assessed in small-scale mating competition experiments.

RESULTS

Feeding mosquito larvae dsRNAs targeting testis genes produced adult males with greatly reduced fertility; several dsRNAs produced males that were highly effective in competing for mates. RNAi-mediated knockdown of the female-specific isoform of doublesex was also effective in producing a highly male-biased population of mosquitoes, thereby overcoming the need to sex-sort insects before release.

CONCLUSIONS

The sequence-specific gene-silencing mechanism of this RNAi technology renders it adaptable for species-specific application across numerous insect species. We envisage its use for traditional large-scale reared releases of mosquitoes and other pest insects, although the technology might also have potential for field-based control of mosquitoes where eggs deposited into a spiked larval site lead to the release of new sterile males.

Role of cell death in the formation of sexual dimorphism in the Drosophila central nervous system

Currently, sex differences in behavior are believed to result from sexually dimorphic neural circuits in the central nervous system (CNS). Drosophila melanogaster is a common model organism for studying the relationship between brain structure, behavior, and genes. Recent studies of sex-specific reproductive behaviors in D. melanogaster have addressed the contribution of sexual differences in the CNS to the control of sex-specific behaviors and the development of sexual dimorphism. For example, sexually dimorphic regions of the CNS are involved in the initiation of male courtship behavior, the generation of the courtship song, and the induction of male-specific muscles in D. melanogaster. In this review, I discuss recent findings about the contribution of cell death to the formation of sexually dimorphic neural circuitry and the regulation of sex-specific cell death by two sex determination factors, Fruitless and Doublesex, in Drosophila.

Systems behavior: of male courtship, the nervous system and beyond in Drosophila

Male courtship in fruit flies is regulated by the same major regulatory genes that also determine general sexual differentiation of the animal. Elaborate genetics has given us insight into the roles of these master genes. These findings have suggested two separate and independent pathways for the regulation of sexual behavior and other aspects of sexual differentiation. Only recently have molecular studies started to look at the downstream effector genes and how they might control sex-specific behavior. These studies have confirmed the essential role of the previously identified male specific products of the fruitless gene in the neuronal circuits in which it is expressed. But there is increasing evidence that a number of non-neuronal tissues and pathways play a pivotal role in modulating this circuit and assuring efficient courtship.

RNA-Seq analysis of splicing in Plasmodium falciparum uncovers new splice junctions, alternative splicing and splicing of antisense transcripts

Over 50% of genes in Plasmodium falciparum, the deadliest human malaria parasite, contain predicted introns, yet experimental characterization of splicing in this organism remains incomplete. We present here a transcriptome-wide characterization of intraerythrocytic splicing events, as captured by RNA-Seq data from four timepoints of a single highly synchronous culture. Gene model-independent analysis of these data in conjunction with publically available RNA-Seq data with HMMSplicer, an in-house developed splice site detection algorithm, revealed a total of 977 new 5' GU-AG 3' and 5 new 5' GC-AG 3' junctions absent from gene models and ESTs (11% increase to the current annotation). In addition, 310 alternative splicing events were detected in 254 (4.5%) genes, most of which truncate open reading frames. Splicing events antisense to gene models were also detected, revealing complex transcriptional arrangements within the parasite's transcriptome. Interestingly, antisense introns overlap sense introns more than would be expected by chance, perhaps indicating a functional relationship between overlapping transcripts or an inherent organizational property of the transcriptome. Independent experimental validation confirmed over 30 new antisense and alternative junctions. Thus, this largest assemblage of new and alternative splicing events to date in Plasmodium falciparum provides a more precise, dynamic view of the parasite's transcriptome.

Fruitless, doublesex and the genetics of social behavior in Drosophila melanogaster

Two genes coding for transcription factors, fruitless and doublesex, have been suggested to play important roles in the regulation of sexually dimorphic patterns of social behavior in Drosophila melanogaster. The generalization that fruitless specified the development of the nervous system and doublesex specified non-neural tissues culminated with claims that fruitless was both necessary and sufficient to establish sex-specific patterns of behavior. Several recent articles refute this notion, however, demonstrating that at a minimum, both fruitless and doublesex are involved in establishing sexually dimorphic features of neural circuitry and behavior in fruit flies.

Can Drosophila melanogaster represent a model system for the detection of reproductive adverse drug reactions?

Once a molecule is identified as a potential drug, the detection of adverse drug reactions is one of the key components of its development and the FDA approval process. We propose using Drosophila melanogaster to screen for reproductive adverse drug reactions in the early stages of drug development. Compared with other non-mammalian models, D. melanogaster has many similarities to the mammalian reproductive system, including putative sex hormones and conserved proteins involved in genitourinary development. Furthermore, the D. melanogaster model would present significant advantages in time efficiency and cost-effectiveness compared with mammalian models. We present data on methotrexate (MTX) reproductive adverse events in multiple animal models, including fruit flies, as proof-of-concept for the use of the D. melanogaster model.

Doublesex and the regulation of sexual dimorphism in Drosophila melanogaster: structure, function, and mutagenesis of a female-specific domain

The DSX (Doublesex) transcription factor regulates somatic sexual differentiation in Drosophila. Female and male isoforms (DSX F and DSX M) are formed due to sex-specific RNA splicing. DNA recognition, mediated by a shared N-terminal zinc module (the DM domain), is enhanced by a C-terminal dimerization element. Sex-specific extension of this element in DSX F and DSX M leads to assembly of distinct transcriptional preinitiation complexes. Here, we describe the structure of the extended C-terminal dimerization domain of DSX F as determined by multidimensional NMR spectroscopy. The core dimerization element is well ordered, giving rise to a dense network of interresidue nuclear Overhauser enhancements. The structure contains dimer-related UBA folds similar to those defined by x-ray crystallographic studies of a truncated domain. Whereas the proximal portion of the female tail extends helix 3 of the UBA fold, the distal tail is disordered. Ala substitutions in the proximal tail disrupt the sex-specific binding of IX (Intersex), an obligatory partner protein and putative transcriptional coactivator; IX-DSX F interaction is, by contrast, not disrupted by truncation of the distal tail. Mutagenesis of the UBA-like dimer of DSX F highlights the importance of steric and electrostatic complementarity across the interface. Two temperature-sensitive mutations at this interface have been characterized in yeast model systems. One weakens a network of solvated salt bridges, whereas the other perturbs the underlying nonpolar interface. These mutations confer graded gene-regulatory activity in yeast within a physiological temperature range and so may provide novel probes for genetic analysis of a sex-specific transcriptional program in Drosophila development.

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.

Sex-specific DoublesexM expression in subsets of Drosophila somatic gonad cells

Background

In Drosophila melanogaster, a pre-mRNA splicing hierarchy controls sexual identity and ultimately leads to sex-specific Doublesex (DSX) transcription factor isoforms. The male-specific DSX^M represses genes involved in female development and activates genes involved in male development. Spatial and temporal control of dsx during embryogenesis is not well documented.

Results

Here we show that DSX^M is specifically expressed in subsets of male somatic gonad cells during embryogenesis. Following testis formation, germ cells remain in contact with DSX^M-expressing cells, including hub cells and premeiotic somatic cyst cells that surround germ cells during spermatogenesis in larval and adult testes.

Conclusion

We show that dsx is transcriptionally regulated in addition to being regulated at the pre-mRNA splicing level by the sex determination hierarchy. The dsx locus is spatially controlled by somatic gonad identity. The continuous expression of DSX^M in cells contacting the germline suggests an ongoing short-range influence of the somatic sex determination pathway on germ cell development.

Drosophila spermatogenesis: insights into testicular cancer

Geneticists have a long history of studying reproduction in the fruitfly, Drosophila melanogaster, and in recent years it has become apparent that many of the genes that regulate invertebrate reproduction have been conserved through vertebrate evolution. As with other higher eukaryotes, spermatogenesis in Drosophila is characterized by a regenerative germline stem cell population that divides asymmetrically to produce mitotic spermatogonia which will eventually differentiate into spermatocytes. Germline tumours consisting of undifferentiated germ cells have been associated with both loss-of-function mutations and ectopic gene expression. While the genesis of these tumours may not be identical to human germ cell tumours many of the genes that regulate stem cell proliferation and aberrant over-proliferation in the Drosophila testis provide candidate molecules that may underlie the genetic programmes that contribute to human testicular oncogenesis.

Regulation of body pigmentation by the Abdominal-B Hox protein and its gain and loss in Drosophila evolution

Hox genes have been implicated in the evolution of many animal body patterns, but the molecular events underlying trait modification have not been elucidated. Pigmentation of the posterior male abdomen is a recently acquired trait in the Drosophila melanogaster lineage. Here, we show that the Abdominal-B (ABD-B) Hox protein directly activates expression of the yellow pigmentation gene in posterior segments. ABD-B regulation of pigmentation evolved through the gain of ABD-B binding sites in a specific cis-regulatory element of the yellow gene of a common ancestor of sexually dimorphic species. Within the melanogaster species group, male-specific pigmentation has subsequently been lost by at least three different mechanisms, including the mutational inactivation of a key ABD-B binding site in one lineage. These results demonstrate how Hox regulation of traits and target genes is gained and lost at the species level and have general implications for the evolution of body form at higher taxonomic levels.

Splicing factor Tra2-beta1 is specifically induced in breast cancer and regulates alternative splicing of the CD44 gene

The human CD44 gene undergoes extensive alternative splicing of multiple variable exons positioned in a cassette in the middle of the gene. Expression of alternative exons is often restricted to certain tissues and could be associated with tumor progression and metastasis of several human malignancies, including breast cancer. Exon v4 contains multiple copies of a C/A-rich exon enhancer sequence required for optimal inclusion of the exon and binding to the nucleic acid-binding proteins YB-1 and human Tra2-beta1. Here, we show that hTra2-beta1, a member of the extended family of serine/arginine-rich (SR) splicing factors, enhances the in vivo inclusion of CD44 exons v4 and v5. It increased inclusion of exons v4 and v5 and acted synergistically with YB-1. Activation required the C/A-rich enhancer within exon v4. Several other SR proteins had none or only a slight effect on CD44 exon inclusion. In contrast, SC35 inhibited exon usage and antagonized the effects of Tra2 or YB-1. In a matched pair analysis of human breast cancers and their corresponding nonpathologic tissue controls, we found a significant induction of Tra2-beta1 in invasive breast cancer, both on the RNA and protein levels. Together with our functional data, these results suggest an important role for Tra2-beta1 in breast cancer. Induction of this splicing factor might be responsible for splicing of CD44 isoforms associated with tumor progression and metastasis.

No evidence of androgenic hormone from the testes of the glowworm, Lampyris noctiluca

The widely accepted concept, stating that insects have no true sex hormones, and that primary as well as secondary sex characteristics are controlled by the genetic inventory of each single cell, is challenged by the report of Naisse, J. [1966a. Contrôle endocrinien de la différenciation sexuelle chez l'Insecte Lampyris noctiluca (Coléoptère Malacoderme Lampyride). I. Rôle androgène des testicules, Arch. Biol. Liège, 77, 139-201] on the discovery of an androgenic hormone in the glowworm, Lampyris noctiluca. This case is of special interest, since it may point to an ancestral mode of sex differentiation in arthropods, considering that androgenic hormones have been discovered and characterized in crustaceans. With the intention to further characterize the androgenic hormone in the glowworm, and to establish a bioassay, we tried to repeat Naisses's transplantation experiments, according to which, androgen producing testes implanted into female larvae should masculinize the female's gonads and all other female features of the sexually strongly dimorphic pupae and beetles. We found, however, that larval development of the glowworm proceeded differently than reported by Naisse, and that sexing of larvae was not possible. Therefore, "blind" transplantations had to be performed. The results of our experiments showed, however, unequivocally, that an androgenic hormone, allegedly synthesized by the apical tissue of larval testes, was not involved in sex differentiation. We found, that in transplants, where testes and ovaries were even located closely to each other, both matured and formed spermatozoa in the testes and vitellogenic oocytes in the ovaries. Masculinization of ovaries was never observed, and the sex of the recipient was always in accordance with the sex of its own gonads. We therefore conclude that Lampyris noctiluca does not synthesize an androgenic hormone in the larval testes, and that sex differentiation is probably regulated as in other insect species. (The apical testis tissue of the glowworm was previously shown to represent progenitors of spermatogonial cyst cells [Balles, S., Maas, U., Sehn, E., Dorn, A., 2002. Testis differentiation in the glowworm, Lampyris noctiluca, with special reference to the apical tissue. J. Morphol. 251, 22-37].).

Dimerization of doublesex is mediated by a cryptic ubiquitin-associated domain fold: implications for sex-specific gene regulation

Male- and female-specific isoforms of the Doublesex (DSX) transcription factor regulate somatic sexual differentiation in Drosophila. The isoforms (DSX(M) and DSX(F)) share an N-terminal DNA binding domain (the DM motif), broadly conserved among metazoan sex-determining pathways. DM-DNA recognition is enhanced by a C-terminal dimerization domain. The crystal structure of this domain, determined at a resolution of 1.6 A, reveals a novel dimeric arrangement of ubiquitin-associated (UBA) folds. Although this alpha-helical motif is well characterized in pathways of DNA repair and subcellular trafficking, to our knowledge this is its first report in a transcription factor. Dimerization is mediated by a non-canonical hydrophobic interface extrinsic to the putative ubiquitin binding surface. Key side chains at this interface, identified by alanine scanning mutagenesis, are conserved among DSX homologs. The mechanism of dimerization is thus unrelated to the low affinity domain swapping observed among ubiquitin-associated CUE domains. The unexpected observation of a ubiquitin-associated fold in DSX extends the repertoire of alpha-helical dimerization elements in transcription factors. The possibility that the ubiquitination machinery participates in the regulation of sexual dimorphism is discussed.

Human RNPS1 and its associated factors: a versatile alternative pre-mRNA splicing regulator in vivo

Human RNPS1 was originally purified and characterized as a pre-mRNA splicing activator, and its role in the postsplicing process has also been proposed recently. To search for factors that functionally interact with RNPS1, we performed a yeast two-hybrid screen with a human cDNA library. Four factors were identified: p54 (also called SRp54; a member of the SR protein family), human transformer 2 beta (hTra2 beta; an exonic splicing enhancer-binding protein), hLucA (a potential component of U1 snRNP), and pinin (also called DRS and MemA; a protein localized in nuclear speckles). The N-terminal region containing the serine-rich (S) domain, the central RNA recognition motif (RRM), and the C-terminal arginine/serine/proline-rich (RS/P) domain of RNPS1 interact with p54, pinin, and hTra2 beta, respectively. Protein-protein binding between RNPS1 and these factors was verified in vitro and in vivo. Overexpression of RNPS1 in HeLa cells induced exon skipping in a model beta-globin pre-mRNA and a human tra-2 beta pre-mRNA. Coexpression of RNPS1 with p54 cooperatively stimulated exon inclusion in an ATP synthase gamma-subunit pre-mRNA. The RS/P domain and RRM are necessary for the exon-skipping activity, whereas the S domain is important for the cooperative effect with p54. RNPS1 appears to be a versatile factor that regulates alternative splicing of a variety of pre-mRNAs.

Human RNPS1 and its associated factors: a versatile alternative pre-mRNA splicing regulator in vivo

Human RNPS1 was originally purified and characterized as a pre-mRNA splicing activator, and its role in the postsplicing process has also been proposed recently. To search for factors that functionally interact with RNPS1, we performed a yeast two-hybrid screen with a human cDNA library. Four factors were identified: p54 (also called SRp54; a member of the SR protein family), human transformer 2 beta (hTra2 beta; an exonic splicing enhancer-binding protein), hLucA (a potential component of U1 snRNP), and pinin (also called DRS and MemA; a protein localized in nuclear speckles). The N-terminal region containing the serine-rich (S) domain, the central RNA recognition motif (RRM), and the C-terminal arginine/serine/proline-rich (RS/P) domain of RNPS1 interact with p54, pinin, and hTra2 beta, respectively. Protein-protein binding between RNPS1 and these factors was verified in vitro and in vivo. Overexpression of RNPS1 in HeLa cells induced exon skipping in a model beta-globin pre-mRNA and a human tra-2 beta pre-mRNA. Coexpression of RNPS1 with p54 cooperatively stimulated exon inclusion in an ATP synthase gamma-subunit pre-mRNA. The RS/P domain and RRM are necessary for the exon-skipping activity, whereas the S domain is important for the cooperative effect with p54. RNPS1 appears to be a versatile factor that regulates alternative splicing of a variety of pre-mRNAs.

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

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

Logjam encodes a predicted EMP24/GP25 protein that is required for Drosophila oviposition behavior

A newly characterized Drosophila melanogaster gene, logjam (loj), functions in female reproduction by modulating oviposition behavior. The locus encodes at least six overlapping transcripts with unique 5' ends. P-element mutants that express very low levels of loj transcripts are unable to oviposit mature eggs. This phenotype can be rescued by the introduction of a transgene expressing the most abundant loj transcript. As for many genes that specify behavioral outputs, loj is present in the adult central nervous system (CNS). Interestingly, it is also observed in vitellogenic egg chambers, suggesting that there may be multiple functions for this gene in egg-laying behavior. loj encodes a predicted protein with homology to the EMP24/GP25 transmembrane components of cytoplasmic vesicles and likely functions in intracellular trafficking.

Quantitative trait loci responsible for variation in sexually dimorphic traits in Drosophila melanogaster

To understand the mechanisms of morphological evolution and species divergence, it is essential to elucidate the genetic basis of variation in natural populations. Sexually dimorphic characters, which evolve rapidly both within and among species, present attractive models for addressing these questions. In this report, we map quantitative trait loci (QTL) responsible for variation in sexually dimorphic traits (abdominal pigmentation and the number of ventral abdominal bristles and sex comb teeth) in a natural population of Drosophila melanogaster. To capture the pattern of genetic variation present in the wild, a panel of recombinant inbred lines was created from two heterozygous flies taken directly from nature. High-resolution mapping was made possible by cytological markers at the average density of one per 2 cM. We have used a new Bayesian algorithm that allows QTL mapping based on all markers simultaneously. With this approach, we were able to detect small-effect QTL that were not evident in single-marker analyses. Our results show that at least for some sexually dimorphic traits, a small number of QTL account for the majority of genetic variation. The three strongest QTL account for >60% of variation in the number of ventral abdominal bristles. Strikingly, a single QTL accounts for almost 60% of variation in female abdominal pigmentation. This QTL maps to the chromosomal region that Robertson et al. have found to affect female abdominal pigmentation in other populations of D. melanogaster. Using quantitative complementation tests, we demonstrate that this QTL is allelic to the bric a brac gene, whose expression has previously been shown to correlate with interspecific differences in pigmentation. Multiple bab alleles that confer distinct phenotypes appear to segregate in natural populations at appreciable frequencies, suggesting that intraspecific and interspecific variation in abdominal pigmentation may share a similar genetic basis.

Genes expressed in the Drosophila head reveal a role for fat cells in sex-specific physiology

The downstream effectors of the Drosophila sex determination cascade are mostly unknown and thought to mediate all aspects of sexual differentiation, physiology and behavior. Here, we employed serial analysis of gene expression (SAGE) to identify male and female effectors expressed in the head, and report 46 sex-biased genes (>4-fold/P < 0.01). We characterized four novel, male- or female-specific genes and found that all are expressed mainly in the fat cells in the head. Tsx (turn on sex-specificity), sxe1 and sxe2 (sex-specific enzyme 1/2) are expressed in males, but not females, and are dependent on the known sex determination pathway, specifically transformer (tra) and its downstream target doublesex (dsx). Female-specific expression of the fourth gene, fit (female-specific independent of transformer), is not controlled by tra and dsx, suggesting an alternative pathway for the regulation of some effector genes. Our results indicate that fat cells in the head express sex-specific effectors, thereby generating distinct physiological conditions in the male and female head. We suggest that these differences have consequences on the male and female brain by modulating sex-specific neuronal processes.

Sex comes in from the cold: the integration of sex and pattern

There has recently been a revolution in our understanding of how the Drosophila sex-determination hierarchy generates somatic sexual dimorphism. Most significantly, the sex hierarchy has been shown to modulate the activities of well-known signaling molecules (FGF, Wnt and TGF beta proteins) and transcription factors (BAB and DAC) to direct various sex-specific aspects of growth and differentiation. As some of the genes encoding these proteins are also the targets of Hox gene action, these and other findings are revealing the levels at which the sex determination and Hox patterning pathways are integrated to control growth, morphogenesis and differentiation.

Androgens regulate the mammalian homologues of invertebrate sex determination genes tra-2 and fox-1

Androgens, like other steroid hormones, exert profound effects on cell growth and survival by modulating the expression of target genes. In vertebrates, androgens play a critical role downstream of the testis determination pathway, influencing the expression of sexually dimorphic traits. Among cells of the nervous system, motor neurons respond to trophic effects of androgen stimulation, with a subpopulation of spinal motor neurons exhibiting sexually dimorphic survival. To study the mechanisms of androgen action in these cells, we performed a subtractive screen for genes upregulated by androgen in a motor neuron cell line. We show androgen-inducible expression of two RNA-binding proteins that are the mammalian homologues of invertebrate sex determination genes. Androgens upregulate the expression of tra-2alpha, an enhancer of RNA splicing homologous to Drosophila tra-2, and promote redistribution of the protein from a diffuse to a speckled pattern within the nucleus. Similarly, androgens upregulate the expression of a novel gene homologous to Caenorhabditis elegans fox-1. These data indicate that androgens exert their effects, in part, by modulating the expression and function of genes involved in RNA processing, and identify homologues of invertebrate sex determination genes as androgen-responsive genes in mammals.

Genetic control and evolution of sexually dimorphic characters in Drosophila

Sexually dimorphic abdominal pigmentation and segment morphology evolved recently in the melanogaster species group of the fruitfly Drosophila. Here we show that these traits are controlled by the bric à brac [corrected] (bab) gene, which integrates regulatory inputs from the homeotic and sex-determination pathways. bab expression is modulated segment- and sex-specifically in sexually dimorphic species, but is uniform in sexually monomorphic species. We suggest that bab has an ancestral homeotic function, and that regulatory changes at the bab locus played a key role in the evolution of sexual dimorphism. Pigmentation patterns specified by bab affect mating preferences, suggesting that sexual selection has contributed to the evolution of bab regulation.

Multi-layered regulation of courtship behaviour

The fruitless gene governs courtship in male, but not female, Drosophila, yet it is expressed and specifically spliced in the brains of both sexes. New experiments reveal that a splice-recognition site retained in the mature message in females provides the basis for sex-specific translational repression.

Function of RRM domains of Drosophila melanogaster ELAV: Rnp1 mutations and rrm domain replacements with ELAV family proteins and SXL

Members of the ELAV family of proteins contain three RNA recognition motifs (RRMs), which are highly conserved. ELAV, a Drosophila melanogaster member of this family, provides a vital function and exhibits a predominantly nuclear localization. To investigate if the RNA-binding property of each of the ELAV RRMs is required for ELAV's in vivo function, amino acid residues critical in RNA binding for each RRM were individually mutated. A stringent genetic complementation test revealed that when the mutant protein was the sole source of ELAV, RNA-binding ability of each RRM was essential to ELAV function. To assess the degree to which each domain was specific for ELAV function and which domains perhaps performed a function common to related ELAV proteins, we substituted an ELAV RRM with the corresponding RRM from RBP9, the D. melanogaster protein most homologous to ELAV; HuD, a human ELAV family protein; and SXL, which, although evolutionarily related, is not an ELAV family member. This analysis revealed that RRM3 replacements were fully functional, but RRM1 and RRM2 replacements were largely nonfunctional. Under less stringent conditions RRM1 and RRM2 replacements from SXL and RRM1 replacement from RBP9 were able to provide supplemental function in the presence of a mutant hypomorphic ELAV protein.

Evidence for redundancy but not trans factor-cis element coevolution in the regulation of Drosophila Yp genes

In Drosophila melanogaster and the endemic Hawaiian species D. grimshawi three Yolk protein (Yp) genes are expressed in a similar sex- and tissue-specific pattern. In contrast, DNA sequence comparisons of promoter/enhancer regions show low levels of similarity. We tested the functional significance of these observations by transforming D. melanogaster with the genomic region that includes the divergently transcribed D. grimshawi DgYp1 and DgYp2 genes; we found that the introduced genes were expressed in female fat body and in ovaries but not in males. Moreover, we found D. grimshawi proteins in the hemolymph and accumulating in ovaries. Using reporter constructs we showed that the intergenic region from D. grimshawi was sufficient to drive accurate expression, but some low level of ectopic expression was seen in males. Transforming D. melanogaster with constructs bearing deletions within the D. grimshawi intergenic region revealed only subtle effects in the overall level of expression, suggesting a high level of redundancy. Testing mutants in the sex-specific regulator doublesex revealed that it is capable of repressing the DgYp genes in males. Together, these data show that D. melanogaster trans-acting factors can regulate the in vivo pattern of DgYp expression and support the notion of a redundant and complex system of cis-acting elements.

hermaphrodite and doublesex function both dependently and independently to control various aspects of sexual differentiation in Drosophila

The hermaphrodite (her) gene is necessary for sexual differentiation in Drosophila. Our characterization of her's zygotic function suggests that one set of female-specific terminal differentiation genes, the yolk protein (yp) genes, is transcriptionally activated by two separate pathways. One is a female-specific pathway, which is positively regulated by the female-specific doublesex protein (DSXF). The other is a non-sex-specific pathway, that is positively regulated by HER. The HER pathway is prevented from functioning in males by the action of the male-specific doublesex protein (DSXM). The HER and DSX pathways also function independently to control downstream target genes in the precursor cells that give rise to the vaginal teeth and dorsal anal plate of females, and the lateral anal plates of males. However, a female-specific pathway that is dependent on both DSXF and HER controls the female-specific differentiation of the foreleg bristles and tergites 5 and 6, and the male-specific differentiation of these tissues does not require the suppression of HER's function by DSXM.

Human Tra2 proteins are sequence-specific activators of pre-mRNA splicing

The RNA-binding protein Tra2 is an important regulator of sex determination in Drosophila. Recently, two mammalian Tra2 homologs of unknown function have been described. Here, we show that human Tra2 proteins are present in HeLa cell nuclear extracts and that they bind efficiently and specifically to a previously characterized pre-mRNA splicing enhancer element. Indeed, both purified proteins bound preferentially to RNA sequences containing GAA repeats, characteristic of many enhancer elements. Neither Tra2 protein functioned in constitutive splicing in vitro, but both activated enhancer-dependent splicing in a sequence-specific manner and restored it after inhibition with competitor RNA. Our findings indicate that mammalian Tra2 proteins are sequence-specific splicing activators that likely participate in the control of cell-specific splicing patterns.

Her, a gene required for sexual differentiation in Drosophila, encodes a zinc finger protein with characteristics of ZFY-like proteins and is expressed independently of the sex determination hierarchy

The zygotic function of the hermaphrodite (her) gene of Drosophila plays an important role in sexual differentiation. Our molecular genetic characterization of her suggests that her is expressed sex non-specifically and independently of other known sex determination genes and that it acts together with the last genes in the sex determination hierarchy, doublesex and intersex, to control female sexual differentiation. Consistent with such a terminal function in sexual differentiation, her encodes a protein with C2H2-type zinc fingers. The her zinc fingers are atypical and similar to the even-numbered zinc fingers of ZFY and ZFX proteins in humans and other vertebrates.

Expression of the sex determining cascade genesSex-lethalanddoublesexin the phorid flyMegaselia scalaris

Sex-lethal (Sxl) and doublesex (dsx) are known to represent parts of the sex-determining cascade in Drosophila melanogaster. We generated cDNA probes of the homologous genes from Megaselia scalaris, a fly species with an epistatic maleness factor as the primary sex determining signal. In Northern blot hybridization of poly(A)+RNA, the M. scalaris dsx probe detected two bands, one of which had a sex-specific size difference, while the Sxl probe bound to RNAs of equal size in females and males. RT-PCR showed Sxl to be transcribed in gonads of adult females and males but not in somatic tissues. Thus, while dsx appears to have a similar function in M. scalaris and D. melanogaster, Sxl does not. The results suggest that the sex-determining pathway of M. scalaris joins that of D. melanogaster between the Sxl and dsx steps.Key words: RNA-binding domain, zinc finger, differential splicing, Drosophila.

roX1 RNA paints the X chromosome of male Drosophila and is regulated by the dosage compensation system

The Drosophila roX1 gene is X-linked and produces RNAs that are male-specific, somatic, and preferentially expressed in the central nervous system. These RNAs are retained in the nucleus and lack any significant open reading frame. Although all sexually dimorphic characteristics in Drosophila were thought to be controlled by the sex determination pathway through the gene transformer (tra), the expression of roX1 is independent of tra activity. Instead, the dosage compensation system is necessary and sufficient for the expression of roX1. Consistent with a potential function in dosage compensation, roX1 RNAs localize specifically to the male X chromosome. This localization occurs even when roX1 RNAs are expressed from autosomal locations in X-to-autosome translocations. The novel regulation and subnuclear localization of roX1 RNAs makes them candidates for an RNA component of the dosage compensation machinery.

dissatisfaction, a gene involved in sex-specific behavior and neural development of Drosophila melanogaster

Few mutations link well defined behaviors with individual neurons and the activity of specific genes. In Drosophila, recent evidence indicates the presence of a doublesex-independent pathway controlling sexual behavior and neuronal differentiation. We have identified a gene, dissatisfaction (dsf), that affects sex-specific courtship behaviors and neural differentiation in both sexes without an associated general behavioral debilitation. Male and female mutant animals exhibit abnormalities in courtship behaviors, suggesting a requirement for dsf in the brain. Virgin dsf females resist males during courtship and copulation and fail to lay mature eggs. dsf males actively court and attempt copulation with both mature males and females but are slow to copulate because of maladroit abdominal curling. Structural abnormalities in specific neurons indicate a role for dsf in the differentiation of sex-specific abdominal neurons. The egg-laying defect in females correlates with the absence of motor neuronal innervation on uterine muscles, and the reduced abdominal curling in males correlates with alteration in motor neuronal innervation of male ventral abdominal muscles. Epistasis experiments show that dsf acts in a tra-dependent and dsx-independent manner, placing dsf in the dsx-independent portion of the sex determination cascade.

Control of male sexual behavior and sexual orientation in Drosophila by the fruitless gene

Sexual orientation and courtship behavior in Drosophila are regulated by fruitless (fru), the first gene in a branch of the sex-determination hierarchy functioning specifically in the central nervous system (CNS). The phenotypes of new fru mutants encompass nearly all aspects of male sexual behavior. Alternative splicing of fru transcripts produces sex-specific proteins belonging to the BTB-ZF family of transcriptional regulators. The sex-specific fru products are produced in only about 500 of the 10(5) neurons that comprise the CNS. The properties of neurons expressing these fru products suggest that fru specifies the fates or activities of neurons that carry out higher order control functions to elicit and coordinate the activities comprising male courtship behavior.

att, a target for regulation by tra2 in the testes of Drosophila melanogaster, encodes alternative RNAs and alternative proteins

We have identified a gene, alternative testis transcripts (att), which is alternatively expressed, at both the RNA and protein levels, in testes and somatic tissues. The testis-specific RNA differs from somatic RNAs in both promoter usage and RNA processing and is dependent on the function of the transformer 2 gene. The differences between the somatic and testis RNAs have substantial consequences at the protein level. The somatic RNAs encode a protein with homology to the mammalian Graves' disease carrier proteins. The testis RNA lacks the initiation codons used in somatic tissue and encodes two different proteins. One of these begins in a testis-specific exon, uses a reading frame different from that for the somatic protein, and is completely novel. The other protein initiates translation in the frame of the somatic RNA at a Len CUG codon which is within the open reading frame for the somatic protein. This produces a novel truncated version of the Graves' disease carrier protein-like protein that lacks all sequences N terminal to the first transmembrane domain.

Sex-specific and non-sex-specific oligomerization domains in both of the doublesex transcription factors from Drosophila melanogaster

The doublesex gene of Drosophila melanogaster encodes the alternatively spliced, sex-specific transcription factors DSXM and DSXF. These factors regulate male- and female-specific transcription of many genes. For example, female-specific transcription of the yolk protein 1 gene is regulated by DSXM repression in males and DSXF activation in females. In this study we used in vitro interaction assays and the in vivo yeast two-hybrid method to identify and examine oligomerization domains of the DSX proteins. A 66-amino-acid segment common to both proteins (amino acids 39 to 104) contains a sequence-specific DNA binding domain and an oligomerization domain (OD1). The OD1 domain oligomerizes up to at least a pentamer, but only dimers bound to a palindromic regulatory site in the yolk protein 1 gene are detected. Both subunits of the OD1 dimer are in contact with DNA. Another segment of each protein (amino acids 350 to 412 for DSXF and 350 to 427 for DSXM) contains a second oligomerization domain (OD2F and OD2M, respectively). The OD2 domains have both sex-specific and non-sex-specific sequences which are necessary for oligomerization. On the basis of sequence analysis, we predict that OD2 oligomerizes through coiled-coil interactions. We speculate that the common function of OD1 and OD2 is to oligomerize the full-length proteins, whereas their specialized functions are to form a dimeric DNA binding unit and a sex-specific transcriptional activation or repression unit.

Purification and physical properties of the male and female double sex proteins of Drosophila

The double sex gene (dsx) encodes two proteins, DSX(M) and DSX(F), that regulate sex-specific transcription in Drosophila. These proteins bind target sites in DNA from which the male-specific DSX(M) represses and the female-specific DSX(F) activates transcription of yolk protein (Yp) genes. We investigated the physical properties of these DSX proteins, which are identical in their amino-terminal 397 residues but are entirely different in their carboxyl-terminal sequences (DSX(F), 30 amino acids; DSX(M), 152 amino acids). DSX(M) and DSX(F) were overexpressed in cultured insect cells and purified to near homogeneity. Gel filtration chromatography and glycerol gradient sedimentation showed that at low concentrations both proteins are dimers of highly asymmetrical shape. The axial ratios are approximately 18:1 (DSX(M), 860 X 48 angstroms; DSX(F), 735 X 43 angstroms). At higher concentrations, the proteins form tetramers. Through use of a novel, double crosslinking assay (protein-DNA plus protein-protein), we demonstrated that a DNA regulatory site binds to both monomers of the DSX dimer and to only two monomers of the tetramer. Furthermore, binding another DNA molecule to what we presume is the second and identical site in the tetramer dramatically shifts the equilibrium from tetramers to dimers. These oligomerization and DNA binding properties are indistinguishable between the male and female proteins.

Sex-specific differentiation of a male-specific abdominal muscle, the Muscle of Lawrence, is abnormal in hydroxyurea-treated and in fruitless male flies

A prominent sex-specific abdominal muscle in male Drosophila is the Muscle of Lawrence (MOL), which is induced by male-specific innervation. We have examined MOL development in wild-type males, in males fed hydroxyurea to ablate the muscle precursors and in fruitless mutants, in which the MOL muscle develops aberrantly. One striking feature of MOLs in wild-type males was the presence of additional muscle nuclei compared with neighboring muscles or MOL-homologues in females. We tested whether muscle length and the sex-specific expression of a reporter gene depended critically on the number of nuclei present within a MOL fiber. MOL fibers developing from a reduced myoblast pool in hydroxyurea-affected hemisegments were recognizable by their attachment points and still contained more nuclei than did neighboring medial fibers, suggesting that these MOL fibers were able to actively recruit myoblasts nearly as well as wild-type MOLs. However, many of the hydroxyurea-affected MOL fibers were incapable of the normal male-specific expression of a muscle-specific reporter gene. We suggest that early events in MOL development, such as finding the correct muscle attachment points, are relatively insensitive to the number of MOL nuclei compared with later events, such as the sex-specific expression of a reporter gene. In fruitless mutant males, MOL-position fibers are smaller and had substantially fewer nuclei compared to wild-type MOLs. Since the number and distribution of muscle precursors was the same in fruitless mutant and wild-type animals, we propose that one fru+ function is to direct the male-specific recruitment of myoblasts into MOL-myotubes. However, fruitless+ must have more than one role in MOL fiber development, since simple reduction in the number of muscle nuclei, as demonstrated by the hydroxyurea ablations, is insufficient to account for all of the MOL muscle phenotypes in fruitless mutant males.

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

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

Integrating sex- and tissue-specific regulation within a single Drosophila enhancer

We have investigated the integration of sex- and tissue-specific transcriptional regulation in Drosophila. A single copy of the o-r enhancer from yolk protein genes directs female- and fat body-specific transcription. It consists of four protein-binding sites: dsxA, which binds male (DSXM) and female (DSXF) proteins encoded by the doublesex gene; aef1, which binds the AEF1 repressor; bzip1, which binds the DmC/EBP activator encoded by the slbo gene; and ref1, which binds an unknown activator. Multimeric and mutated binding sites were used in protein binding, germ-line transformation, and genetic experiments to examine the independent and combinatorial activities of the proteins and DNA sites. DSXF activates from dsxA by sterically excluding AEF1 repressor from the aef1 site and synergistically activating transcription together with a protein at bzip1. Sex specificity in fat bodies arises from the opposite effect of DSXM, which represses activity of the protein at bzip1. Tissue specificity is regulated by all four DNA sites. Separately, bzip1 and ref1 activate transcription in ovarian somatic cells and all nongonadal tissues, respectively, whereas together they activate only in fat bodies. The aef1 site represses ectopic transcription in ovaries and dsxA antirepresses this activity in fat bodies. Thus, in the organism, ref1 and bzip1 act combinatorially to direct the fundamental tissue specificity, aef1 and dsxA modulate this tissue specificity, and dsxA adds sex specificity.

Function of Drosophila ovo+ in germ-line sex determination depends on X-chromosome number

Germ-line sex determination in Drosophila melanogaster requires an assessment of the number of X chromosomes as measured against autosomal standards (XX = female, X = male) and signaling from the soma. Both of these sex determination cues are required for female-specific Sex-lethal+ function in germ cells. The ovo+ locus encodes zinc finger protein(s) required for female-specific splicing of Sex-lethal+ pre-mRNA, making ovo+ a candidate function acting between the two principal cues and Sex-lethal+. We have made ovo reporter genes and find that they show high activity in the germ line of females and low activity in the germ line of males. XY flies transformed into somatic females do not show high levels of reporter activity, while XX flies transformed into somatic males do. This shows that high level ovo+ expression depends on the number of X chromosomes, not the somatic sexual signals. The requirement for ovo+ function is restricted to XX flies. Mutations in ovo have no effect on XY males, X0 males or XY females, but have pronounced effects on germ cell viability in XX females, XX females with sex transformed germ lines, and XX males indicating that ovo+ gene products are required for events occurring only in flies with two X chromosomes.

The mating of a fly

Courtship in Drosophila is influenced by a wide variety of genes, in that many different kinds of pleiotropic mutations lead to defective courtship. This may seem to be a truism, but the broad temporal and spatial expression of most of the fly's "neuro genes" makes it difficult to exclude elements of such genes' actions as materially underlying reproductive behavior. "Courtship genes" that seem to play more particular roles were originally identified as sensory, learning, or rhythm mutations; their reproductive abnormalities have been especially informative for revealing components of male or female actions that might otherwise have gone unnoticed. Further behavioral mutations seemed originally to be courtship-specific, turned out not to have that property, and have led to a broadened perspective on the nature and action of Drosophila's sex-determination genes.

Behavioral and neurobiological implications of sex-determining factors in Drosophila

The function of the central nervous system as it controls sex-specific behaviors in Drosophila has been studied with renewed intensity, in the context of genetic factors that influence the development of sexually differentiated aspects of this insect. Three categories of genetic variations that cause anomalies in courtship and mating behaviors are discussed: (1) mutants isolated with regard to courtship defects, of which putatively courtship-specific variants such as the fruitless mutant are a subset; (2) general behavioral and neurological variants (including sensory and learning mutants), whose defects include subnormal reproductive performance; and (3) mutations of genes within the sex-determination regulatory hierarchy of Drosophila, the analysis of which has included studies of reproductive behavior. Recent studies of mutations in two of these categories have provided new insights into the control of neuronally based aspects of sex-specific behavior. The doublesex gene, the final factor acting in the sex-determination hierarchy, had been previously thought to regulate all aspects of sexual differentiation. Yet, it has been recently shown that doublesex does not control at least one neuronally-determined feature of sex-specific anatomy--a muscle in the male's abdomen, whose normal development is, however, dependent on the action of fruitless. These considerations prompted us to examine further (and in some cases re-examine) the influences exerted by sex-determination hierarchy genes on behavior. Our results--notably those obtained from assessments of doublesex mutations' effects on general reproductive actions and on a particular component of the courtship sequence (male "singing" behavior)--lead to the suggestion that there is a previously unrecognized branch within the sex-determination hierarchy, which controls the differentiation of the male- and female- specific phenotypes of Drosophila. This new branch separates from the doublesex-related one immediately before the action of that gene (just after transformer and transformer-2) and appears to control as least some aspects of neuronally determined sexual differentiation of males.