Doublesex establishes sexual dimorphism in the Drosophila central nervous system in an isoform-dependent manner by directing cell number

Neurobiology of egg-laying behavior in Drosophila: neural control of the female reproductive system

Egg-laying is one of the key aspects of female reproductive behavior in insects. Egg-laying has been studied since the dawn of Drosophila melanogaster as a model organism. The female's internal state, hormones, and external factors, such as nutrition, light, and social environment, affect egg-laying output. However, only recently, neurobiological features of egg-laying behavior have been studied in detail. fruitless and doublesex, two key players in the sex determination pathway, have become focal points in identifying neurons of reproductive significance in both central and peripheral nervous systems. The reproductive tract and external terminalia house sensory neurons that carry the sensory information of egg maturation, mating and egg-laying. These sensory signals include the presence of male accessory gland products and mechanical stimuli. The abdominal neuromere houses neurons that receive information from the reproductive tract, including sex peptide abdominal ganglion neurons (SAGs), and send their information to the brain. In the brain, neuronal groups like aDNs and pC1 clusters modulate egg-laying decision-making, and other neurons like oviINs and oviDNs are necessary for egg-laying itself. Lastly, motor neurons involved in egg-laying, which are mostly octopaminergic, reside in the abdominal neuromere and orchestrate the muscle movements required for laying the egg. Egg-laying neuronal control is important in various evolutionary processes like cryptic female choice, and using different Drosophila species can provide intriguing avenues for the future of the field.

Lagging Brain Gene Expression Patterns of Drosophila melanogaster Young Adult Males Confound Comparisons Between Sexes

Many species, including fruit flies (Drosophila melanogaster), are sexually dimorphic. Phenotypic variation in morphology, physiology, and behavior can affect development, reproduction, health, and aging. Therefore, designating sex as a variable and sex-blocking should be considered when designing experiments. The brain regulates phenotypes throughout the lifespan by balancing survival and reproduction, and sex-specific development at each life stage is likely. Changes in morphology and physiology are governed by differential gene expression, a quantifiable molecular marker for age- and sex-specific variations. We assessed the fruit fly brain transcriptome at three adult ages for gene expression signatures of sex, age, and sex-by-age: 6698 genes were differentially expressed between sexes, with the most divergence at 3 days. Between ages, 31.1% of 6084 differentially expressed genes (1890 genes) share similar expression patterns from 3 to 7 days in females, and from 7 to 14 days in males. Most of these genes (90.5%, 1712) were upregulated and enriched for chemical stimulus detection and/or cilium regulation. Our data highlight an important delay in male brain gene regulation compared to females. Because significant delays in expression could confound comparisons between sexes, studies of sexual dimorphism at phenotypically comparable life stages rather than chronological age should be more biologically relevant.

Contribution of neurons that express fruitless and Clock transcription factors to behavioral rhythms and courtship

Animals need to integrate information across neuronal networks that direct reproductive behaviors and circadian rhythms. In Drosophila, the master regulatory transcription factors that direct courtship behaviors and circadian rhythms are co-expressed in a small set of neurons. In this study we investigate the role of these neurons in both males and females. We find sex-differences in the number of these fruitless and Clock -expressing neurons ( fru ∩ Clk neurons) that is regulated by male-specific Fru. We assign the fru ∩ Clk neurons to the electron microscopy connectome that provides high resolution structural information. We also discover sex-differences in the number of fru -expressing neurons that are post-synaptic targets of Clk -expressing neurons, with more post-synaptic targets in males. When fru ∩ Clk neurons are activated or silenced, males have a shorter period length. Activation of fru ∩ Clk neurons also changes the rate a courtship behavior is performed. We find that activation and silencing fru ∩ Clk neurons impacts the molecular clock in the sLNv master pacemaker neurons, in a cell-nonautonomous manner. These results reveal how neurons that subserve the two processes, reproduction and circadian rhythms, can impact behavioral outcomes in a sex-specific manner.

Changes in the cellular makeup of motor patterning circuits drive courtship song evolution in Drosophila

How evolutionary changes in genes and neurons encode species variation in complex motor behaviors is largely unknown. Here, we develop genetic tools that permit a neural circuit comparison between the model species Drosophila melanogaster and the closely related species D. yakuba, which has undergone a lineage-specific loss of sine song, one of the two major types of male courtship song in Drosophila. Neuroanatomical comparison of song-patterning neurons called TN1 across the phylogeny demonstrates a link between the loss of sine song and a reduction both in the number of TN1 neurons and the neurites supporting the sine circuit connectivity. Optogenetic activation confirms that TN1 neurons in D. yakuba have lost the ability to drive sine song, although they have maintained the ability to drive the singing wing posture. Single-cell transcriptomic comparison shows that D. yakuba specifically lacks a cell type corresponding to TN1A neurons, the TN1 subtype that is essential for sine song. Genetic and developmental manipulation reveals a functional divergence of the sex determination gene doublesex in D. yakuba to reduce TN1 number by promoting apoptosis. Our work illustrates the contribution of motor patterning circuits and cell type changes in behavioral evolution and uncovers the evolutionary lability of sex determination genes to reconfigure the cellular makeup of neural circuits.

The role of fruitless in specifying courtship behaviors across divergent Drosophila species

Sex-specific behaviors are critical for reproduction and species survival. The sex-specifically spliced transcription factor fruitless (fru) helps establish male courtship behaviors in invertebrates. Forcing male-specific fru (fruM) splicing in Drosophila melanogaster females produces male-typical behaviors while disrupting female-specific behaviors. However, whether fru's joint role in specifying male and inhibiting female behaviors is conserved across species is unknown. We used CRISPR-Cas9 to force FruM expression in female Drosophila virilis, a species in which males and females produce sex-specific songs. In contrast to D. melanogaster, in which one fruM allele is sufficient to generate male behaviors in females, two alleles are needed in D. virilis females. D. virilis females expressing FruM maintain the ability to sing female-typical song as well as lay eggs, whereas D. melanogaster FruM females cannot lay eggs. These results reveal potential differences in fru function between divergent species and underscore the importance of studying diverse behaviors and species for understanding the genetic basis of sex differences.

Environmental experiences shape sexually dimorphic neuronal circuits and behaviour

Dimorphic traits, shaped by both natural and sexual selection, ensure optimal fitness and survival of the organism. This includes neuronal circuits that are largely affected by different experiences and environmental conditions. Recent evidence suggests that sexual dimorphism of neuronal circuits extends to different levels such as neuronal activity, connectivity and molecular topography that manifest in response to various experiences, including chemical exposures, starvation and stress. In this review, we propose some common principles that govern experience-dependent sexually dimorphic circuits in both vertebrate and invertebrate organisms. While sexually dimorphic neuronal circuits are predetermined, they have to maintain a certain level of fluidity to be adaptive to different experiences. The first layer of dimorphism is at the level of the neuronal circuit, which appears to be dictated by sex-biased transcription factors. This could subsequently lead to differences in the second layer of regulation namely connectivity and synaptic properties. The third regulator of experience-dependent responses is the receptor level, where dimorphic expression patterns determine the primary sensory encoding. We also highlight missing pieces in this field and propose future directions that can shed light onto novel aspects of sexual dimorphism with potential benefits to sex-specific therapeutic approaches. Thus, sexual identity and experience simultaneously determine behaviours that ultimately result in the maximal survival success.

Let's talk about sex: Mechanisms of neural sexual differentiation in Bilateria

In multicellular organisms, sexed gonads have evolved that facilitate release of sperm versus eggs, and bilaterian animals purposefully combine their gametes via mating behaviors. Distinct neural circuits have evolved that control these physically different mating events for animals producing eggs from ovaries versus sperm from testis. In this review, we will describe the developmental mechanisms that sexually differentiate neural circuits across three major clades of bilaterian animals-Ecdysozoa, Deuterosomia, and Lophotrochozoa. While many of the mechanisms inducing somatic and neuronal sex differentiation across these diverse organisms are clade-specific rather than evolutionarily conserved, we develop a common framework for considering the developmental logic of these events and the types of neuronal differences that produce sex-differentiated behaviors. This article is categorized under: Congenital Diseases > Stem Cells and Development Neurological Diseases > Stem Cells and Development.

Changes in the cellular makeup of motor patterning circuits drive courtship song evolution in Drosophila

How evolutionary changes in genes and neurons encode species variation in complex motor behaviors are largely unknown. Here, we develop genetic tools that permit a neural circuit comparison between the model species Drosophila melanogaster and the closely-related species D. yakuba, who has undergone a lineage-specific loss of sine song, one of the two major types of male courtship song in Drosophila. Neuroanatomical comparison of song patterning neurons called TN1 across the phylogeny demonstrates a link between the loss of sine song and a reduction both in the number of TN1 neurons and the neurites serving the sine circuit connectivity. Optogenetic activation confirms that TN1 neurons in D. yakuba have lost the ability to drive sine song, while maintaining the ability to drive the singing wing posture. Single-cell transcriptomic comparison shows that D. yakuba specifically lacks a cell type corresponding to TN1A neurons, the TN1 subtype that is essential for sine song. Genetic and developmental manipulation reveals a functional divergence of the sex determination gene doublesex in D. yakuba to reduce TN1 number by promoting apoptosis. Our work illustrates the contribution of motor patterning circuits and cell type changes in behavioral evolution, and uncovers the evolutionary lability of sex determination genes to reconfigure the cellular makeup of neural circuits.

Single-cell transcriptome profiles of Drosophila fruitless-expressing neurons from both sexes

Drosophila melanogaster reproductive behaviors are orchestrated by fruitless neurons. We performed single-cell RNA-sequencing on pupal neurons that produce sex-specifically spliced fru transcripts, the fru P1-expressing neurons. Uniform Manifold Approximation and Projection (UMAP) with clustering generates an atlas containing 113 clusters. While the male and female neurons overlap in UMAP space, more than half the clusters have sex differences in neuron number, and nearly all clusters display sex-differential expression. Based on an examination of enriched marker genes, we annotate clusters as circadian clock neurons, mushroom body Kenyon cell neurons, neurotransmitter- and/or neuropeptide-producing, and those that express doublesex. Marker gene analyses also show that genes that encode members of the immunoglobulin superfamily of cell adhesion molecules, transcription factors, neuropeptides, neuropeptide receptors, and Wnts have unique patterns of enriched expression across the clusters. In vivo spatial gene expression links to the clusters are examined. A functional analysis of fru P1 circadian neurons shows they have dimorphic roles in activity and period length. Given that most clusters are comprised of male and female neurons indicates that the sexes have fru P1 neurons with common gene expression programs. Sex-specific expression is overlaid on this program, to build the potential for vastly different sex-specific behaviors.

A male-specific doublesex isoform reveals an evolutionary pathway of sexual development via distinct alternative splicing mechanisms

The doublesex/mab-3 related transcription factor (Dmrt) genes regulate sexual development in metazoans. Studies of the doublesex (dsx) gene in insects, in particular Drosophila melanogaster, reveal that alternative splicing of dsx generates sex-specific Dsx isoforms underlying sexual differentiation. Such a splicing-based mechanism underlying sex-specific Dmrt function is thought to be evolved from a transcription-based mechanism used in non-insect species, but how such transition occurs during evolution is not known. Here we identified a male-specific dsx transcript (dsx^M2) through intron retention (IR), in addition to previously identified dsx^M and dsx^F transcripts through alternative polyadenylation (APA) with mutually exclusive exons. We found that Dsx^M2 had similarly masculinizing function as Dsx^M. We also found that the IR-based mechanism generating sex-specific dsx transcripts was conserved from flies to cockroaches. Further analysis of these dsx transcripts suggested an evolutionary pathway from sexually monomorphic to sex-specific dsx via the sequential use of IR-based and APA-based alternative splicing.

An ancestral male-specific doublesex isoform, dsxM2, is identified via intron retention, with a masculinizing function weaker than the modern dsxM

The doublesex gene regulates dimorphic sexual and aggressive behaviors in Drosophila

Most animal species display dimorphic sexual behaviors and male-biased aggressiveness. Current models have focused on the male-specific product from the fruitless (fru^M) gene, which controls male courtship and male-specific aggression patterns in fruit flies, and describe a male-specific mechanism underlying sexually dimorphic behaviors. Here we show that the doublesex (dsx) gene, which expresses male-specific Dsx^M and female-specific Dsx^F transcription factors, functions in the nervous system to control both male and female sexual and aggressive behaviors. We find that Dsx is not only required in central brain neurons for male and female sexual behaviors, but also functions in approximately eight pairs of male-specific neurons to promote male aggressiveness and approximately two pairs of female-specific neurons to inhibit female aggressiveness. Dsx^F knockdown females fight more frequently, even with males. Our findings reveal crucial roles of dsx, which is broadly conserved from worms to humans, in a small number of neurons in both sexes to establish dimorphic sexual and aggressive behaviors.

Secondary reversion to sexual monomorphism associated with tissue-specific loss of doublesex expression

Animal evolution is characterized by frequent turnover of sexually dimorphic traits-new sex-specific characters are gained, and some ancestral sex-specific characters are lost, in many lineages. In insects, sexual differentiation is predominantly cell autonomous and depends on the expression of the doublesex (dsx) transcription factor. In most cases, cells that transcribe dsx have the potential to undergo sex-specific differentiation, while those that lack dsx expression do not. Consistent with this mode of development, comparative research has shown that the origin of new sex-specific traits can be associated with the origin of new spatial domains of dsx expression. In this report, we examine the opposite situation-a secondary loss of the sex comb, a male-specific grasping structure that develops on the front legs of some drosophilid species. We show that while the origin of the sex comb is linked to an evolutionary gain of dsx expression in the leg, sex comb loss in a newly identified species of Lordiphosa (Drosophilidae) is associated with a secondary loss of dsx expression. We discuss how the developmental control of sexual dimorphism affects the mechanisms by which sex-specific traits can evolve.

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.

Nucleoporin107 mediates female sexual differentiation via Dsx

We recently identified a missense mutation in Nucleoporin107 (Nup107; D447N) underlying XX-ovarian-dysgenesis, a rare disorder characterized by underdeveloped and dysfunctional ovaries. Modeling of the human mutation in Drosophila or specific knockdown of Nup107 in the gonadal soma resulted in ovarian-dysgenesis-like phenotypes. Transcriptomic analysis identified the somatic sex-determination gene doublesex (dsx) as a target of Nup107. Establishing Dsx as a primary relevant target of Nup107, either loss or gain of Dsx in the gonadal soma is sufficient to mimic or rescue the phenotypes induced by Nup107 loss. Importantly, the aberrant phenotypes induced by compromising either Nup107 or dsx are reminiscent of BMP signaling hyperactivation. Remarkably, in this context, the metalloprotease AdamTS-A, a transcriptional target of both Dsx and Nup107, is necessary for the calibration of BMP signaling. As modulation of BMP signaling is a conserved critical determinant of soma-germline interaction, the sex and tissue specific deployment of Dsx-F by Nup107 seems crucial for the maintenance of the homeostatic balance between the germ cells and somatic gonadal cells.

The sex determination gene doublesex regulates expression and secretion of the basement membrane protein Collagen IV

The highly conserved doublesex (dsx) and doublesex/mab-3 related (Dmrt) genes control sexually dimorphic traits across animals. The dsx gene encodes sex-specific transcription factors, Dsx^M in males and Dsx^F in females, which function differentially and often oppositely to establish sexual dimorphism. Here, we report that mutations in dsx, or overexpression of dsx, result in abnormal distribution of the basement membrane (BM) protein Collagen IV in the fat body. We find that Dsx isoforms regulate the expression of Collagen IV in the fat body and its secretion into the BM of other tissues. We identify the procollagen lysyl hydroxylase (dPlod) gene, which is involved in the biosynthesis of Collagen IV, as a direct target of Dsx. We further show that Dsx regulates Collagen IV through dPlod-dependent and independent pathways. These findings reveal how Dsx isoforms function in the secretory fat body to regulate Collagen IV and remotely establish sexual dimorphism.

From fruitless to sex: On the generation and diversification of an innate behavior

Male sexual behavior in Drosophila melanogaster, largely controlled by the fruitless (fru) gene encoding the male specific Fru^M protein, is among the best studied animal behaviors. Although substantial studies suggest that Fru^M specifies a neuronal circuitry governing all aspects of male sexual behaviors, recent findings show that Fru^M is not absolutely necessary for such behaviors. We propose that another regulatory gene doublesex encoding the male-specific Dsx^M protein builds a core neuronal circuitry that possesses the potential for courtship, which could be either induced through adult social experience or innately manifested during development by Fru^M expression in a broader neuronal circuitry. Fru^M expression levels and patterns determine the modes of courtship behavior from innate heterosexual, homosexual, bisexual, to learned courtship. We discuss how Fru^M expression is regulated by hormones and social experiences and tunes functional flexibility of the sex circuitry. We propose that regulatory genes hierarchically build the potential for innate and learned aspects of courtship behaviors, and expression changes of these regulatory genes among different individuals and species with different social experiences ultimately lead to behavioral diversification.

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.

Developmental mechanisms of sex differences: from cells to organisms

Male-female differences in many developmental mechanisms lead to the formation of two morphologically and physiologically distinct sexes. Although this is expected for traits with prominent differences between the sexes, such as the gonads, sex-specific processes also contribute to traits without obvious male-female differences, such as the intestine. Here, we review sex differences in developmental mechanisms that operate at several levels of biological complexity - molecular, cellular, organ and organismal - and discuss how these differences influence organ formation, function and whole-body physiology. Together, the examples we highlight show that one simple way to gain a more accurate and comprehensive understanding of animal development is to include both sexes.

Role of the Forkhead Transcription Factors Fd4 and Fd5 During Drosophila Leg Development

Appendage development requires the coordinated function of signaling pathways and transcription factors to pattern the leg along the three main axes: the antero-posterior (AP), proximo-distal (PD), and dorso-ventral (DV). The Drosophila leg DV axis is organized by two morphogens, Decapentaplegic (Dpp), and Wingless (Wg), which direct dorsal and ventral cell fates, respectively. However, how these signals regulate the differential expression of its target genes is mostly unknown. In this work, we found that two members of the Drosophila forkhead family of transcription factors, Fd4 and Fd5 (also known as fd96Ca and fd96Cb), are identically expressed in the ventro-lateral domain of the leg imaginal disc in response to Dpp signaling. Here, we analyze the expression regulation and function of these genes during leg development. We have generated specific mutant alleles for each gene and a double fd4/fd5 mutant chromosome to study their function during development. We highlight the redundant role of the fd4/fd5 genes during the formation of the sex comb, a male specific structure that appears in the ventro-lateral domain of the prothoracic leg.

A double-negative gene regulatory circuit underlies the virgin behavioral state

Virgin females of many species conduct distinctive behaviors, compared with post-mated and/or pregnant individuals. In Drosophila, this post-mating switch is initiated by seminal factors, implying that the default female state is virgin. However, we recently showed that loss of miR-iab-4/8-mediated repression of the transcription factor Homothorax (Hth) within the abdominal ventral nerve cord (VNC) causes virgins to execute mated behaviors. Here, we use genomic analysis of mir-iab-4/8 deletion and hth-microRNA (miRNA) binding site mutants (hth[BSmut]) to elucidate doublesex (dsx) as a critical downstream factor. Dsx and Hth proteins are highly complementary in CNS, and Dsx is downregulated in miRNA/hth[BSmut] mutants. Moreover, virgin behavior is highly dose sensitive to developmental dsx function. Strikingly, depletion of Dsx from very restricted abdominal neurons (SAG-1 cells) abrogates female virgin conducts, in favor of mated behaviors. Thus, a double-negative regulatory pathway in the VNC (miR-iab-4/8 ⫞ Hth ⫞ Dsx) specifies the virgin behavioral state.

Garaulet et al. use transcriptomic analysis to reveal new downstream elements in a post-transcriptional cascade, via miR-iab-4/8 and Homothorax, that affects patterning of the CNS. This genetic circuit regulates the accumulation of a secondary target (Doublesex), whose level in specific neurons determines the behavior of adult virgin flies.

Analysis of cell-type-specific chromatin modifications and gene expression in Drosophila neurons that direct reproductive behavior

Examining the role of chromatin modifications and gene expression in neurons is critical for understanding how the potential for behaviors are established and maintained. We investigate this question by examining Drosophila melanogaster fru P1 neurons that underlie reproductive behaviors in both sexes. We developed a method to purify cell-type-specific chromatin (Chromatag), using a tagged histone H2B variant that is expressed using the versatile Gal4/UAS gene expression system. Here, we use Chromatag to evaluate five chromatin modifications, at three life stages in both sexes. We find substantial changes in chromatin modification profiles across development and fewer differences between males and females. Additionally, we find chromatin modifications that persist in different sets of genes from pupal to adult stages, which may point to genes important for cell fate determination in fru P1 neurons. We generated cell-type-specific RNA-seq data sets, using translating ribosome affinity purification (TRAP). We identify actively translated genes in fru P1 neurons, revealing novel stage- and sex-differences in gene expression. We also find chromatin modification enrichment patterns that are associated with gene expression. Next, we use the chromatin modification data to identify cell-type-specific super-enhancer-containing genes. We show that genes with super-enhancers in fru P1 neurons differ across development and between the sexes. We validated that a set of genes are expressed in fru P1 neurons, which were chosen based on having a super-enhancer and TRAP-enriched expression in fru P1 neurons.

Interplay between sex determination cascade and major signaling pathways during Drosophila eye development: Perspectives for future research

Understanding molecular mechanisms of sexually dimorphic organ growth is a fundamental problem of developmental biology. Recent quantitative studies showed that the Drosophila compound eye is a convenient model to study the determination of the final organ size. In Drosophila, females have larger eyes than males and this is evident even after correction for the larger body size. Moreover, female eyes include more ommatidia (photosensitive units) than male eyes and this difference is specified at the third larval instar in the eye primordia called eye imaginal discs. This may result in different visual capabilities between the two sexes and have behavioral consequences. Despite growing evidence on the genetic bases of eye size variation between different Drosophila species and strains, mechanisms responsible for within-species sexual dimorphism still remain elusive. Here, we discuss a presumptive crosstalk between the sex determination cascade and major signaling pathways during dimorphic eye development. Male- and female-specific isoforms of Doublesex (Dsx) protein are known to control sex-specific differentiation in the somatic tissues. However, no data on Dsx function during eye disc growth and patterning are currently available. Remarkably, Sex lethal (Sxl), the sex determination switch protein, was shown to directly affect Hedgehog (Hh) and Notch (N) signaling in the Drosophila wing disc. The similarity of signaling pathways involved in the wing and eye disc growth suggests that Sxl might be integrated into regulation of eye development. Dsx role in the eye disc requires further investigation. We discuss currently available data on sex-biased gene expression in the Drosophila eye and highlight perspectives for future studies.

A sex-specific switch between visual and olfactory inputs underlies adaptive sex differences in behavior

Although males and females largely share the same genome and nervous system, they differ profoundly in reproductive investments and require distinct behavioral, morphological, and physiological adaptations. How can the nervous system, while bound by both developmental and biophysical constraints, produce these sex differences in behavior? Here, we uncover a novel dimorphism in Drosophila melanogaster that allows deployment of completely different behavioral repertoires in males and females with minimum changes to circuit architecture. Sexual differentiation of only a small number of higher order neurons in the brain leads to a change in connectivity related to the primary reproductive needs of both sexes-courtship pursuit in males and communal oviposition in females. This study explains how an apparently similar brain generates distinct behavioral repertoires in the two sexes and presents a fundamental principle of neural circuit organization that may be extended to other species.

Doublesex regulates fruitless expression to promote sexual dimorphism of the gonad stem cell niche

Doublesex (Dsx) and Fruitless (Fru) are the two downstream transcription factors that actuate Drosophila sex determination. While Dsx assists Fru to regulate sex-specific behavior, whether Fru collaborates with Dsx in regulating other aspects of sexual dimorphism remains unknown. One important aspect of sexual dimorphism is found in the gonad stem cell (GSC) niches, where male and female GSCs are regulated to create large numbers of sperm and eggs. Here we report that Fru is expressed male-specifically in the GSC niche and plays important roles in the development and maintenance of these cells. Unlike previously-studied aspects of sex-specific Fru expression, which are regulated by Transformer (Tra)-mediated alternative splicing, we show that male-specific expression of fru in the gonad is regulated downstream of dsx, and is independent of tra. fru genetically interacts with dsx to support maintenance of the niche throughout development. Ectopic expression of fru inhibited female niche formation and partially masculinized the ovary. fru is also required autonomously for cyst stem cell maintenance and cyst cell survival. Finally, we identified a conserved Dsx binding site upstream of fru promoter P4 that regulates fru expression in the niche, indicating that fru is likely a direct target for transcriptional regulation by Dsx. These findings demonstrate that fru acts outside the nervous system to influence sexual dimorphism and reveal a new mechanism for regulating sex-specific expression of fru that is regulated at the transcriptional level by Dsx, rather than by alternative splicing by Tra.

Identification and characterization of sexually dimorphic neurons that express the sex-determining gene doublesex in the brain of silkmoth Bombyx mori

Sexual differences in behavior are generated by sexually dimorphic neural circuits in animals. In insects, a highly conserved sex-determining gene doublesex (dsx) plays essential roles in the development of sexual dimorphisms. In the present study, to elucidate the neural basis of sexual differences in behaviors of silkmoth Bombyx mori, we investigated Bombyx mori dsx (Bmdsx) expression in the brains through development. In the brain, Bmdsx was differentially expressed in sex- and developmental stage-dependent manners. BmDSX protein-expressing cells were located in the dorsomedial region of the pupal and adult brains, and constituted two and one neural clusters in males and females, respectively. The number of BmDSX-positive cells was developmentally regulated and peaked at the early to middle pupal stages, suggesting that the sexually dimorphic neural circuits are established during this period. The detection of a neural activity marker protein BmHR38 suggested that the BmDSX-positive cells are not active during sexual behavior in both male and female moths, even though the cells in the vicinity of the BmDSX-positive cell clusters are active. These results imply that Bmdsx plays roles in the development of sexually dimorphic neural circuits, but the neural circuits are not related to sexual behavior in silkmoths.

miRNAs and Neural Alternative Polyadenylation Specify the Virgin Behavioral State

How are diverse regulatory strategies integrated to impose appropriately patterned gene expression that underlie in vivo phenotypes? Here, we reveal how coordinated miRNA regulation and neural-specific alternative polyadenylation (APA) of a single locus controls complex behaviors. Our entry was the unexpected observation that deletion of Bithorax complex (BX-C) miRNAs converts virgin female flies into a subjective post-mated behavioral state, normally induced by seminal proteins following copulation. Strikingly, this behavioral switch is directly attributable to misregulation of homothorax (hth). We localize specific CNS abdominal neurons where de-repressed Hth compromises virgin behavior in BX-C miRNA mutants. Moreover, we use genome engineering to demonstrate that precise mutation of hth 3' UTR sites for BX-C miRNAs or deletion of its neural 3' UTR extension containing most of these sites both induce post-mated behaviors in virgins. Thus, facilitation of miRNA-mediated repression by neural APA is required for virgin females to execute behaviors appropriate to their internal state.

Sex-determining genes distinctly regulate courtship capability and target preference via sexually dimorphic neurons

For successful mating, a male animal must execute effective courtship behaviors toward a receptive target sex, which is female. Whether the courtship execution capability and upregulation of courtship toward females are specified through separable sex-determining genetic pathways remains uncharacterized. Here, we found that one of the two Drosophila sex-determining genes, doublesex (dsx), specifies a male-specific neuronal component that serves as an execution mechanism for courtship behavior, whereas fruitless (fru) is required for enhancement of courtship behavior toward females. The dsx-dependent courtship execution mechanism includes a specific subclass within a neuronal cluster that co-express dsx and fru. This cluster contains at least another subclass that is specified cooperatively by both dsx and fru. Although these neuronal populations can also promote aggressive behavior toward male flies, this capacity requires fru-dependent mechanisms. Our results uncover how sex-determining genes specify execution capability and female-specific enhancement of courtship behavior through separable yet cooperative neurogenetic mechanisms.

Signatures of sex: Sex differences in gene expression in the vertebrate brain

Women and men differ in disease prevalence, symptoms, and progression rates for many psychiatric and neurological disorders. As more preclinical studies include both sexes in experimental design, an increasing number of sex differences in physiology and behavior have been reported. In the brain, sex-typical behaviors are thought to result from sex-specific patterns of neural activity in response to the same sensory stimulus or context. These differential firing patterns likely arise as a consequence of underlying anatomic or molecular sex differences. Accordingly, gene expression in the brains of females and males has been extensively investigated, with the goal of identifying biological pathways that specify or modulate sex differences in brain function. However, there is surprisingly little consensus on sex-biased genes across studies and only a handful of robust candidates have been pursued in the follow-up experiments. Furthermore, it is not known how or when sex-biased gene expression originates, as few studies have been performed in the developing brain. Here we integrate molecular genetic and neural circuit perspectives to provide a conceptual framework of how sex differences in gene expression can arise in the brain. We detail mechanisms of gene regulation by steroid hormones, highlight landmark studies in rodents and humans, identify emerging themes, and offer recommendations for future research. This article is categorized under: Nervous System Development > Vertebrates: General Principles Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Gene Expression and Transcriptional Hierarchies > Sex Determination.

Recent neurogenetic findings in insect courtship behaviour

Insect courtship parades consist of series of innate and stereotyped behaviours that become hardwired-in during the development of the nervous system. As such, insect courtship behaviour provides an excellent model for probing the principles of neuronal assembly, which underlie patterns of behaviour. Here, we present the main advances of recent studies - in species all the way from flies to planthoppers - and we envisage how these could lead to further propitious findings.

The neurogenetics of sexually dimorphic behaviors from a postdevelopmental perspective

Most sexually reproducing animal species are characterized by two morphologically and behaviorally distinct sexes. The genetic, molecular and cellular processes that produce sexual dimorphisms are phylogenetically diverse, though in most cases they are thought to occur early in development. In some species, however, sexual dimorphisms are manifested after development is complete, suggesting the intriguing hypothesis that sex, more generally, might be considered a continuous trait that is influenced by both developmental and postdevelopmental processes. Here, we explore how biological sex is defined at the genetic, neuronal and behavioral levels, its effects on neuronal development and function, and how it might lead to sexually dimorphic behavioral traits in health and disease. We also propose a unifying framework for understanding neuronal and behavioral sexual dimorphisms in the context of both developmental and postdevelopmental, physiological timescales. Together, these two temporally separate processes might drive sex-specific neuronal functions in sexually mature adults, particularly as it pertains to behavior in health and disease.

The Hox gene Abdominal-B uses DoublesexF as a cofactor to promote neuroblast apoptosis in the Drosophila central nervous system

Highly conserved DM domain-containing transcription factors (Doublesex/MAB-3/DMRT1) are responsible for generating sexually dimorphic features. In the Drosophila central nervous system, a set of Doublesex (Dsx)-expressing neuroblasts undergo apoptosis in females whereas their male counterparts proliferate and give rise to serotonergic neurons crucial for adult mating behaviour. Our study demonstrates that the female-specific isoform of Dsx collaborates with Hox gene Abdominal-B (Abd-B) to bring about this apoptosis. Biochemical results suggest that proteins AbdB and Dsx interact through their highly conserved homeodomain and DM domain, respectively. This interaction is translated into a cooperative binding of the two proteins on the apoptotic enhancer in the case of females but not in the case of males, resulting in female-specific activation of apoptotic genes. The capacity of AbdB to use the sex-specific isoform of Dsx as a cofactor underlines the possibility that these two classes of protein are capable of cooperating in selection and regulation of target genes in a tissue- and sex-specific manner. We propose that this interaction could be a common theme in generating sexual dimorphism in different tissues across different species.

A small number of cholinergic neurons mediate hyperaggression in female Drosophila

In the Drosophila model of aggression, males and females fight in same-sex pairings, but a wide disparity exists in the levels of aggression displayed by the 2 sexes. A screen of Drosophila Flylight Gal4 lines by driving expression of the gene coding for the temperature sensitive dTRPA1 channel, yielded a single line (GMR26E01-Gal4) displaying greatly enhanced aggression when thermoactivated. Targeted neurons were widely distributed throughout male and female nervous systems, but the enhanced aggression was seen only in females. No effects were seen on female mating behavior, general arousal, or male aggression. We quantified the enhancement by measuring fight patterns characteristic of female and male aggression and confirmed that the effect was female-specific. To reduce the numbers of neurons involved, we used an intersectional approach with our library of enhancer trap flp-recombinase lines. Several crosses reduced the populations of labeled neurons, but only 1 cross yielded a large reduction while maintaining the phenotype. Of particular interest was a small group (2 to 4 pairs) of neurons in the approximate position of the pC1 cluster important in governing male and female social behavior. Female brains have approximately 20 doublesex (dsx)-expressing neurons within pC1 clusters. Using dsx ^FLP instead of 357 ^FLP for the intersectional studies, we found that the same 2 to 4 pairs of neurons likely were identified with both. These neurons were cholinergic and showed no immunostaining for other transmitter compounds. Blocking the activation of these neurons blocked the enhancement of aggression.

Modular tissue-specific regulation of doublesex underpins sexually dimorphic development in Drosophila

The ability of a single genome to produce distinct and often dramatically different male and female forms is one of the wonders of animal development. In Drosophila melanogaster, most sexually dimorphic traits are controlled by sex-specific isoforms of the doublesex (dsx) transcription factor, and dsx expression is mostly limited to cells that give rise to sexually dimorphic traits. However, it is unknown how this mosaic of sexually dimorphic and monomorphic organs arises. Here, we characterize the cis-regulatory sequences that control dsx expression in the foreleg, which contains multiple types of sex-specific sensory organs. We find that separate modular enhancers are responsible for dsx expression in each sexually dimorphic organ. Expression of dsx in the sex comb is co-regulated by two enhancers with distinct spatial and temporal specificities that are separated by a genitalia-specific enhancer. The sex comb-specific enhancer from D. willistoni, a species that primitively lacks sex combs, is not active in the foreleg. Thus, the mosaic of sexually dimorphic and monomorphic organs depends on modular regulation of dsx transcription by dedicated cell type-specific enhancers.

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.

Recent advances in neuropeptide signaling in Drosophila, from genes to physiology and behavior

This review focuses on neuropeptides and peptide hormones, the largest and most diverse class of neuroactive substances, known in Drosophila and other animals to play roles in almost all aspects of daily life, as w;1;ell as in developmental processes. We provide an update on novel neuropeptides and receptors identified in the last decade, and highlight progress in analysis of neuropeptide signaling in Drosophila. Especially exciting is the huge amount of work published on novel functions of neuropeptides and peptide hormones in Drosophila, largely due to the rapid developments of powerful genetic methods, imaging techniques and innovative assays. We critically discuss the roles of peptides in olfaction, taste, foraging, feeding, clock function/sleep, aggression, mating/reproduction, learning and other behaviors, as well as in regulation of development, growth, metabolic and water homeostasis, stress responses, fecundity, and lifespan. We furthermore provide novel information on neuropeptide distribution and organization of peptidergic systems, as well as the phylogenetic relations between Drosophila neuropeptides and those of other phyla, including mammals. As will be shown, neuropeptide signaling is phylogenetically ancient, and not only are the structures of the peptides, precursors and receptors conserved over evolution, but also many functions of neuropeptide signaling in physiology and behavior.

A matter of timing

A genetic pathway that times development works together with the sex-determination pathway to control the timing of sexually dimorphic neural development in C. elegans.

Sex differences in Drosophila behavior: Qualitative and Quantitative Dimorphism

The importance of sex as a biological variable is being recognized by more and more researchers, including those using Drosophila melanogaster as a model organism. Differences between the two sexes are not confined to well-known reproductive behaviors, but include other behaviors and physiological characteristics that are considered "common" to both sexes. It is possible to categorize sexual dimorphisms into "qualitative" and "quantitative" differences, and this review focuses on recent advances in elucidating genetic and neurophysiological basis of both qualitative and quantitative sex differences in Drosophila behavior. While sex-specific behaviors are often mediated by sexually dimorphic neural circuits, quantitative sexual dimorphism is caused by sex-specific modulation of a common neuronal substrate.

Decoding Sex Differences in the Brain, One Worm at a Time

Sex differences in the brain are prominent features across the animal kingdom. Understanding the anatomical and regulatory mechanisms behind these differences is critical for both explaining sexually dimorphic behaviors and developing sex-targeted treatments for neurological disorders. Clinical studies considering sex biases and basic research on animal models have provided much evidence for the existence of sex differences in the brain and, in a larger sense, sexual dimorphisms in the nervous system. However, due to the complexity of structure and dimorphic behaviors, it is yet unclear precisely how neuronal sexual dimorphisms are regulated on a molecular or cellular level. This commentary reviews available tools for investigating sexual dimorphisms using a simple model organism, the roundworm Caenorhabditis elegans ( C. elegans), which enables one to study gene regulation at single-cell resolution with a number of cutting-edge molecular and genetic technologies. I highlight the doublesex/mab-3 family of transcription factors, first discovered in invertebrates, and their roles in a potentially universal regulatory mechanism underlying neuronal sexual dimorphisms. Studies of these transcription factors using C. elegans, fruit flies, and vertebrates will promote our understanding of fundamental mechanisms behind sex differences in the brain.

Chinmo prevents transformer alternative splicing to maintain male sex identity

Reproduction in sexually dimorphic animals relies on successful gamete production, executed by the germline and aided by somatic support cells. Somatic sex identity in Drosophila is instructed by sex-specific isoforms of the DMRT1 ortholog Doublesex (Dsx). Female-specific expression of Sex-lethal (Sxl) causes alternative splicing of transformer (tra) to the female isoform traF. In turn, TraF alternatively splices dsx to the female isoform dsxF. Loss of the transcriptional repressor Chinmo in male somatic stem cells (CySCs) of the testis causes them to "feminize", resembling female somatic stem cells in the ovary. This somatic sex transformation causes a collapse of germline differentiation and male infertility. We demonstrate this feminization occurs by transcriptional and post-transcriptional regulation of traF. We find that chinmo-deficient CySCs upregulate tra mRNA as well as transcripts encoding tra-splice factors Virilizer (Vir) and Female lethal (2)d (Fl(2)d). traF splicing in chinmo-deficient CySCs leads to the production of DsxF at the expense of the male isoform DsxM, and both TraF and DsxF are required for CySC sex transformation. Surprisingly, CySC feminization upon loss of chinmo does not require Sxl but does require Vir and Fl(2)d. Consistent with this, we show that both Vir and Fl(2)d are required for tra alternative splicing in the female somatic gonad. Our work reveals the need for transcriptional regulation of tra in adult male stem cells and highlights a previously unobserved Sxl-independent mechanism of traF production in vivo. In sum, transcriptional control of the sex determination hierarchy by Chinmo is critical for sex maintenance in sexually dimorphic tissues and is vital in the preservation of fertility.

Evolving doublesex expression correlates with the origin and diversification of male sexual ornaments in the Drosophila immigrans species group

Male ornaments and other sex-specific traits present some of the most dramatic examples of evolutionary innovations. Comparative studies of similar but independently evolved traits are particularly important for identifying repeated patterns in the evolution of these traits. Male-specific modifications of the front legs have evolved repeatedly in Drosophilidae and other Diptera. The best understood of these novel structures is the sex comb of Drosophila melanogaster and its close relatives. Here, we examine the evolution of another male foreleg modification, the sex brush, found in the distantly related Drosophila immigrans species group. Similar to the sex comb, we find that the origin of the sex brush correlates with novel, spatially restricted expression of the doublesex (dsx) transcription factor, the primary effector of the Drosophila sex determination pathway. The diversity of Dsx expression patterns in the immigrans species group closely reflects the differences in the presence, position, and size of the sex brush. Together with previous work on sex comb evolution, these observations suggest that tissue-specific activation of dsx expression may be a common mechanism responsible for the evolution of sexual dimorphism and particularly for the origin of novel male-specific ornaments.

Drosophila female-specific Ilp7 motoneurons are generated by Fruitless-dependent cell death in males and by a double-assurance survival role for Transformer in females

Female-specific Ilp7 neuropeptide-expressing motoneurons (FS-Ilp7 motoneurons) are required in Drosophila for oviduct function in egg laying. Here, we uncover cellular and genetic mechanisms underlying their female-specific generation. We demonstrate that programmed cell death (PCD) eliminates FS-Ilp7 motoneurons in males, and that this requires male-specific splicing of the sex-determination gene fruitless (fru) into the Fru^MC isoform. However, in females, fru alleles that only generate Fru^M isoforms failed to kill FS-Ilp7 motoneurons. This blockade of Fru^M-dependent PCD was not attributable to doublesex gene function but to a non-canonical role for transformer (tra), a gene encoding the RNA splicing activator that regulates female-specific splicing of fru and dsx transcripts. In both sexes, we show that Tra prevents PCD even when the Fru^M isoform is expressed. In addition, we found that Fru^MC eliminated FS-Ilp7 motoneurons in both sexes, but only when Tra was absent. Thus, Fru^MC-dependent PCD eliminates female-specific neurons in males, and Tra plays a double-assurance function in females to establish and reinforce the decision to generate female-specific neurons.

Courtship Love Songs: doublesex Makes the Connection

During courtship, Drosophila melanogaster males sing to females a song composed of rhythmic pulses and sine song. In this issue of Developmental Cell, Shirangi et al. (2016) show that a cluster of doublesex-expressing neurons directs the production of the sine song component through functional linkages to wing motoneurons.

Doublesex Regulates the Connectivity of a Neural Circuit Controlling Drosophila Male Courtship Song

It is unclear how regulatory genes establish neural circuits that compose sex-specific behaviors. The Drosophila melanogaster male courtship song provides a powerful model to study this problem. Courting males vibrate a wing to sing bouts of pulses and hums, called pulse and sine song, respectively. We report the discovery of male-specific thoracic interneurons-the TN1A neurons-that are required specifically for sine song. The TN1A neurons can drive the activity of a sex-non-specific wing motoneuron, hg1, which is also required for sine song. The male-specific connection between the TN1A neurons and the hg1 motoneuron is regulated by the sexual differentiation gene doublesex. We find that doublesex is required in the TN1A neurons during development to increase the density of the TN1A arbors that interact with dendrites of the hg1 motoneuron. Our findings demonstrate how a sexual differentiation gene can build a sex-specific circuit motif by modulating neuronal arborization.

Cell Class-Lineage Analysis Reveals Sexually Dimorphic Lineage Compositions in the Drosophila Brain

The morphology and physiology of neurons are directed by developmental decisions made within their lines of descent from single stem cells. Distinct stem cells may produce neurons having shared properties that define their cell class, such as the type of secreted neurotransmitter. The relationship between cell class and lineage is complex. Here we developed the transgenic cell class-lineage intersection (CLIn) system to assign cells of a particular class to specific lineages within the Drosophila brain. CLIn also enables birth-order analysis and genetic manipulation of particular cell classes arising from particular lineages. We demonstrated the power of CLIn in the context of the eight central brain type II lineages, which produce highly diverse progeny through intermediate neural progenitors. We mapped 18 dopaminergic neurons from three distinct clusters to six type II lineages that show lineage-characteristic neurite trajectories. In addition, morphologically distinct dopaminergic neurons are produced within a given lineage, and they arise in an invariant sequence. We also identified type II lineages that produce doublesex- and fruitless-expressing neurons and examined whether female-specific apoptosis in these lineages accounts for the lower number of these neurons in the female brain. Blocking apoptosis in these lineages resulted in more cells in both sexes with males still carrying more cells than females. This argues that sex-specific stem cell fate together with differential progeny apoptosis contribute to the final sexual dimorphism.

Activation of Latent Courtship Circuitry in the Brain of Drosophila Females Induces Male-like Behaviors

Courtship in Drosophila melanogaster offers a powerful experimental paradigm for the study of innate sexually dimorphic behaviors [1, 2]. Fruit fly males exhibit an elaborate courtship display toward a potential mate [1, 2]. Females never actively court males, but their response to the male’s display determines whether mating will actually occur. Sex-specific behaviors are hardwired into the nervous system via the actions of the sex determination genes doublesex (dsx) and fruitless (fru) [1]. Activation of male-specific dsx/fru⁺ P1 neurons in the brain initiates the male’s courtship display [3, 4], suggesting that neurons unique to males trigger this sex-specific behavior. In females, dsx⁺ neurons play a pivotal role in sexual receptivity and post-mating behaviors [1, 2, 5, 6, 7, 8, 9]. Yet it is still unclear how dsx⁺ neurons and dimorphisms in these circuits give rise to the different behaviors displayed by males and females. Here, we manipulated the function of dsx⁺ neurons in the female brain to investigate higher-order neurons that drive female behaviors. Surprisingly, we found that activation of female dsx⁺ neurons in the brain induces females to behave like males by promoting male-typical courtship behaviors. Activated females display courtship toward conspecific males or females, as well other Drosophila species. We uncovered specific dsx⁺ neurons critical for driving male courtship and identified pheromones that trigger such behaviors in activated females. While male courtship behavior was thought to arise from male-specific central neurons, our study shows that the female brain is equipped with latent courtship circuitry capable of inducing this male-specific behavioral program.

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Activation of brain dsx-pC1 neurons promote male-like courtship in females

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Activated females court conspecific males and females and other Drosophila species

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Methyl pheromones trigger male courtship behaviors in activated females

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The female brain is equipped with latent circuitry underlying male-like behavior

Neurons That Underlie Drosophila melanogaster Reproductive Behaviors: Detection of a Large Male-Bias in Gene Expression in fruitless-Expressing Neurons

Male and female reproductive behaviors in Drosophila melanogaster are vastly different, but neurons that express sex-specifically spliced fruitless transcripts (fru P1) underlie these behaviors in both sexes. How this set of neurons can generate such different behaviors between the two sexes is an unresolved question. A particular challenge is that fru P1-expressing neurons comprise only 2-5% of the adult nervous system, and so studies of adult head tissue or whole brain may not reveal crucial differences. Translating Ribosome Affinity Purification (TRAP) identifies the actively translated pool of mRNAs from fru P1-expressing neurons, allowing a sensitive, cell-type-specific assay. We find four times more male-biased than female-biased genes in TRAP mRNAs from fru P1-expressing neurons. This suggests a potential mechanism to generate dimorphism in behavior. The male-biased genes may direct male behaviors by establishing cell fate in a similar context of gene expression observed in females. These results suggest a possible global mechanism for how distinct behaviors can arise from a shared set of neurons.

Gene networks and developmental context: the importance of understanding complex gene expression patterns in evolution

Animal development is the product of distinct components and interactions-genes, regulatory networks, and cells-and it exhibits emergent properties that cannot be inferred from the components in isolation. Often the focus is on the genotype-to-phenotype map, overlooking the process of development that turns one into the other. We propose a move toward micro-evolutionary analysis of development, incorporating new tools that enable cell type resolution and single-cell microscopy. Using the sex determination pathway in Drosophila to illustrate potential avenues of research, we highlight some of the questions that these emerging technologies can address. For example, they provide an unprecedented opportunity to study heterogeneity within cell populations, and the potential to add the dimension of time to gene regulatory network analysis. Challenges still remain in developing methods to analyze this data and to increase the throughput. However this line of research has the potential to bridge the gaps between previously more disparate fields, such as population genetics and development, opening up new avenues of research.

The Sex Determination Gene transformer Regulates Male-Female Differences in Drosophila Body Size

Almost all animals show sex differences in body size. For example, in Drosophila, females are larger than males. Although Drosophila is widely used as a model to study growth, the mechanisms underlying this male-female difference in size remain unclear. Here, we describe a novel role for the sex determination gene transformer (tra) in promoting female body growth. Normally, Tra is expressed only in females. We find that loss of Tra in female larvae decreases body size, while ectopic Tra expression in males increases body size. Although we find that Tra exerts autonomous effects on cell size, we also discovered that Tra expression in the fat body augments female body size in a non cell-autonomous manner. These effects of Tra do not require its only known targets doublesex and fruitless. Instead, Tra expression in the female fat body promotes growth by stimulating the secretion of insulin-like peptides from insulin producing cells in the brain. Our data suggest a model of sex-specific growth in which body size is regulated by a previously unrecognized branch of the sex determination pathway, and identify Tra as a novel link between sex and the conserved insulin signaling pathway.

Female-biased sexual size dimorphism is common in invertebrates, yet the mechanisms underlying increased female body size remain unclear. We uncovered a key role for sex determination gene transformer (tra) in promoting increased growth in females. Interestingly, we found that sex differences in body size are regulated by Tra in a pathway that is separate of the canonical sex determination pathway, and of other aspects of sexual dimorphism. Instead, Tra function in the fat body regulates growth in a non cell-autonomous manner by regulating the secretion of insulin-like peptides from the brain. This novel Tra-insulin link we describe may have implications for other sexually dimorphic phenotypes in Drosophila (eg. lifespan, stress resistance), many of which are also regulated by insulin.

The Wright stuff: reimagining path analysis reveals novel components of the sex determination hierarchy in Drosophila melanogaster

BACKGROUND

The Drosophila sex determination hierarchy is a classic example of a transcriptional regulatory hierarchy, with sex-specific isoforms regulating morphology and behavior. We use a structural equation modeling approach, leveraging natural genetic variation from two studies on Drosophila female head tissues--DSPR collection (596 F1-hybrids from crosses between DSPR sub-populations) and CEGS population (75 F1-hybrids from crosses between DGRP/Winters lines to a reference strain w1118)--to expand understanding of the sex hierarchy gene regulatory network (GRN). This approach is completely generalizable to any natural population, including humans.

RESULTS

We expanded the sex hierarchy GRN adding novel links among genes, including a link from fruitless (fru) to Sex-lethal (Sxl) identified in both populations. This link is further supported by the presence of fru binding sites in the Sxl locus. 754 candidate genes were added to the pathway, including the splicing factors male-specific lethal 2 and Rm62 as downstream targets of Sxl which are well-supported links in males. Independent studies of doublesex and transformer mutants support many additions, including evidence for a link between the sex hierarchy and metabolism, via Insulin-like receptor.

CONCLUSIONS

The genes added in the CEGS population were enriched for genes with sex-biased splicing and components of the spliceosome. A common goal of molecular biologists is to expand understanding about regulatory interactions among genes. Using natural alleles we can not only identify novel relationships, but using supervised approaches can order genes into a regulatory hierarchy. Combining these results with independent large effect mutation studies, allows clear candidates for detailed molecular follow-up to emerge.