Male-specific fruitless specifies the neural substrates of Drosophila courtship behaviour

A receptor that mediates the post-mating switch in Drosophila reproductive behaviour

Mating in many species induces a dramatic switch in female reproductive behaviour. In most insects, this switch is triggered by factors present in the male's seminal fluid. How these factors exert such profound effects in females is unknown. Here we identify a receptor for the Drosophila melanogaster sex peptide (SP, also known as Acp70A), the primary trigger of post-mating responses in this species. Females that lack the sex peptide receptor (SPR, also known as CG16752), either entirely or only in the nervous system, fail to respond to SP and continue to show virgin behaviours even after mating. SPR is expressed in the female's reproductive tract and central nervous system. The behavioural functions of SPR map to the subset of neurons that also express the fruitless gene, a key determinant of sex-specific reproductive behaviour. SPR is highly conserved across insects, opening up the prospect of new strategies to control the reproductive and host-seeking behaviours of agricultural pests and human disease vectors.

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.

Recurring ethanol exposure induces disinhibited courtship in Drosophila

Alcohol has a strong causal relationship with sexual arousal and disinhibited sexual behavior in humans; however, the physiological support for this notion is largely lacking and thus a suitable animal model to address this issue is instrumental. We investigated the effect of ethanol on sexual behavior in Drosophila. Wild-type males typically court females but not males; however, upon daily administration of ethanol, they exhibited active intermale courtship, which represents a novel type of behavioral disinhibition. The ethanol-treated males also developed behavioral sensitization, a form of plasticity associated with addiction, since their intermale courtship activity was progressively increased with additional ethanol experience. We identified three components crucial for the ethanol-induced courtship disinhibition: the transcription factor regulating male sex behavior Fruitless, the ABC guanine/tryptophan transporter White and the neuromodulator dopamine. fruitless mutant males normally display conspicuous intermale courtship; however, their courtship activity was not enhanced under ethanol. Likewise, white males showed negligible ethanol-induced intermale courtship, which was not only reinstated but also augmented by transgenic White expression. Moreover, inhibition of dopamine neurotransmission during ethanol exposure dramatically decreased ethanol-induced intermale courtship. Chronic ethanol exposure also affected a male's sexual behavior toward females: it enhanced sexual arousal but reduced sexual performance. These findings provide novel insights into the physiological effects of ethanol on sexual behavior and behavioral plasticity.

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.

Methods for homologous recombination in Drosophila

We present detailed protocols for two methods of gene targeting in Drosophila. The first, ends-out targeting, is identical in concept to gene replacement techniques used routinely in mammalian and yeast cells. In Drosophila, the targeted gene is replaced by the marker gene white + (although options exist to generate unmarked targeted alleles). This approach is simple in both the molecular cloning and the genetic manipulations. Ends-out will likely serve most investigators' purposes to generate simple gene deletions or reporter gene "knock-ins." The second method, ends-in targeting, targets a wild-type gene with an engineered mutated copy and generates a duplication structure at the target locus. This duplication can subsequently be reduced to one copy, removing the wild-type gene and leaving only the introduced mutation. Although more complicated in the cloning and genetic manipulations (see Note 1), this approach has the benefit that the mutations may be introduced with no other remnant of the targeting procedure. This "surgical" approach will appeal to investigators who desire minimal perturbation to the genome, such as single nucleotide mutation. Although both approaches appear to be approximately equally efficient (see Note 2), each method has separate strengths and drawbacks. The choice of which approach is best depends on the researcher's goal.

Specific subgroups of FruM neurons control sexually dimorphic patterns of aggression in Drosophila melanogaster

A great challenge facing neuroscience is to understand how genes, molecules, cells, circuits, and systems interact to generate social behavior. Fruit flies (Drosophila melanogaster) offer a powerful model system to address questions of this magnitude. These animals display genetically specified, sexually dimorphic patterns of fighting behavior via sex-specific splicing of the fruitless gene. Here, we show that sexually dimorphic behavioral patterns displayed during aggression are controlled by specific subgroups of neurons expressing male forms of fruitless proteins (Fru(M)). Using the GAL4/UAS system to manipulate transformer expression, we feminized or masculinized different populations of neurons in fly nervous systems. With a panneuronal elav-GAL4 driver, male patterns of fighting behavior were transferred into females and female patterns into males. We screened 60 Gal4 lines that express the yeast transcription factor in different patterns in fly central nervous systems and found five that showed abnormal same-sex courtship behavior. The sexually dimorphic fighting patterns, however, were completely switched only in one and partially switched in a second of these lines. In the other three lines, female patterns of aggression were seen despite a switch in courtship preference. A tight correspondence was seen between Fru(M) expression and how flies fight in several subgroups of neurons usually expressing these proteins: Expression is absent when flies fight like females and present when flies fight like males, thereby beginning a separation between courtship and aggression among these neurons.

Neurogenetics: singing in the brain

The fruitless gene is well-known to play a key role in determining the sexual identity of the fruitfly's nervous system, but new results show that doublesex is also required in thoracic neurons to generate normal male lovesongs.

The sex-determination genes fruitless and doublesex specify a neural substrate required for courtship song

Courtship song is a critical component of male courtship behavior in Drosophila, making the female more receptive to copulation and communicating species-specific information [1-6]. Sex mosaic studies have shown that the sex of certain regions of the central nervous system (CNS) is critical to song production [7]. Our examination of one of these regions, the mesothoracic ganglion (Msg), revealed the coexpression of two sex-determination genes, fruitless (fru) and doublesex (dsx). Because both genes are involved in creating a sexually dimorphic CNS [8, 9] and are necessary for song production [10-13], we investigated the individual contributions of fru and dsx to the specification of a male CNS and song production. We show a novel requirement for dsx in specifying a sexually dimorphic population of fru-expressing neurons in the Msg. Moreover, by using females constitutively expressing the male-specific isoforms of fru (Fru(M)), we show a critical requirement for the male isoform of dsx (Dsx(M)), alongside Fru(M), in the specification of courtship song. Therefore, although Fru(M) expression is sufficient for the performance of many male-specific behaviors [14], we have shown that without Dsx(M), the determination of a male-specific CNS and thus a full complement of male behaviors are not realized.

A rapid genome-wide response to Drosophila melanogaster social interactions

BACKGROUND

The actions and reactions integral to mate recognition and reproduction are examples of multifaceted behaviors for which we are only beginning to comprehend the underlying genetic and molecular complexity. I hypothesized that social interactions, such as those involved in reproductive behaviors, would lead to immediate and assayable changes in gene expression. Such changes may have important effects on individual reproductive success and fitness through alterations in physiology or via short-term or long-term changes in nervous system function.

RESULTS

I used Affymetrix Drosophila Genome arrays to identify genes whose expression profiles would change rapidly due to the social interactions occurring during Drosophila melanogaster courtship. I identified 43 loci with significant expression profile changes during a 5-min exposure period. These results indicate that social interactions can lead to extremely rapid changes in mRNA abundance.

CONCLUSION

The known functions of the up-regulated genes identified in this study include nervous system signaling and spermatogenesis, while the majority of down-regulated loci are implicated in immune signaling. Expression of two of the up-regulated genes, Odorant-binding protein 99b (Obp99b) and female-specific independent of transformer (fit), is controlled by the Drosophila sex-determination gene hierarchy, which regulates male and female mating behaviors and somatic differentiation. Therefore, additional identified loci may represent other long-elusive targets of Drosophila sex-determination genes.

A functional circuit underlying male sexual behaviour in the female mouse brain

In mice, pheromone detection is mediated by the vomeronasal organ and the main olfactory epithelium. Male mice that are deficient for Trpc2, an ion channel specifically expressed in VNO neurons and essential for VNO sensory transduction, are impaired in sex discrimination and male-male aggression. We report here that Trpc2-/- female mice show a reduction in female-specific behaviour, including maternal aggression and lactating behaviour. Strikingly, mutant females display unique characteristics of male sexual and courtship behaviours such as mounting, pelvic thrust, solicitation, anogenital olfactory investigation, and emission of complex ultrasonic vocalizations towards male and female conspecific mice. The same behavioural phenotype is observed after VNO surgical removal in adult animals, and is not accompanied by disruption of the oestrous cycle and sex hormone levels. These findings suggest that VNO-mediated pheromone inputs act in wild-type females to repress male behaviour and activate female behaviours. Moreover, they imply that functional neuronal circuits underlying male-specific behaviours exist in the normal female mouse brain.

Molecular Architecture of Smell and Taste inDrosophila

The chemical senses-smell and taste-allow animals to evaluate and distinguish valuable food resources from dangerous substances in the environment. The central mechanisms by which the brain recognizes and discriminates attractive and repulsive odorants and tastants, and makes behavioral decisions accordingly, are not well understood in any organism. Recent molecular and neuroanatomical advances in Drosophila have produced a nearly complete picture of the peripheral neuroanatomy and function of smell and taste in this insect. Neurophysiological experiments have begun to provide insight into the mechanisms by which these animals process chemosensory cues. Given the considerable anatomical and functional homology in smell and taste pathways in all higher animals, experimental approaches in Drosophila will likely provide broad insights into the problem of sensory coding. Here we provide a critical review of the recent literature in this field and comment on likely future directions.

Comprehensive maps of Drosophila higher olfactory centers: spatially segregated fruit and pheromone representation

In Drosophila, ∼50 classes of olfactory receptor neurons (ORNs) send axons to 50 corresponding glomeruli in the antennal lobe. Uniglomerular projection neurons (PNs) relay olfactory information to the mushroom body (MB) and lateral horn (LH). Here, we combine single-cell labeling and image registration to create high-resolution, quantitative maps of the MB and LH for 35 input PN channels and several groups of LH neurons. We find (1) PN inputs to the MB are stereotyped as previously shown for the LH; (2) PN partners of ORNs from different sensillar groups are clustered in the LH; (3) fruit odors are represented mostly in the posterior-dorsal LH, whereas candidate pheromone-responsive PNs project to the anterior-ventral LH; (4) dendrites of single LH neurons each overlap with specific subsets of PN axons. Our results suggest that the LH is organized according to biological values of olfactory input.

Taste and pheromone perception in the fruit fly Drosophila melanogaster

Taste is an essential sense for detection of nutrient-rich food and avoidance of toxic substances. The Drosophila melanogaster gustatory system provides an excellent model to study taste perception and taste-elicited behaviors. "The fly" is unique in the animal kingdom with regard to available experimental tools, which include a wide repertoire of molecular-genetic analyses (i.e., efficient production of transgenics and gene knockouts), elegant behavioral assays, and the possibility to conduct electrophysiological investigations. In addition, fruit flies, like humans, recognize sugars as a food source, but avoid bitter tasting substances that are often toxic to insects and mammals alike. This paper will present recent research progress in the field of taste and contact pheromone perception in the fruit fly. First, we shall describe the anatomical properties of the Drosophila gustatory system and survey the family of taste receptors to provide an appropriate background. We shall then review taste and pheromone perception mainly from a molecular genetic perspective that includes behavioral, electrophysiological and imaging analyses of wild type flies and flies with genetically manipulated taste cells. Finally, we shall provide an outlook of taste research in this elegant model system for the next few years.

Internal representations of smell in the Drosophila brain

Recent advances in sensory neuroscience using Drosophila olfaction as a model system have revealed brain maps representing the external world. Once we understand how the brain's built-in capability generates the internal olfactory maps, we can then elaborate how the brain computes and makes decision to elicit complex behaviors. Here, we review current progress in mapping Drosophila olfactory circuits and discuss their relationships with innate olfactory behaviors.

Prospero mutants induce precocious sexual behavior in Drosophila males

Brain maturation, a developmental process influenced by both endogenous and environmental factors, can affect sexual behavior. In vertebrates and invertebrates, sexual maturation is under the influence of hormones and neuromodulators, but the role of developmental genes in this process is still poorly understood. We report that prospero (pros), a gene crucial for nervous system development, can change the age of onset of sexual behavior in Drosophila melanogaster males: adult males carrying a single copy of several pros mutations court females and mate at a younger age than control males. However, these pros mutations had no effect on female sexual receptivity and did not alter other male phenotypes related to mating behavior. The Pros protein was detected in several brain and sensory structures of immature adult males, some of which are normally involved in the regulation of male specific behaviors. Our data suggest that the altered pros expression affects the age of onset of male mating behavior.

Olfactory processing and behavior downstream from highly selective receptor neurons

In both the vertebrate nose and the insect antenna, most olfactory receptor neurons (ORNs) respond to multiple odors. However, some ORNs respond to just a single odor, or at most to a few highly related odors. It has been hypothesized that narrowly tuned ORNs project to narrowly tuned neurons in the brain, and that these dedicated circuits mediate innate behavioral responses to a particular ligand. Here we have investigated neural activity and behavior downstream from two narrowly tuned ORN types in Drosophila melanogaster. We found that genetically ablating either of these ORN types impairs innate behavioral attraction to their cognate ligand. Neurons in the antennal lobe postsynaptic to one of these ORN types are, like their presynaptic ORNs, narrowly tuned to a pheromone. However, neurons postsynaptic to the second ORN type are broadly tuned. These results demonstrate that some narrowly tuned ORNs project to dedicated central circuits, ensuring a tight connection between stimulus and behavior, whereas others project to central neurons that participate in the ensemble representations of many odors.

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

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

Peripheral multidendritic sensory neurons are necessary for rhythmic locomotion behavior in Drosophila larvae

From breathing to walking, rhythmic movements encompass physiological processes important across the entire animal kingdom. It is thought by many that the generation of rhythmic behavior is operated by a central pattern generator (CPG) and does not require peripheral sensory input. Sensory feedback is, however, required to modify or coordinate the motor activity in response to the circumstances of actual movement. In contrast to this notion, we report here that sensory input is necessary for the generation of Drosophila larval locomotion, a form of rhythmic behavior. Blockage of all peripheral sensory inputs resulted in cessation of larval crawling. By conditionally silencing various subsets of larval peripheral sensory neurons, we identified the multiple dendritic (MD) neurons as the neurons essential for the generation of rhythmic peristaltic locomotion. By recording the locomotive motor activities, we further demonstrate that removal of MD neuron input disrupted rhythmic motor firing pattern in a way that prolonged the stereotyped segmental motor firing duration and prevented the propagation of posterior to anterior segmental motor firing. These findings reveal that MD sensory neuron input is a necessary component in the neural circuitry that generates larval locomotion.

Modulation of Drosophila male behavioral choice

The reproductive and defensive behaviors that are initiated in response to specific sensory cues can provide insight into how choices are made between different social behaviors. We manipulated both the activity and sex of a subset of neurons and found significant changes in male social behavior. Results from aggression assays indicate that the neuromodulator octopamine (OCT) is necessary for Drosophila males to coordinate sensory cue information presented by a second male and respond with the appropriate behavior: aggression rather than courtship. In competitive male courtship assays, males with no OCT or with low OCT levels do not adapt to changing sensory cues and court both males and females. We identified a small subset of neurons in the suboesophageal ganglion region of the adult male brain that coexpress OCT and male forms of the neural sex determination factor, Fruitless (Fru(M)). A single Fru(M)-positive OCT neuron sends extensive bilateral arborizations to the suboesophageal ganglion, the lateral accessory lobe, and possibly the posterior antennal lobe, suggesting a mechanism for integrating multiple sensory modalities. Furthermore, eliminating the expression of Fru(M) by transformer expression in OCT/tyramine neurons changes the aggression versus courtship response behavior. These results provide insight into how complex social behaviors are coordinated in the nervous system and suggest a role for neuromodulators in the functioning of male-specific circuitry relating to behavioral choice.

A Map of Olfactory Representation in the Drosophila Mushroom Body

Neural coding for olfactory sensory stimuli has been mapped near completion in the Drosophila first-order center, but little is known in the higher brain centers. Here, we report that the antenna lobe (AL) spatial map is transformed further in the calyx of the mushroom body (MB), an essential olfactory associated learning center, by stereotypic connections with projection neurons (PNs). We found that Kenyon cell (KC) dendrites are segregated into 17 complementary domains according to their neuroblast clonal origins and birth orders. Aligning the PN axonal map with the KC dendritic map and ultrastructural observation suggest a positional ordering such that inputs from the different AL glomeruli have distinct representations in the MB calyx, and these representations might synapse on functionally distinct KCs. Our data suggest that olfactory coding at the AL is decoded in the MB and then transferred via distinct lobes to separate higher brain centers.

Good genes, genetic compatibility and the evolution of polyandry: use of the diallel cross to address competing hypotheses

Genetic benefits can enhance the fitness of polyandrous females through the high intrinsic genetic quality of females' mates or through the interaction between female and male genes. I used a full diallel cross, a quantitative genetics design that involves all possible crosses among a set of genetically homogeneous lines, to determine the mechanism through which polyandrous female decorated crickets (Gryllodes sigillatus) obtain genetic benefits. I measured several traits related to fitness and partitioned the phenotypic variance into components representing the contribution of additive genetic variance ('good genes'), nonadditive genetic variance (genetic compatibility), as well as maternal and paternal effects. The results reveal a significant variance attributable to both nonadditive and additive sources in the measured traits, and their influence depended on which trait was considered. The lack of congruence in sources of phenotypic variance among these fitness-related traits suggests that the evolution and maintenance of polyandry are unlikely to have resulted from one selective influence, but rather are the result of the collective effects of a number of factors.

A role for the adult fat body in Drosophila male courtship behavior

Mating behavior in Drosophila depends critically on the sexual identity of specific regions in the brain, but several studies have identified courtship genes that express products only outside the nervous system. Although these genes are each active in a variety of non-neuronal cell types, they are all prominently expressed in the adult fat body, suggesting an important role for this tissue in behavior. To test its role in male courtship, fat body was feminized using the highly specific Larval serum protein promoter. We report here that the specific feminization of this tissue strongly reduces the competence of males to perform courtship. This effect is limited to the fat body of sexually mature adults as the feminization of larval fat body that normally persists in young adults does not affect mating. We propose that feminization of fat body affects the synthesis of male-specific secreted circulating proteins that influence the central nervous system. In support of this idea, we demonstrate that Takeout, a protein known to influence mating, is present in the hemolymph of adult males but not females and acts as a secreted protein.

Sequential use of mushroom body neuron subsets during drosophila odor memory processing

Drosophila mushroom bodies (MB) are bilaterally symmetric multilobed brain structures required for olfactory memory. Previous studies suggested that neurotransmission from MB neurons is only required for memory retrieval. Our unexpected observation that Dorsal Paired Medial (DPM) neurons, which project only to MB neurons, are required during memory storage but not during acquisition or retrieval, led us to revisit the role of MB neurons in memory processing. We show that neurotransmission from the alpha'beta' subset of MB neurons is required to acquire and stabilize aversive and appetitive odor memory, but is dispensable during memory retrieval. In contrast, neurotransmission from MB alphabeta neurons is only required for memory retrieval. These data suggest a dynamic requirement for the different subsets of MB neurons in memory and are consistent with the notion that recurrent activity in an MB alpha'beta' neuron-DPM neuron loop is required to stabilize memories formed in the MB alphabeta neurons.

Functional and mechanistic insights from genome-wide studies of splicing regulation in the brain

We review here results arising from the systematic functional analysis of Nova, a neuron-specific RNA binding protein targeted in an autoimmune neurological disorder associated with cancer. We have developed a combination of biochemical, genetic and bioinformatic methods to generate a global understanding of Nova's role as a splicing regulator. Genome-wide identification and validation of Nova target RNAs has yielded unexpected insights into the protein's mechanism of action and into the functionally coherent role of Nova in the biology of the neuronal synapse. These studies provide us with a platform for understanding the role of RNA binding proteins in tissue-specific splicing regulation and in disease.

The neural and genetic substrates of sexual behavior in Drosophila

fruitless (fru), originally identified with its mutant conferring male homosexuality, is a neural sex determination gene in Drosophila that produces sexually dimorphic sets of transcripts. In the nervous system, Fru is translated only in males. Fru proteins likely regulate the transcription of a set of downstream genes. The expression of Fru proteins is sufficient to induce male sexual behavior in females. A group of fru-expressing neurons called "mAL" neurons in the brain shows conspicuous sexual dimorphism. mAL is composed of 5 neurons in females and 30 neurons in males. It includes neurons with bilateral projections in males and contralateral projections in females. Terminal arborization patterns are also sexually dimorphic. These three characteristics are feminized in fru mutant males. The inactivation of cell death genes results in the production of additional mAL neurons that are of the male type in the female brain. This suggests that male-specific Fru inhibits mAL neuron death, leading to the formation of a male-specific neural circuit that underlies male sexual behavior. Fru orchestrates a spectrum of downstream genes as a master control gene to establish the maleness of the brain.

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

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

A double-switch system regulates male courtship behavior in male and female Drosophila melanogaster

Current models describe male-specific fruitless (fruM) as a genetic 'switch' regulating sexual behavior in Drosophila melanogaster, and they postulate that female (F) and male (M) doublesex (dsx) products control body sexual morphology. In contradiction to this simple model, we show that dsx, as well as fruM and non-sex-specific retained (retn), affect both male and female sexual behaviors. In females, both retn and dsxF contribute to female receptivity, and both genes act to repress male-like courtship activity in the presence or absence of fruM. In males, consistent with the opposing functions of dsxM and dsxF, dsxM acts as a positive factor for male courtship. retn also acts counter to fruM in the development of the male-specific muscle of Lawrence. Molecularly, retn seems to regulate sexual behavior via a previously described complex that represses zerknullt. Thus, we show that fru and dsx together act as a 'switch' system regulating behavior in the context of other developmental genes, such as retn.

Alternative starts of transcription, several paralogues, and almost-fixed interspecific differences of the gene fruitless in a hemimetabolous insect

In fruit flies, the gene fruitless (fru) governs the establishment of the potential for male sexual behavior. We partially cloned fru from a hemimetabolous insect for the first time and we compared fru among three closely related and acoustically communicating grasshopper species: Chorthippus biguttulus, C. brunneus, and C. mollis. The fru of grasshoppers is organized similarly to fru of holometabolous insects, with a BTB and Zn-finger domains separated by a nonconserved repetitive linker. As in Drosophila, several transcripts of fru are found in grasshoppers. We also present evidence for the coexistence of several copies of fru in the grasshopper genome. Within species these copies are almost identical and carry almost-fixed species-specific differences. This suggests that the paralogous copies of fru in grasshoppers do not evolve independently from each other.

Phenotypes of Drosophila brain neurons in primary culture reveal a role for fascin in neurite shape and trajectory

Subtle cellular phenotypes in the CNS may evade detection by routine histopathology. Here, we demonstrate the value of primary culture for revealing genetically determined neuronal phenotypes at high resolution. Gamma neurons of Drosophila melanogaster mushroom bodies (MBs) are remodeled during metamorphosis under the control of the steroid hormone 20-hydroxyecdysone (20E). In vitro, wild-type gamma neurons retain characteristic morphogenetic features, notably a single axon-like dominant primary process and an arbor of short dendrite-like processes, as determined with microtubule-polarity markers. We found three distinct genetically determined phenotypes of cultured neurons from grossly normal brains, suggesting that subtle in vivo attributes are unmasked and amplified in vitro. First, the neurite outgrowth response to 20E is sexually dimorphic, being much greater in female than in male gamma neurons. Second, the gamma neuron-specific "naked runt" phenotype results from transgenic insertion of an MB-specific promoter. Third, the recessive, pan-neuronal "filagree" phenotype maps to singed, which encodes the actin-bundling protein fascin. Fascin deficiency does not impair the 20E response, but neurites fail to maintain their normal, nearly straight trajectory, instead forming curls and hooks. This is accompanied by abnormally distributed filamentous actin. This is the first demonstration of fascin function in neuronal morphogenesis. Our findings, along with the regulation of human Fascin1 (OMIM 602689) by CREB (cAMP response element-binding protein) binding protein, suggest FSCN1 as a candidate gene for developmental brain disorders. We developed an automated method of computing neurite curvature and classifying neurons based on curvature phenotype. This will facilitate detection of genetic and pharmacological modifiers of neuronal defects resulting from fascin deficiency.

Differential roles of two major brain structures, mushroom bodies and central complex, for Drosophila male courtship behavior

Drosophila male courtship is a complex and robust behavior, the potential for which is genetically built into specific neural circuits in the central nervous system. Previous studies using male-female mosaics and the flies with defects in particular brain structures implicated the critical central regions involved in male courtship behavior. However, their acute physiological roles in courtship regulation still largely remain unknown. Using the temperature-sensitive Dynamin mutation, shibire(ts1), here we demonstrate the significance of two major brain structures, the mushroom bodies and the central complex, in experience-independent aspects of male courtship. We show that blocking of synaptic transmission in the mushroom body intrinsic neurons significantly delays courtship initiation and reduces the courtship activity by shortening the courtship bout length when virgin females are used as a sexual target. Interestingly, however, the same treatment affects neither initiation nor maintenance of courtship toward young males that release courtship-stimulating pheromones different from those of virgin females. In contrast, blocking of synaptic transmission in a central complex substructure, the fan-shaped body, slightly but significantly reduces courtship activity toward both virgin females and young males with little effect on courtship initiation. Taken together, our results indicate that the neuronal activity in the mushroom bodies plays an important role in responding to female-specific sex pheromones that stimulate initiation and maintenance of male courtship behavior, whereas the fan-shaped body neurons are involved in maintenance of male courtship regardless of the nature of courtship-stimulating cues.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Sex- and clock-controlled expression of the neuropeptide F gene in Drosophila

Drosophila neuropeptide F (NPF), a homolog of vertebrate neuropeptide Y, functions in feeding and coordination of behavioral changes in larvae and in modulation of alcohol sensitivity in adults, suggesting diverse roles for this peptide. To gain more insight into adult-specific NPF neuronal functions, we studied how npf expression is regulated in the adult brain. Here, we report that npf expression is regulated in both sex-nonspecific and male-specific manners. Our data show that male-specific npf (ms-npf) expression is controlled by the transformer (tra)-dependent sex-determination pathway. Furthermore, fruitless, one of the major genes functioning downstream of tra, is apparently an upstream regulator of ms-npf transcription. Males lacking ms-npf expression (through tra(F)-mediated feminization) or npf-ablated male flies display significantly reduced male courtship activity, suggesting that one function of ms-npf neurons is to modulate fruitless-regulated sexual behavior. Interestingly, one of the ms-npf neuronal groups belongs to the previously defined clock-controlling dorsolateral neurons. Such ms-npf expression in the dorsolateral neurons is absent in arrhythmic Clock(Jrk) and cycle(02) mutants, suggesting that npf is under dual regulation by circadian and sex-determining factors. Based on these data, we propose that NPF also plays a role in clock-controlled sexual dimorphism in adult Drosophila.

A Command Chemical Triggers an Innate Behavior by Sequential Activation of Multiple Peptidergic Ensembles

BACKGROUND

At the end of each molt, insects shed their old cuticle by performing the ecdysis sequence, an innate behavior consisting of three steps: pre-ecdysis, ecdysis, and postecdysis. Blood-borne ecdysis-triggering hormone (ETH) activates the behavioral sequence through direct actions on the central nervous system.

RESULTS

To elucidate neural substrates underlying the ecdysis sequence, we identified neurons expressing ETH receptors (ETHRs) in Drosophila. Distinct ensembles of ETHR neurons express numerous neuropeptides including kinin, FMRFamides, eclosion hormone (EH), crustacean cardioactive peptide (CCAP), myoinhibitory peptides (MIP), and bursicon. Real-time imaging of intracellular calcium dynamics revealed sequential activation of these ensembles after ETH action. Specifically, FMRFamide neurons are activated during pre-ecdysis; EH, CCAP, and CCAP/MIP neurons are active prior to and during ecdysis; and activity of CCAP/MIP/bursicon neurons coincides with postecdysis. Targeted ablation of specific ETHR ensembles produces behavioral deficits consistent with their proposed roles in the behavioral sequence.

CONCLUSIONS

Our findings offer novel insights into how a command chemical orchestrates an innate behavior by stepwise recruitment of central peptidergic ensembles.

Blueprints for behavior: genetic specification of neural circuitry for innate behaviors

Innate behaviors offer a unique opportunity to use genetic analysis to dissect and characterize the neural substrates of complex behavioral programs. Courtship in Drosophila involves a complex series of stereotyped behaviors that include numerous exchanges of multimodal sensory information over time. As we will discuss in this review, recent work has demonstrated that male-specific expression of Fruitless transcription factors (Fru(M) proteins) is necessary and sufficient to confer the potential for male courtship behaviors. Fru(M) factors program neurons of the male central and peripheral nervous systems whose function is dedicated to sexual behaviors. This circuitry seems to integrate sensory information to define behavioral states and regulate conserved neural elements for sex-specific behavioral output. The principles that govern the circuitry specified by Fru(M) expression might also operate in subcortical networks that govern innate behaviors in mammals.

Sex cells: dissecting the functions of fruitless isoforms

Behavioral and developmental sexual dimorphism in fruitflies is related to male-specific expression of protein isoforms encoded by the fruitless gene. A recent study shows that the wide-scale effects of fruitless are associated with differential functions of individual protein isoforms.

Sex and gonadal steroid modulation of pheromone receptor gene expression in the mouse vomeronasal organ

Non-volatile chemosignals in rodents are detected by unique receptors in the vomeronasal organ of the accessory olfactory system. Although the vomeronasal organ has been implicated in the regulation of sexually dimorphic behavioral and neuroendocrine functions, the underlying cellular mechanisms are undetermined. In previous studies we showed that exposure to soiled male bedding augmented immediate early gene immunoreactivity in neurons of the basal zone of the vomeronasal organ, an effect that depended on gender and sex steroid expression. To determine whether this effect could be due to differences in vomeronasal organ receptor expression, we examined two representatives (VR1 and VR4) from different subfamilies of the V2R family of receptors that are expressed in the basal zone of the vomeronasal organ. Adult Swiss-Webster male and female mice were gonadectomized and implanted with capsules containing 17beta-estradiol, testosterone or neither steroid (control). Two weeks later vomeronasal organs were processed for in situ hybridization using probes from the N-terminal extracellular domains of VR1 and VR4. Expression of both VR1 and VR4 was significantly higher in males than in females. Estradiol, but not testosterone-treated, males had significantly lower levels of VR1 expression in the caudal vomeronasal organ compared with untreated gonadectomized males. In contrast, testosterone enhanced VR4 expression in males relative to similarly treated females. Despite these effects, we found no evidence that vomeronasal organ neurons express either androgen or estrogen receptors. These data show that expression of vomeronasal organ receptors in mice is sexually dimorphic and regulated by sex steroids. Thus, gonadal hormones may affect the response of vomeronasal organ neurons to chemosignals by altering levels of the receptors to which they bind.

Wiring stability of the adult Drosophila olfactory circuit after lesion

Neuronal wiring plasticity in response to experience or injury has been reported in many parts of the adult nervous system. For instance, visual or somatosensory cortical maps can reorganize significantly in response to peripheral lesions, yet a certain degree of stability is essential for neuronal circuits to perform their dedicated functions. Previous studies on lesion-induced neuronal reorganization have primarily focused on systems that use continuous neural maps. Here, we assess wiring plasticity in a discrete neural map represented by the adult Drosophila olfactory circuit. Using conditional expression of toxins, we genetically ablated specific classes of neurons and examined the consequences on their synaptic partners or neighboring classes in the adult antennal lobe. We find no alteration of connection specificity between olfactory receptor neurons (ORNs) and their postsynaptic targets, the projection neurons (PNs). Ablating an ORN class maintains PN dendrites within their glomerular borders, and ORN axons normally innervating an adjacent target do not expand. Likewise, ablating PN classes does not alter their partner ORN axon connectivity. Interestingly, an increase in the contralateral ORN axon terminal density occurs in response to the removal of competing ipsilateral ORNs. Therefore, plasticity in this circuit can occur but is confined within a glomerulus, thereby retaining the wiring specificity of ORNs and PNs. We conclude that, although adult olfactory neurons can undergo plastic changes in response to the loss of competition, the olfactory circuit overall is extremely stable in preserving segregated information channels in this discrete map.

Coding of Odors by a Receptor Repertoire

We provide a systematic analysis of how odor quality, quantity, and duration are encoded by the odorant receptor repertoire of the Drosophila antenna. We test the receptors with a panel of over 100 odors and find that strong responses are sparse, with response density dependent on chemical class. Individual receptors range along a continuum from narrowly tuned to broadly tuned. Broadly tuned receptors are most sensitive to structurally similar odorants. Strikingly, inhibitory responses are widespread among receptors. The temporal dynamics of the receptor repertoire provide a rich representation of odor quality, quantity, and duration. Receptors with similar odor sensitivity often map to widely dispersed glomeruli in the antennal lobe. We construct a multidimensional "odor space" based on the responses of each individual receptor and find that the positions of odors depend on their chemical class, concentration, and molecular complexity. The space provides a basis for predicting behavioral responses to odors.

Genetic and functional subdivision of the Drosophila antennal lobe

Olfactory systems confer the recognition and discrimination of a large number of structurally distinct odor molecules. Recent molecular analysis of odorant receptor (OR) genes and circuits has led to a model of odor coding in which a population of olfactory sensory neurons (OSNs) expressing a single OR converges upon a unique olfactory glomerulus. Activation of the OR can thus be read out by the activation of its cognate glomerulus. Drosophila is a powerful system in which to test this model because the entire repertoire of 62 ORs can be manipulated genetically. However, a complete understanding of how fly olfactory circuits are organized is lacking. Here, we present a nearly complete map of OR projections from OSNs to the antennal lobe (AL) in the fly brain. Four populations of OSNs coexpress two ORs along with Or83b, and a fifth expresses one OR and one gustatory receptor (GR) along with Or83b. One glomerulus receives coconvergent input from two separate populations of OSNs. Three ORs label sexually dimorphic glomeruli implicated in sexual courtship and are thus candidate Drosophila pheromone receptors. This olfactory sensory map provides an experimental framework for relating ORs to glomeruli and ultimately behavior.

Sexual behaviour: do a few dead neurons make the difference?

Why do males and females behave so differently? Sexually dimorphic neural circuitry has just been found in parts of the fly's brain thought to control mating behaviour. Might this explain why males and females have such distinct sexual behaviours?

Wiring specificity in the olfactory system

The fruitfly brain learns about the olfactory world by reading the activity of about 50 distinct channels of incoming information. The receptor neurons that compose each channel have their own distinctive odour response profile governed by a specific receptor molecule. These receptor neurons form highly specific connections in the first olfactory relay of the fly brain, each synapsing with specific second order partner neurons. We use this system to discuss the logic of wiring specificity in the brain and to review the cellular and molecular mechanisms that allow such precise wiring to develop.

RNA binding proteins and the regulation of neuronal synaptic plasticity

Cognitive function and memory rely on synaptic plasticity - the ability of synapses to modify their strength in response to stimulation. Emerging evidence indicates that post-transcriptional gene regulation is necessary for synaptic plasticity at several levels: by increasing proteome diversity through alternative splicing, or by enabling activity-dependent regulation of mRNA localization, translation or degradation in the dendrite. Mouse knockout studies have linked three RNA-binding proteins, fragile X mental retardation protein, cytoplasmic polyadenylation element binding factor and neuro-oncological ventral antigen to specific aspects of synaptic plasticity. Specificity in the regulation of synaptic plasticity might, in part, relate to the functional coherence of proteins encoded by the RNA targets of each RNA-binding protein.

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

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

A cis-regulatory sequence within the yellow locus of Drosophila melanogaster required for normal male mating success

Drosophila melanogaster males perform a courtship ritual consisting of a series of dependent fixed-action patterns. The yellow (y) gene is required for normal male courtship behavior and subsequent mating success. To better characterize the requirement for y in the manifestation of innate male sexual behavior, we measured the male mating success (MMS) of 12 hypomorphic y mutants and matched-outbred-background controls using a y+ rescue element on a freely segregating minichromosome. We found that 4 hypomorphs significantly reduced MMS to varying degrees. Reduced MMS was largely independent of adult pigmentation patterns. These mutations defined a 300-bp regulatory region upstream of the transcription start, the mating-success regulatory sequence (MRS), whose function is required for normal MMS. Visualization of gene action via GFP and a Yellow antibody suggests that the MRS directs y transcription in a small number of cells in the third instar CNS, the developmental stage previously implicated in the role of y with regard to male courtship behavior. The presence of Yellow protein in these cells positively correlates with MMS in a subset of mutants. The MRS contains a regulatory sequence controlling larval pigmentation and a 35-bp sequence that is highly conserved within the genus Drosophila and is predicted to bind known transcription factors.

Sex- and segment-specific modulation of gene expression profiles in Drosophila

Homeotic and sex-determining genes control a wide range of morphological traits by regulating the expression of different target genes in different tissues. The identity of most of these target genes remains unknown, and it is not even clear what fraction of the genome is regulated in a segment- and sex-specific manner. In this report, we examine segment- and sex-specific gene expression in Drosophila pupal legs. The first and second legs in Drosophila have clearly distinguishable bristle patterns. Bristle pattern in the first leg also differs between males and females, whereas the second leg has no overt sexual dimorphism. To identify the genes responsible for these differences, we compared transcriptional profiles between male and female first and second legs during early pupal development. The extent of sexually dimorphic gene expression parallels morphological differences: over 100 genes are expressed sex specifically in the first leg, whereas no sexual differences are seen in the second leg. Segmental differences are less extensive than sexual dimorphism and involve fewer than 14 genes. We have identified a novel gene, CG13857, that is expressed exclusively in the first leg in a pattern that suggests this gene may play an important role in specifying segment- and sex-specific bristle patterns.