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Sex determination (SD) is a crucial developmental process, but its molecular underpinnings are very diverse, both between and within species. SD mechanisms have traditionally been categorized as either genetic (GSD) or environmental (ESD), depending on the type of cue that triggers sexual differentiation. However, mixed systems, with both genetic and environmental components, are more prevalent than previously thought. Here, we show theoretically that environmental effects on expression levels of genes within SD regulatory mechanisms can easily trigger within-species evolutionary divergence of SD mechanisms. This may lead to the stable coexistence of multiple SD mechanisms and to spatial variation in the occurrence of different SD mechanisms along environmental gradients. We applied the model to the SD system of the housefly, a global species with world-wide latitudinal clines in the frequencies of different SD systems, and found that it correctly predicted these clines if specific genes in the housefly SD system were assumed to have temperature-dependent expression levels. We conclude that environmental sensitivity of gene regulatory networks may play an important role in diversification of SD mechanisms. [ABSTRACT FROM AUTHOR]

The ability to transition between different behavioural stages is a widespread phenomenon across the animal kingdom. Such behavioural adaptations are often linked to changes in the sensitivity of those neurons that sense chemical cues associated with the respective behaviours. To identify the genetic mechanisms that regulate neuronal sensitivity, and by that behaviour, typically *omics approaches, such as RNA‐ and protein‐sequencing, are applied to sensory organs of individuals displaying differences in behaviour. In this review, we discuss these genetic mechanisms and how they impact neuronal sensitivity, summarize the correlative and functional evidence for their role in regulating behaviour and discuss future directions. As such, this review can help interpret *omics data by providing a comprehensive list of already identified genes and mechanisms that impact behaviour through changes in neuronal sensitivity. [ABSTRACT FROM AUTHOR]

Heterogeneity in food resources is a major driver of local adaptation and speciation. Dietary specialization typically involves multiple life‐history traits and may thus be limited by the extent to which these traits adapt in concert. Here, we use Drosophila melanogaster, representing an intermediate state in the generalist‐specialist continuum, to explore the scope for dietary specialization. D. melanogaster has a close association with yeast, an essential but heterogeneous food resource. We quantify how different D. melanogaster strains from around the globe respond to different yeast species, across multiple yeast‐dependent life‐history traits including feeding, mating, egg‐laying, egg development and survival. We find that D. melanogaster strains respond to different yeast species in different ways, indicating distinct fly strain–yeast interactions. However, we detect no evidence for trade‐offs: fly performance tends to be positively rather than negatively correlated across yeast species. We also find that the responses to different yeast species are not aligned across traits: different life‐history traits are maximized on different yeast species. Finally, we confirm that D. melanogaster is a resource generalist: it can grow, reproduce and survive on all the yeast species we tested. Together, these findings provide a possible explanation for the limited extent of dietary specialization in D. melanogaster. [ABSTRACT FROM AUTHOR]

Chemicals released into the environment by food, predators and conspecifics play critical roles in Drosophila reproduction. Females and males live in an environment full of smells, whose molecules communicate to them the availability of food, potential mates, competitors or predators. Volatile chemicals derived from fruit, yeast growing on the fruit, and flies already present on the fruit attract Drosophila, concentrating flies at food sites, where they will also mate. Species-specific cuticular hydrocarbons displayed on female Drosophila as they mature are sensed by males and act as pheromones to stimulate mating by conspecific males and inhibit heterospecific mating. The pheromonal profile of a female is also responsive to her nutritional environment, providing an honest signal of her fertility potential. After mating, cuticular and semen hydrocarbons transferred by the male change the female’s chemical profile. These molecules make the female less attractive to other males, thus protecting her mate’s sperm investment. Females have evolved the capacity to counteract this inhibition by ejecting the semen hydrocarbon (along with the rest of the remaining ejaculate) a few hours after mating. Although this ejection can temporarily restore the female’s attractiveness, shortly thereafter another male pheromone, a seminal peptide, decreases the female’s propensity to re-mate, thus continuing to protect the male’s investment. Females use olfaction and taste sensing to select optimal egg-laying sites, integrating cues for the availability of food for her offspring, and the presence of other flies and of harmful species. We argue that taking into account evolutionary considerations such as sexual conflict, and the ecological conditions in which flies live, is helpful in understanding the role of highly species-specific pheromones and blends thereof, as well as an individual’s response to the chemical cues in its environment. [ABSTRACT FROM AUTHOR]

Adult sex ratio determines the level of mate availability and intrasexual competition for each sex. Sex ratio biases have been proposed to enhance the productivity of animal rearing procedures. However, behaviour may change in response to sex ratio manipulations that may counteract potential benefits. We investigated how sex ratios affected mating behaviour of the housefly Musca domestica , a species used in the animal feed industry. We hypothesized a reduced courtship effort and mating latency and increased ejaculate allocation (copulation duration) under male-biased sex ratios, whereas female-biased sex ratios would lead to the opposite effects. However, courtship effort was reduced in female-biased groups, implying reduced male harassment. Mating latency was lower and copulation lasted longer in female-biased groups, which may reduce reproduction time and increase female fecundity and lifespan. Our results indicate that in houseflies, female-biased sex ratios cause behavioural changes in both sexes that could positively contribute to reproductive output. [ABSTRACT FROM AUTHOR]

Summary: Social cues contribute to the circadian entrainment of physiological and behavioral rhythms. These cues supplement the influence of daily and seasonal cycles in light and temperature. In Drosophila, the social environment modulates circadian mechanisms that regulate sex pheromone production and mating behavior. Here we demonstrate that a neuroendocrine pathway, defined by the neuropeptide Pigment-Dispersing Factor (PDF), couples the CNS to the physiological output of peripheral clock cells that produce pheromones, the oenocytes. PDF signaling from the CNS modulates the phase of the oenocyte clock. Despite its requirement for sustaining free-running locomoter activity rhythms, PDF is not necessary to sustain molecular rhythms in the oenocytes. Interestingly, disruption of the PDF signaling pathway reduces male sex pheromones and results in sex-specific differences in mating behavior. Our findings highlight the role of neuropeptide signaling and the circadian system in synchronizing the physiological and behavioral processes that govern social interactions. [ABSTRACT FROM AUTHOR]

Abstract: It is common for females from many species to mate with multiple males within one reproductive cycle. As a result, sperm from different males come into contact in the female reproductive organs, where they compete for ova fertilization. This sperm competition appears to drive the ejaculation of a greater number of sperm than required to fertilize the ova. Both models and experimental observations indicate that males adjust the number of sperm in their ejaculate to the presence of rival males. Here, we show that Drosophila melanogaster males increase sperm allocation immediately after perceiving the presence of other males, but not females. Consistent with previous reports, we show that males use both auditory and olfactory modalities to determine the identity of potential rivals in their environment and we further show that these modalities are required for males to modulate sperm allocation. Our results support the sperm competition risk assessment theory, which predicts that males increase sperm allocation while perceiving the immediate risk of sperm competition, and reconcile previous observations in D. melanogaster that were at odds with this model. [Copyright &y& Elsevier]

The inability to discriminate friend from foe or the ‘one’ among many potential mates can have immediate life-threatening consequences or a long-term evolutionary impact. Successful social interactions depend on the ability to recognize and identify individuals within a social context. Once recognition occurs, a repertoire of behavioral responses becomes available and choices are made as interactions between individuals unfold. The vinegar fly, Drosophila melanogaster, displays a wide range of social activities and patterns of social interaction. If a male fly is unable to recognize other males or distinguish them from females, he may attempt to court both males and females alike, wasting energy and reducing his fitness. We review recent studies on the mechanisms of social recognition in this organism that pertain to both sides of an interaction: the generation of signals by one individual and the receiving and processing of these signals by others. [ABSTRACT FROM AUTHOR]

Social interactions depend on individuals recognizing each other, and in this context many organisms use chemical signals to indicate species and sex. Cuticular hydrocarbon signals are used by insects, including Drosophila melanogaster, to distinguish conspecific individuals from others. These chemicals also contribute to intraspecific courtship and mating interactions. However, the possibility that sex and species identification are linked by common chemical signalling mechanisms has not been formally tested. Here we provide direct evidence that a single compound is used to communicate female identity among D. melanogaster, and to define a reproductive isolation barrier between D. melanogaster and sibling species. A transgenic manipulation eliminated cuticular hydrocarbons by ablating the oenocytes, specialized cells required for the expression of these chemical signals. The resulting oenocyte-less (oe-) females elicited the normal repertoire of courtship behaviours from males, but were actually preferred over wild-type females by courting males. In addition, wild-type males attempted to copulate with oe- males. Thus, flies lacking hydrocarbons are a sexual hyperstimulus. Treatment of virgin females with the aversive male pheromone cis-vaccenyl acetate (cVA) significantly delayed mating of oe- females compared to wild-type females. This difference was eliminated when oe- females were treated with a blend of cVA and the female aphrodisiac (7Z,11Z)-heptacosadiene (7,11-HD), showing that female aphrodisiac compounds can attenuate the effects of male aversive pheromones. 7,11-HD also was shown to have a crucial role in heterospecific encounters. Specifically, the species barrier was lost because males of other Drosophila species courted oe- D. melanogaster females, and D. simulans males consistently mated with them. Treatment of oe- females with 7,11-HD restored the species barrier, showing that a single compound can confer species identity. These results identify a common mechanism for sexual and species recognition regulated by cuticular hydrocarbons. [ABSTRACT FROM AUTHOR]

Summary: Courtship song is a critical component of male courtship behavior in Drosophila, making the female more receptive to copulation and communicating species-specific information . Sex mosaic studies have shown that the sex of certain regions of the central nervous system (CNS) is critical to song production . 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 and are necessary for song production , 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 (FruM), we show a critical requirement for the male isoform of dsx (DsxM), alongside FruM, in the specification of courtship song. Therefore, although FruM expression is sufficient for the performance of many male-specific behaviors , we have shown that without DsxM, the determination of a male-specific CNS and thus a full complement of male behaviors are not realized. [Copyright &y& Elsevier]

Summary: 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. [Copyright &y& Elsevier]

Females control the paternity of their offspring by selectively mating with males they perceive to be of high quality. In species where females mate with multiple males in succession, females may bias offspring paternity by favoring the sperm of one male over another, a process known as cryptic female choice. 1 While evidence of cryptic female choice exists in multiple taxa, the mechanisms underlying this process have remained difficult to unravel. 2 Understanding cryptic female choice requires demonstration of a female-driven post-mating bias in sperm use and paternity and a causal link between this bias and male cues. 3 In this study, we present evidence of cryptic female choice in female Drosophila melanogaster. Through experiments utilizing transgenic males expressing fluorescent sperm, we observed that exposure to attractive males between matings prompts females to expel the ejaculate of their initial mate more rapidly than in the presence of less attractive males. While doing so, females exhibit a bias in sperm storage against their first mate, thereby favoring the paternity of their subsequent mate. Our findings reveal that females adjust the timing of ejaculate expulsion in response to male pheromones in their environment, specifically heptanal and 11- cis -vaccenyl acetate, which are sensed by females through specific odorant receptors. We provide a cryptic female choice mechanism allowing a female to modulate the share of paternity of her first mate depending on the sensing of the quality of potential mates in her environment. These findings showcase that paternity can be influenced by events beyond copulation. [Display omitted] • D. melanogaster females exert cryptic mate choice • Females eject their first mate's ejaculate faster in the presence of attractive males • Females exposed to attractive males bias sperm storage against their first mate • Male pheromones heptanal and cVA are sufficient to shorten ejection latency Doubovetzky et al. present evidence of cryptic female choice in Drosophila melanogaster. Females exposed to pheromones of attractive males expedite ejaculate ejection from their first mate, favoring subsequent mates' sperm. This work highlights that female fruit flies can influence paternity of their offspring through post-copulatory mechanisms. [ABSTRACT FROM AUTHOR]

Summary Male flies put on a multimedia show during courtship involving dance, song, perfume and even vibrations; if a female likes it, she pauses to let him know. Recent studies shed new light on how development and experience contribute to neural mechanisms of female sexual receptivity. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

The fruitless (fru) gene in Drosophila melanogaster is a multifunctional gene that has sex-specific functions in the regulation of male sexual behavior and sex-nonspecific functions affecting adult viability and external morphology. The authors examined fru's sex-nonspecific role in embryonic neural development. fru transcripts from sex-nonspecific promoters are expressed beginning at the earliest stages of neurogenesis, and Fru proteins are present in both neurons and glia. In embryos that lack most or all fru function, FasII- and BP102-positive axons have defasciculation defects and grow along abnormal pathways in the CNS. These defects in axonal projections in fru mutants were rescued by the expression of specific UAS-fru transgenes under the control of a pan-neuronal scabrous-GAL4 driver. Results suggest that one of fru's sex-nonspecific roles is to regulate the pathfinding ability of axons in the embryonic CNS.

Summary: Background: The social life of animals depends on communication between individuals. Recent studies in Drosophila melanogaster demonstrate that various behaviors are influenced by social interactions. For example, courtship is a social interaction mediated by pheromonal signaling that occurs more frequently during certain times of the day than others. In adult flies, sex pheromones are synthesized in cells called oenocytes and displayed on the surface of the cuticle. Although the role of Drosophila pheromones in sexual behavior is well established, little is known about the timing of these signals or how their regulation is influenced by the presence of other flies. Results: We report that oenocytes contain functional circadian clocks that appear to regulate the synthesis of pheromones by controlling the transcription of desaturase1 (desat1), a gene required for production of male cuticular sex pheromones. Moreover, levels of these pheromones vary throughout the day in a pattern that depends on the clock genes and most likely also depends on the circadian control of desat1 in the oenocytes. To assess group dynamics, we manipulated the genotypic composition of social groups (single versus mixed genotypes). This manipulation significantly affects clock gene transcription both in the head and oenocytes, and it also affects the pattern of pheromonal accumulation on the cuticle. Remarkably, we found that flies in mixed social groups mate more frequently than do their counterparts in uniform groups. Conclusions: These results demonstrate that social context exerts a regulatory influence on the expression of chemical signals, while modulating sexual behavior in the fruit fly. [Copyright &y& Elsevier]

Even in well-characterized genomes, many transcripts are considered noncoding RNAs (ncRNAs) simply due to the absence of large open reading frames (ORFs). However, it is now becoming clear that many small ORFs (smORFs) produce peptides with important biological functions. In the process of characterizing the ribosomebound transcriptome of an important cell type of the seminal fluid-producing accessory gland of Drosophila melanogaster, we detected an RNA, previously thought to be noncoding, called male-specific abdominal (msa). Notably, msa is nested in the HOX gene cluster of the Bithorax complex and is known to contain a micro-RNA within one of its introns. We find that this RNA encodes a "micropeptide" (9 or 20 amino acids, MSAmiP) that is expressed exclusively in the secondary cells of the male accessory gland, where it seems to accumulate in nuclei. Importantly, loss of function of this micropeptide causes defects in sperm competition. In addition to bringing insights into the biology of a rare cell type, this work underlines the importance of small peptides, a class of molecules that is now emerging as important actors in complex biological processes. [ABSTRACT FROM AUTHOR]

Sexual conflict occurs because males and females are exposed to different selection pressures. This can affect many aspects of female and male biology, such as physiology, behavior, genetics, and even population ecology. Its broad impact has caused widespread interest in sexual conflict. However, a key aspect of sexual conflict is often confused; it comprises two distinct forms: intralocus and interlocus sexual conflict (IASC and IRSC). Although both are caused by sex differences in selection, they operate via different proximate and ultimate mechanisms. Intralocus sexual conflict and IRSC are often not clearly defined as separate processes in the scientific literature, which impedes a proper understanding of each form as well as of their relative impact on sexual conflict. Furthermore, our current knowledge of the genetics of these phenomena is severely limited. This prevents us from empirically testing numerous theories regarding the role of these two forms of sexual conflict in evolution. Here, we clarify the distinction between IASC and IRSC, by discussing how male and female interests differ, how and when sex‐specific adaptation occurs, and how this may lead to evolutionary change. We then describe a framework for their study, focusing on how future experiments may help identify the genetics underlying these phenomena. Through this, we hope to promote a more critical reflection on IASC and IRSC as well as underline the necessity of genetic and mechanistic studies of these two phenomena. Sexual conflict occurs in two forms: intralocus and interlocus conflict. Each has its own distinct ecological and evolutionary dynamics, but they are sometimes poorly distinguished from one another, which can be confusing to those who are not familiar with sexual conflict. Here, we describe differences between these two types of sexual conflict and discuss how future research may shed further light on them and their genetic bases. [ABSTRACT FROM AUTHOR]

Being part of a group facilitates cooperation between group members but also creates competition for resources. This is a conundrum for gravid females, whose future offspring benefit from being in a group only if there are enough resources relative to group size. Females may therefore be expected to modulate reproductive output depending on social context. In the fruit fly Drosophila melanogaster , females actively attract conspecifics to lay eggs on the same resources, generating groups in which individuals may cooperate or compete. The genetic tractability of this species allows dissecting the mechanisms underlying physiological adaptation to social context. Here, we show that females produce eggs increasingly faster as group size increases. By laying eggs faster when grouped than when isolated, females reduce competition between offspring and increase offspring survival. In addition, grouped females lay eggs during the day, while isolated females lay them at night. We show that responses to the presence of others requires visual input and that flies from any sex, mating status, or species can trigger these responses. The mechanisms of this modulation of egg laying by group is connected to a lifting of the inhibition of light on oogenesis and egg laying, possibly mediated in part by an increase in juvenile hormone activity. Because modulation of reproduction by social context is a hallmark of animals with higher levels of sociality, our findings in a species considered solitary question the validity of this nomenclature and suggest a widespread and profound influence of social context on reproduction. [Display omitted] • Light inhibits egg laying in isolated, but not in grouped, females • Presence of other flies lifts light's inhibition of oogenesis and ovulation • Females speed egg-laying onset in a group to ensure offspring survival • D. melanogaster 's reproduction is fundamentally influenced by social context Bailly et al. show that being grouped induces fruit fly females to lay eggs through the day and night, while isolated females lay eggs at night. Females detect others via vision, which results in faster oogenesis during the day, likely mediated by juvenile hormone. Reproduction in fruit flies is thus fundamentally modulated by the presence of others. [ABSTRACT FROM AUTHOR]

Individuals can make choices based on information learned from others, a phenomenon called social learning. How observers differentiate between which individual they should or should not learn from is, however, poorly understood. Here, we showed that Drosophila melanogaster females can influence the choice of egg-laying site of other females through pheromonal marking. Mated females mark territories of high quality food by ejecting surplus male sperm containing the aggregation pheromone cis-11-vaccenyl acetate (cVA) and, in addition, deposit several sex- and species-specific cuticular hydrocarbon (CHC) pheromones. These pheromonal cues affect the choices of other females, which respond by preferentially laying eggs on the marked food. This system benefits both senders and responders, as communal egg laying increases offspring survival. Virgin females, however, do not elicit a change in the egg-laying decision of mated females, even when food has been supplemented with ejected sperm from mated females, thus indicating the necessity for additional cues. Genetic ablation of either a female's CHC pheromones or those of their mate results in loss of ability of mated females to attract other females. We conclude that mated females use a pheromonal marking system, comprising cVA acquired from male ejaculate with sex- and species-specific CHCs produced by both mates, to indicate egg-laying sites. This system ensures information reliability because mated, but not virgin, females have both the ability to generate the pheromone blend that attracts other flies to those sites and a direct interest in egg-laying site quality. [ABSTRACT FROM AUTHOR]

In Drosophila melanogaster, genes of the sex-determination hierarchy orchestrate the development and differentiation of sex-specific tissues, establishing sex-specific physiology and neural circuitry. One of these sex-determination genes, fruitless (fru), plays a key role in the formation of neural circuits underlying Drosophila male courtship behavior. Conservation of fru gene structure and sex-specific expression has been found in several insect orders, though it is still to be determined whether a male courtship role for the gene is employed in these species due to the lack of mutants and homologous experimental evidence. We have isolated the fru ortholog (Md-fru) from the common housefly, Musca domestica, and show the gene’s conserved genomic structure. We demonstrate that male-specific Md-fru transcripts arise by conserved mechanisms of sex-specific splicing. Here we show that Md-fru, is similarly involved in controlling male courtship behavior. A male courtship behavioral function for Md-fru was revealed by the behavioral and neuroanatomical analyses of a hypomorphic allele, Md-traman, which specifically disrupted the expression of Md-fru in males, leading to severely impaired male courtship behavior. In line with a role in nervous system development, we found that expression of Md-fru was confined to neural tissues in the brain, most prominently in optic neuropil and in peripheral sensory organs. We propose that, like in Drosophila, overt sexual differentiation of the housefly depends on a sex-determining pathway that bifurcates downstream of the Md-tra gene to coordinate dimorphic development of non-neuronal tissues mediated by Md-dsx with that of neuronal tissues largely mediated by Md-fru. [ABSTRACT FROM AUTHOR]

Pheromones regulate male social behaviors in Drosophila, but the identities and behavioral role(s) of these chemosensory signals, and how they interact, are incompletely understood. We found that (z)-7-tricosene, a male-enriched cuticular hydrocarbon that was previously shown to inhibit male-male courtship, was essential for normal levels of aggression. The mechanisms by which (z)-7-tricosene induced aggression and suppressed courtship were independent, but both required the gustatory receptor Gr32a. Sensitivity to (z)-7-tricosene was required for the aggression-promoting effect of 11-cis-vaccenyl acetate (cVA), an olfactory pheromone, but (z)-7-tricosene sensitivity was independent of cVA. (z)-7-tricosene and cVA therefore regulate aggression in a hierarchical manner. Furthermore, the increased courtship caused by depletion of male cuticular hydrocarbons was suppressed by a mutation in the olfactory receptor Or47b. Thus, male social behaviors are controlled by gustatory pheromones that promote aggression and suppress courtship, and whose influences are dominant to olfactory pheromones that enhance these behaviors. [ABSTRACT FROM AUTHOR]

Appropriate displays of aggression rely on the ability to recognize potential competitors. As in most species, Drosophila males fight with other males and do not attack females. In insects, sex recognition is strongly dependent on chemosensory communication, mediated by cuticular hydrocarbons acting as pheromones. While the roles of chemical and other sensory cues in stimulating male to female courtship have been well characterized in Drosophila, the signals that elicit aggression remain unclear. Here we show that when female pheromones or behavior are masculinized, males recognize females as competitors and switch from courtship to aggression. To masculinize female pheromones, a transgene carrying dsRNA for the sex determination factor transformer (traIR) was targeted to the pheromone producing cells, the oenocytes. Shortly after copulation males attacked these females, indicating that pheromonal cues can override other sensory cues. Surprisingly, masculinization of female behavior by targeting traIR to the nervous system in an otherwise normal female also was sufficient to trigger male aggression. Simultaneous masculinization of both pheromones and behavior induced a complete switch in the normal male response to a female. Control males now fought rather than copulated with these females. In a reciprocal experiment, feminization of the oenocytes and nervous system in males by expression of transformer (traF) elicited high levels of courtship and little or no aggression from control males. Finally, when confronted with flies devoid of pheromones, control males attacked male but not female opponents, suggesting that aggression is not a default behavior in the absence of pheromonal cues. Thus, our results show that masculinization of either pheromones or behavior in females is sufficient to trigger male-to-female aggression. Moreover, by manipulating both the pheromonal profile and the fighting patterns displayed by the opponent, male behavioral responses towards males and females can be completely reversed. Therefore, both pheromonal and behavioral cues are used by Drosophila males in recognizing a conspecific as a competitor. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • doublesex -expressing aDN neurons have sexually dimorphic inputs • Male aDNs receive visual inputs via LC10 visual projection neurons • Female aDNs receive olfactory inputs via the lateral horn • aDNs control visual courtship pursuit in males and communal egg laying in females How does the nervous system produce sex differences in behavior? Nojima et al. report a sexually dimorphic neuronal cluster in the Drosophila brain that receives sex-specific sensory information: visual signals required for courtship pursuit in males and olfactory signals required for communal egg laying in females. [ABSTRACT FROM AUTHOR]