Neural control of sexually dimorphic behaviors

Neurons Underlying Aggression-Like Actions That Are Shared by Both Males and Females in Drosophila

Aggression involves both sexually monomorphic and dimorphic actions. How the brain implements these two types of actions is poorly understood. We found that in Drosophila melanogaster, a set of neurons, which we call CL062, previously shown to mediate male aggression also mediate female aggression. These neurons elicit aggression acutely and without the presence of a target. Although the same set of actions is elicited in males and females, the overall behavior is sexually dimorphic. The CL062 neurons do not express fruitless, a gene required for sexual dimorphism in flies, and expressed by most other neurons important for controlling fly aggression. Connectomic analysis in a female electron microscopy dataset suggests that these neurons have limited connections with fruitless expressing neurons that have been shown to be important for aggression and signal to different descending neurons. Thus, CL062 is part of a monomorphic circuit for aggression that functions parallel to the known dimorphic circuits.

The function of juvenile-adult transition axis in female sexual receptivity of Drosophila melanogaster

Female sexual receptivity is essential for reproduction of a species. Neuropeptides play the main role in regulating female receptivity. However, whether neuropeptides regulate female sexual receptivity during the neurodevelopment is unknown. Here, we found the peptide hormone prothoracicotropic hormone (PTTH), which belongs to the insect PG (prothoracic gland) axis, negatively regulated virgin female receptivity through ecdysone during neurodevelopment in Drosophila melanogaster. We identified PTTH neurons as doublesex-positive neurons, they regulated virgin female receptivity before the metamorphosis during the third-instar larval stage. PTTH deletion resulted in the increased EcR-A expression in the whole newly formed prepupae. Furthermore, the ecdysone receptor EcR-A in pC1 neurons positively regulated virgin female receptivity during metamorphosis. The decreased EcR-A in pC1 neurons induced abnormal morphological development of pC1 neurons without changing neural activity. Among all subtypes of pC1 neurons, the function of EcR-A in pC1b neurons was necessary for virgin female copulation rate. These suggested that the changes of synaptic connections between pC1b and other neurons decreased female copulation rate. Moreover, female receptivity significantly decreased when the expression of PTTH receptor Torso was reduced in pC1 neurons. This suggested that PTTH not only regulates female receptivity through ecdysone but also through affecting female receptivity associated neurons directly. The PG axis has similar functional strategy as the hypothalamic-pituitary-gonadal axis in mammals to trigger the juvenile-adult transition. Our work suggests a general mechanism underlying which the neurodevelopment during maturation regulates female sexual receptivity.

Peripheral hearing sensitivity is similar between the sexes in a benthic turtle species despite the larger body size of males

Sexually dimorphic hearing sensitivity has evolved in many vertebrate species, and the sex with a larger body size typically shows more sensitive hearing. However, generalizing this association is controversial. Research on sexually dimorphic hearing sensitivity contributes to an understanding of auditory sense functions, adaptations, and evolution among species. Therefore, the hypothesized association between body size and hearing needs further validation, especially in specific animal groups. In this study, we assessed hearing sensitivity by measuring auditory brainstem responses (ABRs) in both sexes of 3-year-old Chinese softshell turtles (Pelodiscus sinensis). In this species, male bodies are larger than those of female, and individuals spend most of their lives in the mud at the bottom of freshwater habitats. We found that for both sexes, the hearing sensitivity bandwidth was 0.2-0.9 kHz. Although males were significantly larger than females, no significant differences in ABR thresholds or latencies were found between males and females at the same stimulus frequency. These results indicate that P. sinensis hearing is only sensitive to low-frequency (typically <0.9 kHz) sound signals and that sexually dimorphic hearing sensitivity is not a trait that has evolved in P. sinensis. Physiological and environmental reasons may account for P. sinensis acoustic communication via low-frequency sound signals and the lack of sexually dimorphic hearing sensitivity in these benthic turtles. The results of this study refine our understanding of the adaptation and evolution of the vertebrate auditory system.

Sexually dimorphic control of affective state processing and empathic behaviors

Recognizing the affective states of social counterparts and responding appropriately fosters successful social interactions. However, little is known about how the affective states are expressed and perceived and how they influence social decisions. Here, we show that male and female mice emit distinct olfactory cues after experiencing distress. These cues activate distinct neural circuits in the piriform cortex (PiC) and evoke sexually dimorphic empathic behaviors in observers. Specifically, the PiC → PrL pathway is activated in female observers, inducing a social preference for the distressed counterpart. Conversely, the PiC → MeA pathway is activated in male observers, evoking excessive self-grooming behaviors. These pathways originate from non-overlapping PiC neuron populations with distinct gene expression signatures regulated by transcription factors and sex hormones. Our study unveils how internal states of social counterparts are processed through sexually dimorphic mechanisms at the molecular, cellular, and circuit levels and offers insights into the neural mechanisms underpinning sex differences in higher brain functions.

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

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

Neurons underlying aggressive actions that are shared by both males and females in Drosophila

Aggression involves both sexually monomorphic and dimorphic actions. How the brain implements these two types of actions is poorly understood. We found that a set of neurons, which we call CL062, previously shown to mediate male aggression also mediate female aggression. These neurons elicit aggression acutely and without the presence of a target. Although the same set of actions is elicited in males and females, the overall behavior is sexually dimorphic. The CL062 neurons do not express fruitless , a gene required for sexual dimorphism in flies, and expressed by most other neurons important for controlling fly aggression. Connectomic analysis suggests that these neurons have limited connections with fruitless expressing neurons that have been shown to be important for aggression, and signal to different descending neurons. Thus, CL062 is part of a monomorphic circuit for aggression that functions parallel to the known dimorphic circuits.

Sexually dimorphic morphology, feeding behavior and gene expression profiles in cotton aphid Aphis gossypii

BACKGROUND

Sexual dimorphism exists in most insects; however, less is known about sexual dimorphism in aphids. In this study, we identified sexually dimorphic differences in morphology, feeding behavior and gene expression between sexual females and males of the cotton aphid through electron microscopy, electrical penetration graph techniques and RNA sequencing.

RESULTS

All males were alate with a slender reddish-yellow body and abdominal yellow-black stripes, whereas all sexual females were apterous with a pudgy green body. Sensillum types on the antennae were identical between the two sexes, although males had more sensilla, possibly because the antennae are significantly longer in males compared with sexual females. In terms of feeding behavior, males spent more time probing mesophyll cells and the phloem sieve, and salivating into the phloem sieve. By contrast, sexual females spent more time ingesting xylem sap. In total, 510 and 724 genes were specifically expressed in sexual females and males, respectively, and were significantly enriched in signaling pathways related to reproduction for sexual females (e.g. ovarian steroidogenesis, oxytocin signaling pathway) and energy and flight for males (e.g. thermogenesis, insulin signaling pathway). Moreover, 8551 differentially expressed genes were identified between the two sexes, of which the 3720 upregulated genes in sexual females were mostly enriched in signaling pathways of metabolism and energy, such as thermogenesis and the citrate cycle.

CONCLUSION

This study provides insight into sexual dimorphism in aphids and lays a foundation for revealing the molecular mechanism underlying differences between the two sexes in cotton aphid. © 2023 Society of Chemical Industry.

Neural circuit mechanisms linking courtship and reward in Drosophila males

Courtship has evolved to achieve reproductive success in animal species. However, whether courtship itself has a positive value remains unclear. In the present work, we report that courtship is innately rewarding and can induce the expression of appetitive short-term memory (STM) and long-term memory (LTM) in Drosophila melanogaster males. Activation of male-specific P1 neurons is sufficient to mimic courtship-induced preference and memory performance. Surprisingly, P1 neurons functionally connect to a large proportion of dopaminergic neurons (DANs) in the protocerebral anterior medial (PAM) cluster. The acquisition of STM and LTM depends on two distinct subsets of PAM DANs that convey the courtship-reward signal to the restricted regions of the mushroom body (MB) γ and α/β lobes through two dopamine receptors, D1-like Dop1R1 and D2-like Dop2R. Furthermore, the retrieval of STM stored in the MB α'/β' lobes and LTM stored in the MB α/β lobe relies on two distinct MB output neurons. Finally, LTM consolidation requires two subsets of PAM DANs projecting to the MB α/β lobe and corresponding MB output neurons. Taken together, our findings demonstrate that courtship is a potent rewarding stimulus and reveal the underlying neural circuit mechanisms linking courtship and reward in Drosophila males.

Male and female mice display consistent lifelong ability to address potential life-threatening cues using different post-threat coping strategies

BACKGROUND

Sex differences ranging from physiological functions to pathological disorders are developmentally hard-wired in a broad range of animals, from invertebrates to humans. These differences ensure that animals can display appropriate behaviors under a variety of circumstances, such as aggression, hunting, sleep, mating, and parental care, which are often thought to be important in the acquisition of resources, including territory, food, and mates. Although there are reports of an absence of sexual dimorphism in the context of innate fear, the question of whether there is sexual dimorphism of innate defensive behavior is still an open question. Therefore, an in-depth investigation to determine whether there are sex differences in developmentally hard-wired innate defensive behaviors in life-threatening circumstances is warranted.

RESULTS

We found that innate defensive behavioral responses to potentially life-threatening stimuli between males and females were indistinguishable over their lifespan. However, by using 3 dimensional (3D)-motion learning framework analysis, we found that males and females showed different behavioral patterns after escaping to the refuge. Specifically, the defensive "freezing" occurred primarily in males, whereas females were more likely to return directly to exploration. Moreover, there were also no estrous phase differences in innate defensive behavioral responses after looming stimuli.

CONCLUSIONS

Our results demonstrate that visually-evoked innate fear behavior is highly conserved throughout the lifespan in both males and females, while specific post-threat coping strategies depend on sex. These findings indicate that innate fear behavior is essential to both sexes and as such, there are no evolutionary-driven sex differences in defensive ability.

Neural circuit mechanisms that govern inter-male attack in mice

Individuals of many species fight with conspecifics to gain access to or defend critical resources essential for survival and reproduction. Such intraspecific fighting is evolutionarily selected for in a species-, sex-, and environment-dependent manner when the value of resources secured exceeds the cost of fighting. One such example is males fighting for chances to mate with females. Recent advances in new tools open up ways to dissect the detailed neural circuit mechanisms that govern intraspecific, particularly inter-male, aggression in the model organism Mus musculus (house mouse). By targeting and functional manipulating genetically defined populations of neurons and their projections, these studies reveal a core neural circuit that controls the display of reactive male-male attacks in mice, from sensory detection to decision making and action selection. Here, we summarize these critical results. We then describe various modulatory inputs that route into the core circuit to afford state-dependent and top-down modulation of inter-male attacks. While reviewing these exciting developments, we note that how the inter-male attack circuit converges or diverges with neural circuits that mediate other forms of social interactions remain not fully understood. Finally, we emphasize the importance of combining circuit, pharmacological, and genetic analysis when studying the neural control of aggression in the future.

Molecular and neural mechanisms regulating sexual motivation of virgin female Drosophila

During courtship, multiple information sources are integrated in the brain to reach a final decision, i.e., whether or not to mate. The brain functions for this complex behavior can be investigated by genetically manipulating genes and neurons, and performing anatomical, physiological, and behavioral analyses. Drosophila is a powerful model experimental system for such studies, which need to be integrated from molecular and cellular levels to the behavioral level, and has enabled pioneering research to be conducted. In male flies, which exhibit a variety of characteristic sexual behaviors, we have accumulated knowledge of many genes and neural circuits that control sexual behaviors. On the other hand, despite the importance of the mechanisms of mating decision-making in females from an evolutionary perspective (such as sexual selection), research on the mechanisms that control sexual behavior in females has progressed somewhat slower. In this review, we focus on the pre-mating behavior of female Drosophila melanogaster, and introduce previous key findings on the neuronal and molecular mechanisms that integrate sensory information and selective expression of behaviors toward the courting male.

A circuit logic for sexually shared and dimorphic aggressive behaviors in Drosophila

Aggression involves both sexually monomorphic and dimorphic actions. How the brain implements these two types of actions is poorly understood. We have identified three cell types that regulate aggression in Drosophila: one type is sexually shared, and the other two are sex specific. Shared common aggression-promoting (CAP) neurons mediate aggressive approach in both sexes, whereas functionally downstream dimorphic but homologous cell types, called male-specific aggression-promoting (MAP) neurons in males and fpC1 in females, control dimorphic attack. These symmetric circuits underlie the divergence of male and female aggressive behaviors, from their monomorphic appetitive/motivational to their dimorphic consummatory phases. The strength of the monomorphic → dimorphic functional connection is increased by social isolation in both sexes, suggesting that it may be a locus for isolation-dependent enhancement of aggression. Together, these findings reveal a circuit logic for the neural control of behaviors that include both sexually monomorphic and dimorphic actions, which may generalize to other organisms.

Differences in the contributions of sex linkage and androgen regulation to sex-biased gene expression in juvenile and adult sticklebacks

Different evolutionary interests between males and females can lead to the evolution of sexual dimorphism. However, intersex genetic correlations due to the shared genome can constrain the evolution of sexual dimorphism, resulting in intra-locus sexual conflict. One of the mechanisms resolving this conflict is sex linkage, which allows males and females to carry different alleles on sex chromosomes. Another is a regulatory mutation causing sex-biased gene expression, which is often mediated by gonadal steroids in vertebrates. How do these two mechanisms differ in the contributions to the resolution of intra-locus sexual conflict? The magnitude of sexual conflict often varies between the juvenile and adult stages. Because gonadal steroids change in titre during development, we hypothesized that gonadal steroids play a role in sexual dimorphism expression only at certain developmental stages, whereas sex linkage is more important for sexual dimorphism expressed throughout life. Our brain transcriptome analysis of juvenile and adult threespine sticklebacks showed that the majority of genes that were sex-biased in both stages were sex-linked. The relative contribution of androgen-dependent regulation to the sex-biased transcriptome increased and that of sex linkage declined in adults compared to juveniles. The magnitude of the sex differences was greater in sex-linked genes than androgen-responsive genes, suggesting that sex linkage is more effective than androgen regulation in the production of large sex differences in gene expression. Overall, our data are consistent with the hypothesis that sex linkage is effective in resolving sexual conflict throughout life, whereas androgen-dependent regulation can contribute to temporary resolution of sexual conflict.

Ascl1 is required to specify a subset of ventromedial hypothalamic neurons

Despite clear physiological roles, the ventromedial hypothalamus (VMH) developmental programs are poorly understood. Here, we asked whether the proneural gene achaete-scute homolog 1 (Ascl1) contributes to VMH development. Ascl1 transcripts were detected in embryonic day (E) 10.5 to postnatal day 0 VMH neural progenitors. The elimination of Ascl1 reduced the number of VMH neurons at E12.5 and E15.5, particularly within the VMH-central (VMHC) and -dorsomedial (VMHDM) subdomains, and resulted in a VMH cell fate change from glutamatergic to GABAergic. We observed a loss of Neurog3 expression in Ascl1-/- hypothalamic progenitors and an upregulation of Neurog3 when Ascl1 was overexpressed. We also demonstrated a glutamatergic to GABAergic fate switch in Neurog3-null mutant mice, suggesting that Ascl1 might act via Neurog3 to drive VMH cell fate decisions. We also showed a concomitant increase in expression of the central GABAergic fate determinant Dlx1/2 in the Ascl1-null hypothalamus. However, Ascl1 was not sufficient to induce an ectopic VMH fate when overexpressed outside the normal window of competency. Combined, Ascl1 is required but not sufficient to specify the neurotransmitter identity of VMH neurons, acting in a transcriptional cascade with Neurog3.

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

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

Genetic Perturbation of TIA1 Reveals a Physiological Role in Fear Memory

TIA1 is a prion-related RNA-binding protein whose capacity to form various types of intracellular aggregates has been implicated in neurodegenerative disease. However, its role in normal brain function is poorly understood. Here, we show that TIA1 bidirectionally modulates stress-dependent synaptic plasticity in the hippocampus, a brain region involved in fear memory and olfactory discrimination learning. At the behavioral level, conditioned odor avoidance is potentiated by TIA1 deletion, whereas overexpression of TIA1 in the ventral hippocampus inhibits both contextual fear memory and avoidance. However, the latter genetic manipulations have little impact on other hippocampus-dependent tasks. Transcriptional profiling indicates that TIA1 presides over a large network of immune system genes with modulatory roles in synaptic plasticity and long-term memory. Our results uncover a physiological and partly sex-dependent function for TIA1 in fear memory and may provide molecular insight into stress-related psychiatric conditions, such as post-traumatic stress disorder (PTSD) and anxiety.

G-protein signaling is required for increasing germline stem cell division frequency in response to mating in Drosophila males

Adult stem cells divide to renew the stem cell pool and replenish specialized cells that are lost due to death or usage. However, little is known about the mechanisms regulating how stem cells adjust to a demand for specialized cells. A failure of the stem cells to respond to this demand can have serious consequences, such as tissue loss, or prolonged recovery post injury. Here, we challenge the male germline stem cells (GSCs) of Drosophila melanogaster for the production of specialized cells, sperm cells, using mating experiments. We show that repeated mating reduced the sperm pool and increased the percentage of GSCs in M- and S-phase of the cell cycle. The increase in dividing GSCs depended on the activity of the highly conserved G-proteins. Germline expression of RNA-Interference (RNA-i) constructs against G-proteins, or a dominant negative G-protein eliminated the increase in GSC division frequency in mated males. Consistent with a role for the G-proteins in regulating GSC division frequency, RNA-i against seven out of 35 G-protein coupled receptors (GPCRs) within the germline cells also eliminated the capability of males to increase the numbers of dividing GSCs in response to mating.

Evolution of the neural sex-determination system in insects: does fruitless homologue regulate neural sexual dimorphism in basal insects?

In the brain of holometabolous insects such as the fruit fly Drosophila melanogaster, the fruitless gene produces sex-specific gene products under the control of the sex-specific splicing cascade and contributes to the formation of the sexually dimorphic circuits. Similar sex-specific gene products of fruitless homologues have been identified in other holometabolous insects such as mosquitoes and a parasitic wasp, suggesting the fruitless-dependent neural sex-determination system is widely conserved amongst holometabolous insects. However, it remains obscure whether the fruitless-dependent neural sex-determination system is present in basal hemimetabolous insects. To address this issue, identification, characterization, and expression analyses of the fruitless homologue were conducted in the two-spotted cricket, Gryllus bimaculatus, as a model hemimetabolous insect. The Gryllus fruitless gene encodes multiple isoforms with a unique zinc finger domain, and does not encode a sex-specific gene product. The Gryllus Fruitless protein is broadly expressed in the neurones and glial cells in the brain, and there was no prominent sex-related difference in the expression levels of Gryllus fruitless isoforms. The results suggest that the Gryllus fruitless gene is not involved in the neural sex-determination in the cricket brain.

The fight to understand fighting: neurogenetic approaches to the study of aggression in insects

Aggression is an evolutionarily conserved behavior that evolved in the framework of defending or obtaining resources. When expressed out of context, unchecked aggression can have destructive consequences. Model systems that allow examination of distinct neuronal networks at the molecular, cellular, and circuit levels are adding immensely to our understanding of the biological basis of this behavior and should be relatable to other species up to and including man. Investigators have made particular use of insect models to both describe this quantifiable and stereotyped behavior and to manipulate genes and neuron function via numerous genetic and pharmacological tools. This review discusses recent advances in techniques that improve our ability to identify, manipulate, visualize, and compare the genes, neurons, and circuits that are required for the output of this complex and clinically relevant social behavior.

Large-scale GWAS reveals insights into the genetic architecture of same-sex sexual behavior

Twin and family studies have shown that same-sex sexual behavior is partly genetically influenced, but previous searches for specific genes involved have been underpowered. We performed a genome-wide association study (GWAS) on 477,522 individuals, revealing five loci significantly associated with same-sex sexual behavior. In aggregate, all tested genetic variants accounted for 8 to 25% of variation in same-sex sexual behavior, only partially overlapped between males and females, and do not allow meaningful prediction of an individual's sexual behavior. Comparing these GWAS results with those for the proportion of same-sex to total number of sexual partners among nonheterosexuals suggests that there is no single continuum from opposite-sex to same-sex sexual behavior. Overall, our findings provide insights into the genetics underlying same-sex sexual behavior and underscore the complexity of sexuality.

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

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

Sex differences in aggression: Differential roles of 5-HT2, neuropeptide F and tachykinin

Despite the conserved function of aggression across taxa in obtaining critical resources such as food and mates, serotonin's (5-HT) modulatory role on aggressive behavior appears to be largely inhibitory for vertebrates but stimulatory for invertebrates. However, critical gaps exist in our knowledge of invertebrates that need to be addressed before definitively stating opposing roles for 5-HT and aggression. Specifically, the role of 5-HT receptor subtypes are largely unknown, as is the potential interactive role of 5-HT with other neurochemical systems known to play a critical role in aggression. Similarly, the influence of these systems in driving sex differences in aggressive behavior of invertebrates is not well understood. Here, we investigated these questions by employing complementary approaches in a novel invertebrate model of aggression, the stalk-eyed fly. A combination of altered social conditions, pharmacological manipulation and 5-HT2 receptor knockdown by siRNA revealed an inhibitory role of this receptor subtype on aggression. Additionally, we provide evidence for 5-HT2's involvement in regulating neuropeptide F activity, a suspected inhibitor of aggression. However, this function appears to be stage-specific, altering only the initiation stage of aggressive conflicts. Alternatively, pharmacologically increasing systemic concentrations of 5-HT significantly elevated the expression of the neuropeptide tachykinin, which did not affect contest initiation but instead promoted escalation via production of high intensity aggressive behaviors. Notably, these effects were limited solely to males, with female aggression and neuropeptide expression remaining unaltered by any manipulation that affected 5-HT. Together, these results demonstrate a more nuanced role for 5-HT in modulating aggression in invertebrates, revealing an important interactive role with neuropeptides that is more reminiscent of vertebrates. The sex-differences described here also provide valuable insight into the evolutionary contexts of this complex behavior.

Neural control of sexually dimorphic social behaviors

Sexually reproducing animals display sex differences in behavior. Although many of these sex differences in behavior are acquired with experience, sexually dimorphic behaviors such as mating and aggression are innate in the sense that they can be displayed without prior training or experience. In this review, we present recent advances in our understanding of the neural control of such innate sexually dimorphic social behaviors, with a focus on sexual behavior and aggression in flies and mice. We provide a brief overview of fundamental processes that regulate sexual differentiation in these animals to provide a framework within which more recent advances can be understood. We discuss advances in sensory, neuromodulatory, neural circuit, and experiential regulation of sexually dimorphic social behaviors.

Sex differences in Drosophila behavior: Qualitative and Quantitative Dimorphism

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

Molecular mechanisms underlying sexual differentiation of the nervous system

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

Proteomic changes during adult stage in pre-optic, hypothalamus, hippocampus and pituitary regions of female rat brain following neonatal exposure to estradiol-17β

Although neonatal exposure to estrogen or estrogenic compounds results in irreversible changes in the brain function and reproductive abnormalities during adulthood but the underlying mechanisms are still largely unknown. The present study has attempted to compare the protein profiles of sexually dimorphic brain regions of adult female rats which were exposed to estradiol- 17β during neonatal period. The total proteins extracted from pre-optic area (POA), hypothalamus, hippocampus and pituitary of control and neonatally E2 treated female rats was subjected to 2D-SDS-PAGE and differentially expressed proteins were identified by MALDI TOF/TOF-MS. Our results revealed that a total of 21 protein spots which were identified as differentially expressed in all the four regions analyzed; the differential expression was further validated by RT-PCR and western blotting. The differentially expressed proteins such as 14-3-3 zeta/delta (POA), LMNA (hippocampus), Axin2 (hypothalamus), Syntaxin-7 (hippocampus), prolactin and somatotropin (pituitary) which have very important functions in the process of neuronal differentiation, migration, axon outgrowth, formation of dendritic spine density and synaptic plasticity and memory have not been previously reported in association with neonatal estrogen exposure. The affected brain functions are very important for the establishment of sex specific brain morphology and behavior. Our results suggest that the differentially expressed proteins may play an important role in irreversible changes in the brain function as well as reproductive abnormalities observed in the female rats during adulthood.

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

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

Deciphering Drosophila female innate behaviors

Innate responses are often sexually dimorphic. Studies of female specific behaviors have remained niche, but the focus is changing as illustrated by the recent progress in understanding the female courtship responses and egg-laying decisions. In this review, we will cover our current knowledge about female behaviors in these two specific contexts. Recent studies elucidate on how females process the courtship song. They also show that egg-laying decisions are extremely complex, requiring the assessment of food, microbial, predator and social cues. Study of female responses will improve our understanding of how a nervous system processes different challenges.

Odorant binding protein 69a connects social interaction to modulation of social responsiveness in Drosophila

Living in a social environment requires the ability to respond to specific social stimuli and to incorporate information obtained from prior interactions into future ones. One of the mechanisms that facilitates social interaction is pheromone-based communication. In Drosophila melanogaster, the male-specific pheromone cis-vaccenyl acetate (cVA) elicits different responses in male and female flies, and functions to modulate behavior in a context and experience-dependent manner. Although it is the most studied pheromone in flies, the mechanisms that determine the complexity of the response, its intensity and final output with respect to social context, sex and prior interaction, are still not well understood. Here we explored the functional link between social interaction and pheromone-based communication and discovered an odorant binding protein that links social interaction to sex specific changes in cVA related responses. Odorant binding protein 69a (Obp69a) is expressed in auxiliary cells and secreted into the olfactory sensilla. Its expression is inversely regulated in male and female flies by social interactions: cVA exposure reduces its levels in male flies and increases its levels in female flies. Increasing or decreasing Obp69a levels by genetic means establishes a functional link between Obp69a levels and the extent of male aggression and female receptivity. We show that activation of cVA-sensing neurons is sufficeint to regulate Obp69a levels in the absence of cVA, and requires active neurotransmission between the sensory neuron to the second order olfactory neuron. The cross-talk between sensory neurons and non-neuronal auxiliary cells at the olfactory sensilla, represents an additional component in the machinery that promotes behavioral plasticity to the same sensory stimuli in male and female flies.

In this work, we used Drosophila melanogaster as a model organism to explore a basic question in neuroscience: why do different individuals experience the same sensory stimuli, such as smell differently, and moreover, why does one individual experience identical stimuli differently on different occasions? Focusing on sex specific behaviors in fruit flies, we identified odorant binding protein 69a (Obp69a) as a new player in the machinery that promotes behavioral plasticity to the same sensory stimuli in male and female flies.

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

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

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

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

Analysis of locomotor behavior in the German Mouse Clinic

BACKGROUND

Generation and phenotyping of mutant mouse models continues to increase along with the search for the most efficient phenotyping tests. Here we asked if a combination of different locomotor tests is necessary for comprehensive locomotor phenotyping, or if a large data set from an automated gait analysis with the CatWalk system would suffice.

NEW METHOD

First we endeavored to meaningfully reduce the large CatWalk data set by Principal Component Analysis (PCA) to decide on the most relevant parameters. We analyzed the influence of sex, body weight, genetic background and age. Then a combination of different locomotor tests was analyzed to investigate the possibility of redundancy between tests.

RESULT

The extracted 10 components describe 80% of the total variance in the CatWalk, characterizing different aspects of gait. With these, effects of CatWalk version, sex, body weight, age and genetic background were detected. In addition, the PCA on a combination of locomotor tests suggests that these are independent without significant redundancy in their locomotor measures.

COMPARISON WITH EXISTING METHODS

The PCA has permitted the refinement of the highly dimensional CatWalk (and other tests) data set for the extraction of individual component scores and subsequent analysis.

CONCLUSION

The outcome of the PCA suggests the possibility to focus on measures of the front and hind paws, and one measure of coordination in future experiments to detect phenotypic differences. Furthermore, although the CatWalk is sensitive for detecting locomotor phenotypes pertaining to gait, it is necessary to include other tests for comprehensive locomotor phenotyping.

Neuromodulation and Strategic Action Choice in Drosophila Aggression

In this review, I discuss current knowledge and outstanding questions on the neuromodulators that influence aggressive behavior of the fruit fly Drosophila melanogaster. I first present evidence that Drosophila exchange information during an agonistic interaction and choose appropriate actions based on this information. I then discuss the influence of several biogenic amines and neuropeptides on aggressive behavior. One striking characteristic of neuromodulation is that it can configure a neural circuit dynamically, enabling one circuit to generate multiple outcomes. I suggest a consensus effect of each neuromodulatory molecule on Drosophila aggression, as well as effects of receptor proteins where relevant data are available. Lastly, I consider neuromodulation in the context of strategic action choices during agonistic interactions. Genetic components of neuromodulatory systems are highly conserved across animals, suggesting that molecular and cellular mechanisms controlling Drosophila aggression can shed light on neural principles governing action choice during social interactions.

Proteomic analyses of limbic regions in neonatal male, female and androgen receptor knockout mice

Background

It is well-established that organizational effects of sex steroids during early development are fundamental for sex-typical displays of, for example, mating and aggressive behaviors in rodents and other species. Male and female brains are known to differ with respect to neuronal morphology in particular regions of the brain, including the number and size of neurons, and the density and length of dendrites in nuclei of hypothalamus and amygdala. The aim of the present study was to use global proteomics to identify proteins differentially expressed in hypothalamus/amygdala during early development (postnatal day 8) of male, female and conditional androgen receptor knockout (AR^NesDel) male mice, lacking androgen receptors specifically in the brain. Furthermore, verification of selected sexually dimorphic proteins was performed using targeted proteomics.

Results

Our proteomic approach, iTRAQ, allowed us to investigate expression differences in the 2998 most abundantly expressed proteins in our dissected tissues. Approximately 170 proteins differed between the sexes, and 38 proteins between AR^NesDel and control males (p < 0.05). In line with previous explorative studies of sexually dimorphic gene expression we mainly detected subtle protein expression differences (fold changes <1.3). The protein MARCKS (myristoylated alanine rich C kinase substrate), having the largest fold change of the proteins selected from the iTRAQ analyses and of known importance for synaptic transmission and dendritic branching, was confirmed by targeted proteomics as differentially expressed between the sexes.

Conclusions

Overall, our results provide solid evidence that a large number of proteins show sex differences in their brain expression and could potentially be involved in brain sexual differentiation. Furthermore, our finding of a sexually dimorphic expression of MARCKS in the brain during development warrants further investigation on the involvement in sexual differentiation of this protein.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-016-0332-1) contains supplementary material, which is available to authorized users.

Capturing and Manipulating Activated Neuronal Ensembles with CANE Delineates a Hypothalamic Social-Fear Circuit

We developed a technology (capturing activated neuronal ensembles [CANE]) to label, manipulate, and transsynaptically trace neural circuits that are transiently activated in behavioral contexts with high efficiency and temporal precision. CANE consists of a knockin mouse and engineered viruses designed to specifically infect activated neurons. Using CANE, we selectively labeled neurons that were activated by either fearful or aggressive social encounters in a hypothalamic subnucleus previously known as a locus for aggression, and discovered that social-fear and aggression neurons are intermixed but largely distinct. Optogenetic stimulation of CANE-captured social-fear neurons (SFNs) is sufficient to evoke fear-like behaviors in normal social contexts, whereas silencing SFNs resulted in reduced social avoidance. CANE-based mapping of axonal projections and presynaptic inputs to SFNs further revealed a highly distributed and recurrent neural network. CANE is a broadly applicable technology for dissecting causality and connectivity of spatially intermingled but functionally distinct ensembles.

Soccer players awarded one or more red cards exhibit lower 2D:4D ratios

Anatomical, cognitive and behavioral sex differences are widely recognized in many species. It has been proposed that some of these differences might result from the organizing effects of prenatal sex steroids. In humans, males usually exhibit higher levels of physical aggression and prowess. In this study, we analyze the relationship between second-to-fourth digit (2D:4D) ratios-a proxy for prenatal androgen levels-and foul play and sporting performance in a sample of junior soccer players from a professional Uruguayan soccer club. Our results show that the most aggressive players (i.e., those awarded one or more red cards) have a more masculine finger pattern (lower 2D:4D ratio), while no relationship could be found between sporting performance and 2D:4D ratios. The results are discussed in the context of previous findings. Aggr. Behav. 42:417-426, 2016. © 2015 Wiley Periodicals, Inc.

The role of ghrelin signalling for sexual behaviour in male mice

Ghrelin, a gut-brain signal, is well known to regulate energy homeostasis, food intake and appetite foremost via hypothalamic ghrelin receptors (GHS-R1A). In addition, ghrelin activates the reward systems in the brain, namely the mesolimbic dopamine system, and regulates thereby the rewarding properties of addictive drugs as well as of palatable foods. Given that the mesolimbic dopamine system mandates the reinforcing properties of addictive drugs and natural rewards, such as sexual behaviour, we hypothesize that ghrelin plays an important role for male sexual behaviour, a subject for the present studies. Herein we show that ghrelin treatment increases, whereas pharmacological suppression (using the GHSR-1A antagonist JMV2959) or genetic deletion of the GHS-R1A in male mice decreases the sexual motivation for as well as sexual behaviour with female mice in oestrus. Pre-treatment with L-dopa (a dopamine precursor) prior to treatment with JMV2959 significantly increased the preference for female mouse compared with vehicle treatment. On the contrary, treatment with 5-hydroxythyptohan (a precursor for serotonin) prior to treatment with JMV2959 decreased the sexual motivation compared to vehicle. In separate experiments, we show that ghrelin and GHS-R1A antagonism do not affect the time spent over female bedding as measured in the androgen-dependent bedding test. Collectively, these data show that the hunger hormone ghrelin and its receptor are required for normal sexual behaviour in male mice and that the effects of the ghrelin signalling system on sexual behaviour involve dopamine neurotransmission.

Aversion and attraction through olfaction

Sensory cues that predict reward or punishment are fundamental drivers of animal behavior. For example, attractive odors of palatable food or a potential mate predict reward, while aversive odors of pathogen-laced food or a predator predict punishment. Aversive and attractive odors can be detected by intermingled sensory neurons that express highly related olfactory receptors and display similar central projections. These findings raise basic questions of how innate odor valence is extracted from olfactory circuits, how such circuits are developmentally endowed and modulated by state, and how innate and learned odor responses are related. Here, we review odors, receptors and neural circuits associated with stimulus valence, discussing salient principles derived from studies on nematodes, insects and vertebrates. Understanding the organization of neural circuitry that mediates odor aversion and attraction will provide key insights into how the brain functions.

Central neural circuitry mediating courtship song perception in male Drosophila

Animals use acoustic signals across a variety of social behaviors, particularly courtship. In Drosophila, song is detected by antennal mechanosensory neurons and further processed by second-order aPN1/aLN(al) neurons. However, little is known about the central pathways mediating courtship hearing. In this study, we identified a male-specific pathway for courtship hearing via third-order ventrolateral protocerebrum Projection Neuron 1 (vPN1) neurons and fourth-order pC1 neurons. Genetic inactivation of vPN1 or pC1 disrupts song-induced male-chaining behavior. Calcium imaging reveals that vPN1 responds preferentially to pulse song with long inter-pulse intervals (IPIs), while pC1 responses to pulse song closely match the behavioral chaining responses at different IPIs. Moreover, genetic activation of either vPN1 or pC1 induced courtship chaining, mimicking the behavioral response to song. These results outline the aPN1-vPN1-pC1 pathway as a labeled line for the processing and transformation of courtship song in males.

DOI:http://dx.doi.org/10.7554/eLife.08477.001

The seemingly simple fruit fly engages in an intricate courtship ritual before it mates. Male flies use their wings to ‘sing’ a complex song that makes females more willing to mate. The song also encourages nearby males to start courting, and these males may then intervene to compete for the female. Each species of fruit fly has its own song, and it is important for both males and females to detect the right song.

The sounds of the courtship song are detected by vibration-sensitive neurons on the flies' antennae. These neurons send signals to the fly's brain. But little is known about how this information is then processed, or how information about the song can be integrated with other courtship cues.

Zhou et al. have now identified a pathway of neurons in male flies that is responsible for hearing the courtship song. This pathway stretches from the antennae to neurons deep within the brain. These neural pathways are different in males and females, suggesting that the two sexes use different circuits of neurons for hearing courtship songs. Zhou et al. then used genetic techniques to show that males need every neuron in this pathway to hear courtship songs.

Further experiments revealed that stimulating the ‘deep layer’ neurons caused male flies to respond as if they are hearing the courtship song. These neurons are likely to integrate the song with information from other senses and may encode a general signal for arousal.

These findings now pave the way to deepen our understanding of how information from different senses—for example, courtship songs, visual cues, and pheromones—can be integrated to drive specific behaviors. The next challenge is to explore how species-specific songs are detected and recognized, a goal that has yet to be achieved in any species.

DOI:http://dx.doi.org/10.7554/eLife.08477.002

Locomotion Induced by Spatial Restriction in Adult Drosophila

Drosophila adults display an unwillingness to enter confined spaces but the behaviors induced by spatial restriction in Drosophila are largely unknown. We developed a protocol for high-throughput analysis of locomotion and characterized features of locomotion in a restricted space. We observed intense and persistent locomotion of flies in small circular arenas (diameter 1.27 cm), whereas locomotion was greatly reduced in large circular arenas (diameter 3.81 cm). The increased locomotion induced by spatial restriction was seen in male flies but not female flies, indicating sexual dimorphism of the response to spatial restriction. In large arenas, male flies increased locomotion in arenas previously occupied by male but not female individuals. In small arenas, such pre-conditioning had no effect on male flies, which showed intense and persistent locomotion similar to that seen in fresh arenas. During locomotion with spatial restriction, wildtype Canton-S males traveled slower and with less variation in speed than the mutant w1118 carrying a null allele of white gene. In addition, wildtype flies showed a stronger preference for the boundary than the mutant in small arenas. Genetic analysis with a series of crosses revealed that the white gene was not associated with the phenotype of boundary preference in wildtype flies.

Cyclic Regulation of Sensory Perception by a Female Hormone Alters Behavior

Females may display dramatically different behavior depending on their state of ovulation. This is thought to occur through sex-specific hormones acting on behavioral centers in the brain. Whether incoming sensory activity also differs across the ovulation cycle to alter behavior has not been investigated. Here, we show that female mouse vomeronasal sensory neurons (VSNs) are temporarily and specifically rendered "blind" to a subset of male-emitted pheromone ligands during diestrus yet fully detect and respond to the same ligands during estrus. VSN silencing occurs through the action of the female sex-steroid progesterone. Not all VSNs are targeted for silencing; those detecting cat ligands remain continuously active irrespective of the estrous state. We identify the signaling components that account for the capacity of progesterone to target specific subsets of male-pheromone responsive neurons for inactivation. These findings indicate that internal physiology can selectively and directly modulate sensory input to produce state-specific behavior. PAPERCLIP.

Specification of select hypothalamic circuits and innate behaviors by the embryonic patterning gene dbx1

The hypothalamus integrates information required for the production of a variety of innate behaviors such as feeding, mating, aggression, and predator avoidance. Despite an extensive knowledge of hypothalamic function, how embryonic genetic programs specify circuits that regulate these behaviors remains unknown. Here, we find that in the hypothalamus the developmentally regulated homeodomain-containing transcription factor Dbx1 is required for the generation of specific subclasses of neurons within the lateral hypothalamic area/zona incerta (LH) and the arcuate (Arc) nucleus. Consistent with this specific developmental role, Dbx1 hypothalamic-specific conditional-knockout mice display attenuated responses to predator odor and feeding stressors but do not display deficits in other innate behaviors such as mating or conspecific aggression. Thus, activity of a single developmentally regulated gene, Dbx1, is a shared requirement for the specification of hypothalamic nuclei governing a subset of innate behaviors. VIDEO ABSTRACT.

Constraints on the evolution of a doublesex target gene arising from doublesex's pleiotropic deployment

"Regulatory evolution," that is, changes in a gene's expression pattern through changes at its regulatory sequence, rather than changes at the coding sequence of the gene or changes of the upstream transcription factors, has been increasingly recognized as a pervasive evolution mechanism. Many somatic sexually dimorphic features of Drosophila melanogaster are the results of gene expression regulated by the doublesex (dsx) gene, which encodes sex-specific transcription factors (DSX(F) in females and DSX(M) in males). Rapid changes in such sexually dimorphic features are likely a result of changes at the regulatory sequence of the target genes. We focused on the Flavin-containing monooxygenase-2 (Fmo-2) gene, a likely direct dsx target, to elucidate how sexually dimorphic expression and its evolution are brought about. We found that dsx is deployed to regulate the Fmo-2 transcription both in the midgut and in fat body cells of the spermatheca (a female-specific tissue), through a canonical DSX-binding site in the Fmo-2 regulatory sequence. In the melanogaster group, Fmo-2 transcription in the midgut has evolved rapidly, in contrast to the conserved spermathecal transcription. We identified two cis-regulatory modules (CRM-p and CRM-d) that direct sexually monomorphic or dimorphic Fmo-2 transcription, respectively, in the midguts of these species. Changes of Fmo-2 transcription in the midgut from sexually dimorphic to sexually monomorphic in some species are caused by the loss of CRM-d function, but not the loss of the canonical DSX-binding site. Thus, conferring transcriptional regulation on a CRM level allows the regulation to evolve rapidly in one tissue while evading evolutionary constraints posed by other tissues.

Mechanosensory interactions drive collective behaviour in Drosophila

Collective behaviour enhances environmental sensing and decision-making in groups of animals^1,2. Experimental and theoretical investigations of schooling fish, flocking birds and human crowds have demonstrated that simple interactions between individuals can explain emergent group dynamics^3,4. These findings imply the existence of neural circuits that support distributed behaviours, but the molecular and cellular identities of relevant sensory pathways are unknown. Here we show that Drosophila melanogaster exhibits collective responses to an aversive odour: individual flies weakly avoid the stimulus, but groups show enhanced escape reactions. Using high-resolution behavioural tracking, computational simulations, genetic perturbations, neural silencing and optogenetic activation we demonstrate that this collective odour avoidance arises from cascades of appendage touch interactions between pairs of flies. Inter-fly touch sensing and collective behaviour require the activity of distal leg mechanosensory sensilla neurons and the mechanosensory channel NOMPC^5,6. Remarkably, through these inter-fly encounters, wild-type flies can elicit avoidance behaviour in mutant animals that cannot sense the odour – a basic form of communication. Our data highlight the unexpected importance of social context in the sensory responses of a solitary species and open the door to a neural circuit level understanding of collective behaviour in animal groups.

Tachykinin-expressing neurons control male-specific aggressive arousal in Drosophila

Males of most species are more aggressive than females, but the neural mechanisms underlying this dimorphism are not clear. Here, we identify a neuron and a gene that control the higher level of aggression characteristic of Drosophila melanogaster males. Males, but not females, contain a small cluster of FruM(+) neurons that express the neuropeptide tachykinin (Tk). Activation and silencing of these neurons increased and decreased, respectively, intermale aggression without affecting male-female courtship behavior. Mutations in both Tk and a candidate receptor, Takr86C, suppressed the effect of neuronal activation, whereas overexpression of Tk potentiated it. Tk neuron activation overcame reduced aggressiveness caused by eliminating a variety of sensory or contextual cues, suggesting that it promotes aggressive arousal or motivation. Tachykinin/Substance P has been implicated in aggression in mammals, including humans. Thus, the higher aggressiveness of Drosophila males reflects the sexually dimorphic expression of a neuropeptide that controls agonistic behaviors across phylogeny.

Drosophila pheromone-sensing neurons expressing the ppk25 ion channel subunit stimulate male courtship and female receptivity

As in many species, gustatory pheromones regulate the mating behavior of Drosophila. Recently, several ppk genes, encoding ion channel subunits of the DEG/ENaC family, have been implicated in this process, leading to the identification of gustatory neurons that detect specific pheromones. In a subset of taste hairs on the legs of Drosophila, there are two ppk23-expressing, pheromone-sensing neurons with complementary response profiles; one neuron detects female pheromones that stimulate male courtship, the other detects male pheromones that inhibit male-male courtship. In contrast to ppk23, ppk25, is only expressed in a single gustatory neuron per taste hair, and males with impaired ppk25 function court females at reduced rates but do not display abnormal courtship of other males. These findings raised the possibility that ppk25 expression defines a subset of pheromone-sensing neurons. Here we show that ppk25 is expressed and functions in neurons that detect female-specific pheromones and mediates their stimulatory effect on male courtship. Furthermore, the role of ppk25 and ppk25-expressing neurons is not restricted to responses to female-specific pheromones. ppk25 is also required in the same subset of neurons for stimulation of male courtship by young males, males of the Tai2 strain, and by synthetic 7-pentacosene (7-P), a hydrocarbon normally found at low levels in both males and females. Finally, we unexpectedly find that, in females, ppk25 and ppk25-expressing cells regulate receptivity to mating. In the absence of the third antennal segment, which has both olfactory and auditory functions, mutations in ppk25 or silencing of ppk25-expressing neurons block female receptivity to males. Together these results indicate that ppk25 identifies a functionally specialized subset of pheromone-sensing neurons. While ppk25 neurons are required for the responses to multiple pheromones, in both males and females these neurons are specifically involved in stimulating courtship and mating.

Cellular and behavioral functions of fruitless isoforms in Drosophila courtship

Background

Male-specific products of the fruitless (fru) gene control the development and function of neuronal circuits that underlie male-specific behaviors in Drosophila, including courtship. Alternative splicing generates at least three distinct Fru isoforms, each containing a different zinc-finger domain. Here, we examine the expression and function of each of these isoforms.

Results

We show that most fru⁺ cells express all three isoforms, yet each isoform has a distinct function in the elaboration of sexually dimorphic circuitry and behavior. The strongest impairment in courtship behavior is observed in fru^C mutants, which fail to copulate, lack sine song, and do not generate courtship song in the absence of visual stimuli. Cellular dimorphisms in the fru circuit are dependent on Fru^C rather than other single Fru isoforms. Removal of Fru^C from the neuronal classes vAB3 or aSP4 leads to cell-autonomous feminization of arborizations and loss of courtship in the dark.

Conclusions

These data map specific aspects of courtship behavior to the level of single fru isoforms and fru⁺ cell types—an important step toward elucidating the chain of causality from gene to circuit to behavior.

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fru A, B, and C isoforms have largely overlapping expression in the male fly CNS

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All three fru isoforms contribute to male courtship, with fru^C being the most critical

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Fru^C specifies sexual dimorphisms in neuron number and arborizations

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Fru^C is required in defined neuronal classes for male-specific anatomy and behavior