Gr33a modulates Drosophila male courtship preference

Molecular Functions and Physiological Roles of Gustatory Receptors of the Silkworm Bombyx mori

Complete elucidation of members of the gustatory receptor (Gr) family in lepidopteran insects began in the silkworm Bombyx mori. Grs of lepidopteran insects were initially classified into four subfamilies based on the results of phylogenetic studies and analyses of a few ligands. However, with further ligand analysis, it has become clear that plant secondary metabolites are important targets not only for Grs in the bitter subfamily but also for the Drosophila melanogaster Gr43a orthologue subfamily and Grs in the sugar subfamily. Gene knockout experiments showed that B. mori Gr6 (BmGr6) and BmGr9 are involved in the recognition of the feeding-promoting compounds chlorogenic acid and isoquercetin in mulberry leaves by the maxillary palps, suggesting that these Grs are responsible for palpation-dependent host recognition without biting. On the other hand, BmGr expression was also confirmed in nonsensory organs. Midgut enteroendocrine cells that produce specific neuropeptides were shown to express specific BmGrs, suggesting that BmGrs are involved in the induction of endocrine secretion in response to changes in the midgut contents. Furthermore, gene knockout experiments indicated that BmGr6 is indeed involved in the secretion of myosuppressin. On the other hand, BmGr9 was shown to induce signal transduction that is not derived from the intracellular signaling cascade mediated by G proteins but from the fructose-regulated cation channel of BmGr9 itself. Cryogenic electron microscopy revealed the mechanism by which the ion channel of the BmGr9 homotetramer opens upon binding of fructose to the ligand-binding pocket. Research on BmGrs has contributed greatly to our understanding of the functions and roles of Grs in insects.

Evolution of chemosensory tissues and cells across ecologically diverse Drosophilids

Chemosensory tissues exhibit significant between-species variability, yet the evolution of gene expression and cell types underlying this diversity remain poorly understood. To address these questions, we conducted transcriptomic analyses of five chemosensory tissues from six Drosophila species and integrated the findings with single-cell datasets. While stabilizing selection predominantly shapes chemosensory transcriptomes, thousands of genes in each tissue have evolved expression differences. Genes that have changed expression in one tissue have often changed in multiple other tissues but at different past epochs and are more likely to be cell type-specific than unchanged genes. Notably, chemosensory-related genes have undergone widespread expression changes, with numerous species-specific gains/losses including novel chemoreceptors expression patterns. Sex differences are also pervasive, including a D. melanogaster-specific excess of male-biased expression in sensory and muscle cells in its forelegs. Together, our analyses provide new insights for understanding evolutionary changes in chemosensory tissues at both global and individual gene levels.

The Effect of Chromosomes on Courtship Behavior in Sibling Species of the Drosophila virilis Group

Prezygotic isolation mechanisms, particularly courtship behavior, play a significant role in the formation of reproductive barriers. The action of these mechanisms leads to the coexistence of numerous closely related insect species with specific adaptations in a shared or adjacent territory. The genetic basis of these mechanisms has been studied using closely related Drosophila species, such as the D. virilis group. However, the investigation of individual courtship behavior elements has been limited until recently, and the effect of genotype on the species-specific features of courtship as a whole has not been thoroughly examined. It should be noted that courtship behavior is not a typical quantitative trait that can be easily measured or quantified in both females and males, similar to traits like wing length or bristle number. Each courtship element involves the participation of both female and male partners, making the genetic analysis of this behavior complex. As a result, the traditional approach of genetic analysis for quantitative traits, which involves variance decomposition in a set of crosses, including parental species, F1 and F2 hybrids, and backcrosses of F1 to parental species, is not suitable for analyzing courtship behavior. To address this, we employed a modified design by introducing what we refer to as 'reference partners' during the testing of hybrid individuals from F1, F2, and backcrosses. These reference partners represented one of the parental species. This approach allowed us to categorize all possible test combinations into four groups based on the reference partner's sex (female or male) and their constant genotype towards one of the parental species (D. virilis or D. americana). The genotype of the second partner in the within-group test combinations varied from completely conspecific to completely heterospecific, based on the parental chromosomal sets. To assess the contribution of partner genotypes to the variability of courtship-element parameters, we employed structural equation modeling (SEM) instead of the traditional analysis of variance (ANOVA). SEM enabled us to estimate the regression of the proportion of chromosomes of a specific species type on the value of each courtship-element parameter in partners with varying genotypes across different test combinations. The aim of the current study was to analyze the involvement of sex chromosomes and autosomes in the formation of courtship structure in D. virilis and D. americana. The genetic analysis was complemented by video recording and formalization of courtship-ritual elements. D. virilis was found to be more sensitive to mate stimuli compared to D. americana. The majority of species-specific parameters, such as latency and duration of courtship elements (e.g., male and female song, following, licking, and circling), were shown to be influenced by the D. virilis genotype. However, not all of these parameters significantly impact copulation success, with the male song, licking, and following being the most significant. In females, the female song was found to have a significant relationship only with copulation duration. The influence of the female genotype on the species-specific parameters of courtship elements is primarily related to autosomes, while the male genotype is associated with the X chromosomes. The study suggests that sexual selection primarily occurs through acoustic and chemoreceptor channels.

CRISPR/Cas9-mediated genome editing of gustatory receptor NlugGr23a causes male sterility in the brown planthopper Nilaparvata lugens

Gustatory receptors (Grs) have an essential role in chemical recognition so as to evaluate food quality. Insect Grs also participate in non-gustatory functions, such as olfaction, temperature sensing, and mating. In this study, we knocked out NlugGr23a, a putative fecundity-related Gr, using the CRISPR/Cas9 system in the brown planthopper Nilaparvata lugens, a serious insect pest of rice. Surprisingly, homozygous NlugGr23a mutant (NlugGr23a^-/-) males were sterile but their sperm were motile and morphologically normal. DAPI staining of mutant sperm inseminated eggs showed that most of NlugGr23a^-/- sperm failed to fertilize eggs, even if they were capable of entering into the egg as a result of their arrested development prior to male pronucleus formation. Immunohistochemistry demonstrated the expression of NlugGr23a in testis. Moreover, prior mating by NlugGr23a^-/- males suppressed female fertility. To our knowledge, it is the first report that a chemoreceptor is implicated in male sterility and provides a potential molecular target for genetic pest control alternatives.

Social Behavioral Deficits with Loss of Neurofibromin Emerge from Peripheral Chemosensory Neuron Dysfunction

Neurofibromatosis type 1 (NF1) is a neurodevelopmental disorder associated with social and communicative disabilities. The cellular and circuit mechanisms by which loss of neurofibromin 1 (Nf1) results in social deficits are unknown. Here, we identify social behavioral dysregulation with Nf1 loss in Drosophila. These deficits map to primary dysfunction of a group of peripheral sensory neurons. Nf1 regulation of Ras signaling in adult ppk23⁺ chemosensory cells is required for normal social behaviors in flies. Loss of Nf1 attenuates ppk23⁺ neuronal activity in response to pheromones, and circuit-specific manipulation of Nf1 expression or neuronal activity in ppk23⁺ neurons rescues social deficits. This disrupted sensory processing gives rise to persistent changes in behavior beyond the social interaction, indicating a sustained effect of an acute sensory misperception. Together our data identify a specific circuit mechanism through which Nf1 regulates social behaviors and suggest social deficits in NF1 arise from propagation of sensory misinformation.

Identification and preliminary characterization of chemosensory-related proteins in the gall fly, Procecidochares utilis by transcriptomic analysis

Chemoreception is critical for insect behaviors such as foraging, host searching and oviposition. The process of chemoreception is mediated by a series of proteins, including odorant-binding proteins (OBPs), gustatory receptors (GRs), odorant receptors (ORs), ionotropic receptors (IRs), chemosensory proteins (CSPs) and sensory neuron membrane proteins (SNMPs). The tephritid stem gall fly, Procecidochares utilis Stone, is a type of egg parasitic insect, which is an effective biological control agent for the invasive weed Ageratina adenophora in many countries. However, the study of molecular components related to the olfactory system of P. utilis has not been investigated. Here, we conducted the developmental transcriptome (egg, first-third instar larva, pupa, female and male adult) of P. utilis using next-generation sequencing technology and identified a total of 133 chemosensory genes, including 40 OBPs, 29 GRs, 24 ORs, 28 IRs, 6 CSPs, and 6 SNMPs. The sequences of these candidate chemosensory genes were confirmed by BLAST, and phylogenetic analysis was performed. Quantitative real-time PCR (qRT-PCR) confirmed that the expression levels of the candidate OBPs varied at the different developmental stages of P. utilis with most OBPs expressed mainly in the pupae, female and male adults but scarcely in eggs and larvae, which was consistent with the differentially expressed genes (DEGs) analysis using the fragments per kilobase per million fragments (FPKM) value. Our results provide a significant contribution towards the knowledge of the set of chemosensory proteins and help advance the use of P. utilis as biological control agents.

Two alternative splicing variants of a sugar gustatory receptor modulate fecundity through different signalling pathways in the brown planthopper, Nilaparvata lugens

Insect gustatory receptors play crucial roles in multiple physiological behaviours. Although the alternative splicing of some gustatory receptors has been observed in insect species, differences in their ligands and functions have rarely been reported. Here, we cloned NlGr10a and NlGr10b, two alternative splicing variants of a sugar gustatory receptor gene in the brown planthopper (BPH), Nilaparvata lugens (Stål), and found that their ligands were different by calcium imaging assay. The ligands of NlGr10a were fructose and cellobiose, and the ligand of NlGr10b was arabinose. Subsequently, the RNAi results showed that knockdown of both splicing variants decreased the number of eggs laid by BPH females, and the egg hatching rate after knockdown of NlGr10a was significantly lower than that after knockdown of NlGr10b. Furthermore, NlGr10a promoted the fecundity of BPH through the AMPK- and AKT-NlVg/NlVgR signalling pathways, whereas NlGr10b promoted the fecundity only through the AMPK- and AKT-NlVg signalling pathways. These findings broaden our understanding of the difference in the ligands and functions of alternative splicing variants of gustatory receptors in insects.

A determining factor for insect feeding preference in the silkworm, Bombyx mori

Feeding preference is critical for insect adaptation and survival. However, little is known regarding the determination of insect feeding preference, and the genetic basis is poorly understood. As a model lepidopteran insect with economic importance, the domesticated silkworm, Bombyx mori, is a well-known monophagous insect that predominantly feeds on fresh mulberry leaves. This species-specific feeding preference provides an excellent model for investigation of host-plant selection of insects, although the molecular mechanism underlying this phenomenon remains unknown. Here, we describe the gene GR66, which encodes a putative bitter gustatory receptor (GR) that is responsible for the mulberry-specific feeding preference of B. mori. With the aid of a transposon-based, clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein-9 nuclease (Cas9) system, the GR66 locus was genetically mutated, and homozygous mutant silkworm strains with truncated gustatory receptor 66 (GR66) proteins were established. GR66 mutant larvae acquired new feeding activity, exhibiting the ability to feed on a number of plant species in addition to mulberry leaves, including fresh fruits and grain seeds that are not normally consumed by wild-type (WT) silkworms. Furthermore, a feeding choice assay revealed that the mutant larvae lost their specificity for mulberry. Overall, our findings provide the first genetic and phenotypic evidences that a single bitter GR is a major factor affecting the insect feeding preference.

LPS perception through taste-induced reflex in Drosophila melanogaster

In flies, grooming serves several purposes, including protection against pathogens and parasites. Previously, we found Escherichia coli or lipopolysaccharides (LPS) can induce grooming behavior via activation of contact chemoreceptors on Drosophila wing. This suggested that specific taste receptors may contribute to this detection. In this study, we examined the perception of commercially available LPS on Drosophila wing chemoreceptors in grooming reflex. Behavioral tests conducted with bitter, sweet and salty gustation such as caffeine, sucrose and salt, using flies carrying a defect in one of their taste receptors related to the detection of bitter molecules (Gr66a, Gr33a), sugars (Gr5a, Gr64f), or salt (IR76b). LPS and tastants of each category were applied to wing sensilla of these taste defectflies and to wild-type Canton Special (CS) flies. Our results indicate that the grooming reflex induced by LPS requires a wide range of gustatory genes, and the inactivation of any of tested genes expressing cells causes a significant reduction of the behavior. This suggests that, while the grooming reflex is strongly regulated by cues perceived as aversive, other sapid cues traditionally related to sweet and salty tastes are also contributing to this behavior.

Myc inhibits JNK-mediated cell death in vivo

The proto-oncogene Myc is well known for its roles in promoting cell growth, proliferation and apoptosis. However, in this study, we found from a genetic screen that Myc inhibits, rather than promotes, cell death triggered by c-Jun N-terminal kinase (JNK) signaling in Drosophila. Firstly, expression of Drosophila Myc (dMyc) suppresses, whereas loss of dMyc enhances, ectopically activated JNK signaling-induced cell death. Secondly, dMyc impedes physiologically activated JNK pathway-mediated cell death. Thirdly, loss of dMyc triggers JNK pathway activation and JNK-dependent cell death. Finally, the mammalian cMyc gene, when expressed in Drosophila, impedes activated JNK signaling-induced cell death. Thus, besides its well-studied apoptosis promoting function, Myc also antagonizes JNK-mediated cell death in Drosophila, and this function is likely conserved from fly to human.

Insulin signaling in female Drosophila links diet and sexual attractiveness

Appropriate sexual selection or individual sexual attractiveness is closely associated with the reproductive success of a species. Here, we report that young male flies exhibit innate courtship preference for female flies that are raised on higher-yeast diets and that have greater body weight and fecundity, but reduced locomotor activity and shortened lifespan. Male flies discriminate among females that have been fed diets that contain 3 different yeast concentrations-1, 5, and 20% yeast- via gustatory, but not visual or olfactory, perception. Female flies that are raised on higher-yeast diets exhibit elevated expression levels of Drosophila insulin-like peptides (di lps), and we demonstrate that hypomorphic mutations of di lp2, 3, 5 or foxo, as well as oenocyte-specific gene disruption of the insulin receptor, all abolish this male courtship preference for high yeast-fed females. Moreover, our data demonstrate that disrupted di lp signaling can alter the expression profile of some cuticular hydrocarbons (CHCs) in female flies, and that genetic inhibition of an enzyme involved in the biosynthesis of CHCs in oenocytes, elongase F, also eliminates the male courtship preference. Together, our findings provide mechanistic insights that link female reproductive potential to sexual attractiveness, thereby encouraging adaptive mating and optimal reproductive success.-Lin, W.-S., Yeh, S.-R., Fan, S.-Z., Chen, L.-Y., Yen, J.-H., Fu, T.-F., Wu, M.-S., Wang, P.-Y. Insulin signaling in female Drosophila links diet and sexual attractiveness.

Or47b plays a role in Drosophila males' preference for younger mates

Reproductive behaviour is important for animals to keep their species existing on Earth. A key question is how to generate more and healthier progenies by choosing optimal mates. In Drosophila melanogaster, males use multiple sensory cues, including vision, olfaction and gustation, to achieve reproductive success. These sensory inputs are important, yet not all these different modalities are simultaneously required for courtship behaviour to occur. Moreover, the roles of these sensory inputs for male courtship choice remain largely unknown. Here, we demonstrate that males court younger females with greater preference and that olfactory inputs are indispensable for this male courtship choice. Specifically, the olfactory receptor Or47b is required for males to discriminate younger female mates from older ones. In combination with our previous work indicating that gustatory perception is necessary for this preference behaviour, our current study demonstrates the requirement of both olfaction and gustation in Drosophila males' courtship preference, thus providing new insights into the role of sensory cues in reproductive behaviour and success.

Establishment of a Drosophila AD model

Alzheimer's disease (AD) is the most common form of dementia that affects people's health greatly. Though amyloid precursor protein (APP) has been implicated in the pathogenesis of AD, the exact role of APP and its underlying mechanism in AD progression have remained largely elusive. Drosophila melanogaster has been extensively used as a model organism to study a wide range of human diseases including AD. In this protocol, we expressed full length human APP in the Drosophila nervous system and examined its effect on locomotion and choice ability. We found that expression of APP produced locomotion defects in larvae as measured by plate crawling ability assay (PCA), and in adult flies as monitored by plate cycling ability assay (CLA). In addition, expression of APP results in male courtship choice (MCC) defect, since wild-type males court preferentially toward young virgin females over old ones, while APP-expressing males failed to show this preference. This protocol enables us to screen for novel AD candidate genes as well as therapeutic compounds to ameliorate the disease.

Sexual circuitry in Drosophila

The sexual behavior of Drosophila melanogaster is an outstanding paradigm to understand the molecular and neuronal basis of sophisticated animal actions. We discuss recent advances in our knowledge of the genetic hardwiring of the underlying neuronal circuitry, and how pertinent sensory cues are differentially detected and integrated in the male and female brain. We also consider how experience influences these circuits over short timescales, and the evolution of these pathways over longer timescales to endow species-specific sexual displays and responses.

The neural basis for insect pheromonal communication

Insects rely on chemosensory signals to drive a multitude of behavioral decisions. From conspecific and mate recognition to aggression, the proper detection and processing of these chemical signals - termed pheromones - is crucial for insects' fitness. While the identities and physiological impacts of diverse insect pheromones have been known for many years, how these important molecules are perceived and processed by the nervous system to produce evolutionarily beneficial behaviors is still mostly unknown. Here we present an overview of the current state of research into the peripheral and central nervous system mechanisms that process and drive behavioral responses to diverse pheromonal cues.