Evolved differences in larval social behavior mediated by novel pheromones

Shaping the environment - Drosophila suzukii larvae construct their own niche

In holometabolous insects, the choice of oviposition substrate by the adult needs to be coordinated with the developmental needs of the larva. Drosophila suzukii female flies possess an enlarged serrated ovipositor, which has enabled them to conquer the ripening fruit as an oviposition niche. They insert their eggs through the skin of priced small fruits. However, this specialization seems to clash with the nutritional needs for larval development since ripening fruits have a low protein content and are high in sugars. In this work, we studied how D. suzukii larvae develop in and interact with the blueberry. We show that despite its hardness and composition, D. suzukii's first instar larvae are able to use the ripening fruit by engaging in niche construction. They display unique physical and behavioral characteristics that allow them to process the hard-ripening fruit and provoke an improvement in its composition that better suits larval nutritional needs.

Spatial and temporal coordination of Duox/TrpA1/Dh31 and IMD pathways is required for the efficient elimination of pathogenic bacteria in the intestine of Drosophila larvae

Multiple gut antimicrobial mechanisms are coordinated in space and time to efficiently fight foodborne pathogens. In Drosophila melanogaster, production of reactive oxygen species (ROS) and antimicrobial peptides (AMPs) together with intestinal cell renewal play a key role in eliminating gut microbes. A complementary mechanism would be to isolate and treat pathogenic bacteria while allowing colonization by commensals. Using real-time imaging to follow the fate of ingested bacteria, we demonstrate that while commensal Lactiplantibacillus plantarum freely circulate within the intestinal lumen, pathogenic strains such as Erwinia carotovora or Bacillus thuringiensis, are blocked in the anterior midgut where they are rapidly eliminated by antimicrobial peptides. This sequestration of pathogenic bacteria in the anterior midgut requires the Duox enzyme in enterocytes, and both TrpA1 and Dh31 in enteroendocrine cells. Supplementing larval food with hCGRP, the human homolog of Dh31, is sufficient to block the bacteria, suggesting the existence of a conserved mechanism. While the immune deficiency (IMD) pathway is essential for eliminating the trapped bacteria, it is dispensable for the blockage. Genetic manipulations impairing bacterial compartmentalization result in abnormal colonization of posterior midgut regions by pathogenic bacteria. Despite a functional IMD pathway, this ectopic colonization leads to bacterial proliferation and larval death, demonstrating the critical role of bacteria anterior sequestration in larval defense. Our study reveals a temporal orchestration during which pathogenic bacteria, but not innocuous, are confined in the anterior part of the midgut in which they are eliminated in an IMD-pathway-dependent manner.

Population parameters of Drosophila larval cooperative foraging

Cooperative foraging behavior can be advantageous when there is a common exploitable resource. By cooperating, members of the group can take advantage of the potential of increased efficiency of working together as well as equitable distribution of the product. An experimental signature of cooperative foraging is an Allee effect where at a certain number of individuals, there is a peak of fitness. What happens when there are intruders especially ones that do not contribute to any work required for foraging? Drosophila larvae secrete digestive enzymes and exodigest food. Under crowded conditions in liquid food these larvae form synchronized feeding clusters which provides a fitness benefit. A key for this synchronized feeding behavior is the visually guided alignment between adjacent larvae in a feeding cluster. Larvae who do not align their movements are excluded from the groups and subsequently lose the benefit. This may be a way of editing the group to include only known members. To test the model, the fitness benefit from cooperative behavior was further investigated to establish an Allee effect for a number of strains including those who cannot exodigest or cluster. In a standard lab vial, about 40 larvae is the optimal number for fitness. Combinations of these larvae were also examined. The expectation was that larvae who do not contribute to exodigestion are obligate cheaters and would be expelled. Indeed, obligate cheaters gain greatly from the hosts but paradoxically, so do the hosts. Clusters that include cheaters are more stable. Therefore, clustering and the benefits from it are dependent on more than just the contribution to exodigestion. This experimental system should provide a rich future model to understand the metrics of cooperative behavior.

The scent of offspring: chemical profiles of larvae change during development and affect parental behavior in a burying beetle

Chemical cues and signals, especially in insects, play a pivotal role in mediating interactions between individuals. Past studies have largely focused on adult semiochemicals and have neglected those of juvenile stages. Especially in the context of parental care, the larval odor might have a profound impact on parenting behavior, guiding parents in how much resources they should allocate to the different developmental stages. However, whether ontogenetic changes occur in subsocial species and whether larval-emitted scents influence parent-offspring interactions is largely unknown. Using 3 different sampling techniques, we analyzed the cuticular and VOC profile of the 3 larval instars of the burying beetle Nicrophorus vespilloides, which is known for its elaborate parental care. We found distinct differences in the cuticular and VOC profiles across the 3 larval stages. Second-instar larvae, which receive more frequent feedings from parents than the other larval stages, released greater amounts of acetophenone, methyl geranate, and octanoic acid isopropyl ester than the first and third instar. Additionally, using a newly developed bioassay with automated video tracking, we found that adding the odor of second-instar larvae to first-instar larvae increased the number of maternal feeding trips. Our results suggest that the odor produced by larvae plays an important role in mediating parent-offspring interactions. Given these findings, burying beetles might emerge as a promising candidate for identifying a potential begging pheromone.

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.

One genome, multiple phenotypes: decoding the evolution and mechanisms of environmentally induced developmental plasticity in insects

Plasticity in developmental processes gives rise to remarkable environmentally induced phenotypes. Some of the most striking and well-studied examples of developmental plasticity are seen in insects. For example, beetle horn size responds to nutritional state, butterfly eyespots are enlarged in response to temperature and humidity, and environmental cues also give rise to the queen and worker castes of eusocial insects. These phenotypes arise from essentially identical genomes in response to an environmental cue during development. Developmental plasticity is taxonomically widespread, affects individual fitness, and may act as a rapid-response mechanism allowing individuals to adapt to changing environments. Despite the importance and prevalence of developmental plasticity, there remains scant mechanistic understanding of how it works or evolves. In this review, we use key examples to discuss what is known about developmental plasticity in insects and identify fundamental gaps in the current knowledge. We highlight the importance of working towards a fully integrated understanding of developmental plasticity in a diverse range of species. Furthermore, we advocate for the use of comparative studies in an evo-devo framework to address how developmental plasticity works and how it evolves.

Cross-generation pheromonal communication drives Drosophila oviposition site choice

In a heterogeneous and changing environment, oviposition site selection strongly affects the survival and fitness of the offspring.^1,2 Similarly, competition between larvae affects their prospects.³ However, little is known about the involvement of pheromones in regulating these processes.^4,5,6,7,8 Here, we show that mated females of Drosophila melanogaster prefer to lay eggs on substrates containing extracts of conspecific larvae. After analyzing these extracts chemically, we test each compound in an oviposition assay and find that mated females display a dose-dependent preference to lay eggs on substrates spiked with (Z)-9-octadecenoic acid ethyl ester (OE). This egg-laying preference relies on gustatory receptor Gr32a and tarsal sensory neurons expressing this receptor. The concentration of OE also regulates larval place choice in a dose-dependent manner. Physiologically, OE activates female tarsal Gr32a⁺ neurons. In conclusion, our results reveal a cross-generation communication strategy essential for oviposition site selection and regulation of larval density.

Comparative transcriptomic assessment of the chemosensory receptor repertoire of Drosophila suzukii adult and larval olfactory organs

The spotted wing Drosophila, Drosophila suzukii, has emerged within the past decade as an invasive species on a global scale, and is one of the most economically important pests in fruit and berry production in Europe and North America. Insect ecology, to a strong degree, depends on the chemosensory modalities of smell and taste. Extensive research on the sensory receptors of the olfactory and gustatory systems in Drosophila melanogaster provide an excellent frame of reference to better understand the fundamentals of the chemosensory systems of D. suzukii. This knowledge may enhance the development of semiochemicals for sustainable management of D. suzukii, which is urgently needed. Here, using a transcriptomic approach we report the chemosensory receptor expression profiles in D. suzukii female and male antennae, and for the first time, in larval heads including the dorsal organ that houses larval olfactory sensory neurons. In D. suzukii adults, we generally observed a lack of sexually dimorphic expression levels in male and female antennae. While there was generally conservation of antennal expression of odorant and ionotropic receptor orthologues for D. melanogaster and D. suzukii, gustatory receptors showed more distinct species-specific profiles. In larval head tissues, for all three receptor gene families, there was also a greater degree of species-specific gene expression patterns. Analysis of chemosensory receptor repertoires in the pest species, D. suzukii relative to those of the genetic model D. melanogaster enables comparative studies of the chemosensory, physiology, and ecology of D. suzukii.

Effect of Irradiation on Reproduction of Female Spodoptera litura (Fabr.) (Lepidoptera: Noctuidae) in Relation to the Inherited Sterility Technique

Radiobiological investigations on the reproductive behavior of female Spodoptera litura (Fabr.) were conducted with the aim of determining the suitable radio-sterilizing dose for females in order to release them along with sub-sterile males for effective implementation of the Inherited Sterility technique against this pest. Calling and copulation duration significantly increased, while mating success, oviposition, fertility and longevity significantly decreased with increasing radiation dose (100-200 Gy) compared to control. In view of the effect of irradiation on mating behavior and reproductive viability of female S. litura, 130 Gy was identified as a suitable radio-sterilization dose. Further molecular studies were conducted to corroborate this dose for female sterilization, along with a higher dose of 200 Gy in order to validate the gradational response of ionizing radiation. GC-MS analysis indicated decreased sex pheromone titer at 130 Gy, which was more pronounced at 200 Gy. Pheromone-associated genes, PBAN and PBAN-R showed decreased expression at 130 Gy, and were drastically reduced at 200 Gy. The fertility-related Vg gene also showed a negative correlation with radiation exposure. Based on these radiation responses of female S. litura, 130 Gy might be considered a suitable dose for complete female sterility and its inclusion in sterile insect programs against S. litura.

Optogenetic and thermogenetic manipulation of defined neural circuits and behaviors in Drosophila

Neural network dynamics underlying flexible animal behaviors remain elusive. The fruit fly Drosophila melanogaster is considered an excellent model in behavioral neuroscience because of its simple neuroanatomical architecture and the availability of various genetic methods. Moreover, Drosophila larvae's transparent body allows investigators to use optical methods on freely moving animals, broadening research directions. Activating or inhibiting well-defined events in excitable cells with a fine temporal resolution using optogenetics and thermogenetics led to the association of functions of defined neural populations with specific behavioral outputs such as the induction of associative memory. Furthermore, combining optogenetics and thermogenetics with state-of-the-art approaches, including connectome mapping and machine learning-based behavioral quantification, might provide a complete view of the experience- and time-dependent variations of behavioral responses. These methodologies allow further understanding of the functional connections between neural circuits and behaviors such as chemosensory, motivational, courtship, and feeding behaviors and sleep, learning, and memory.

Social environment affects sensory gene expression in ant larvae

Social insects depend on communication to regulate social behaviour. This also applies to their larvae, which are commonly exposed to social interactions and can react to social stimulation. However, how social insect larvae sense their environment is not known. Using RNAseq, we characterized expression of sensory-related genes in larvae of the ant Formica fusca, upon exposure to two social environments: isolation without contact to other individuals, and stimulation via the presence of other developing individuals. Expression of key sensory-related genes was higher following social stimulation, and larvae expressed many of the same sensory-related genes as adult ants and larvae of other insects, including genes belonging to the major insect chemosensory gene families. Our study provides first insights into the molecular changes associated with social information perception in social insect larvae.

Drosophila sechellia: A Genetic Model for Behavioral Evolution and Neuroecology

Defining the mechanisms by which animals adapt to their ecological niche is an important problem bridging evolution, genetics, and neurobiology. We review the establishment of a powerful genetic model for comparative behavioral analysis and neuroecology, Drosophila sechellia. This island-endemic fly species is closely related to several cosmopolitan generalists, including Drosophila melanogaster, but has evolved extreme specialism, feeding and reproducing exclusively on the noni fruit of the tropical shrub Morinda citrifolia. We first describe the development and use of genetic approaches to facilitate genotype/phenotype associations in these drosophilids. Next, we survey the behavioral, physiological, and morphological adaptations of D. sechellia throughout its life cycle and outline our current understanding of the genetic and cellular basis of these traits. Finally, we discuss the principles this knowledge begins to establish in the context of host specialization, speciation, and the neurobiology of behavioral evolution and consider open questions and challenges in the field.

A Tale of Two Transcriptomic Responses in Agricultural Pests via Host Defenses and Viral Replication

Autographa californica Multiple Nucleopolyhedrovirus (AcMNPV) is a baculovirus that causes systemic infections in many arthropod pests. The specific molecular processes underlying the biocidal activity of AcMNPV on its insect hosts are largely unknown. We describe the transcriptional responses in two major pests, Spodoptera frugiperda (fall armyworm) and Trichoplusia ni (cabbage looper), to determine the host-pathogen responses during systemic infection, concurrently with the viral response to the host. We assembled species-specific transcriptomes of the hemolymph to identify host transcriptional responses during systemic infection and assessed the viral transcript abundance in infected hemolymph from both species. We found transcriptional suppression of chitin metabolism and tracheal development in infected hosts. Synergistic transcriptional support was observed to suggest suppression of immune responses and induction of oxidative stress indicating disease progression in the host. The entire AcMNPV core genome was expressed in the infected host hemolymph with a proportional high abundance detected for viral transcripts associated with replication, structure, and movement. Interestingly, several of the host genes that were targeted by AcMNPV as revealed by our study are also targets of chemical insecticides currently used commercially to control arthropod pests. Our results reveal an extensive overlap between biological processes represented by transcriptional responses in both hosts, as well as convergence on highly abundant viral genes expressed in the two hosts, providing an overview of the host-pathogen transcriptomic landscape during systemic infection.

The genetic architecture of larval aggregation behavior in Drosophila

Many insect species exhibit basal social behaviors such as aggregation, which play important roles in their feeding and mating ecologies. However, the evolutionary, genetic, and physiological mechanisms that regulate insect aggregation remain unknown for most species. Here, we used natural populations of Drosophila melanogaster to identify the genetic architecture that drives larval aggregation feeding behavior. By using quantitative and reverse genetic approaches, we have identified a complex neurogenetic network that plays a role in regulating the decision of larvae to feed in either solitude or as a group. Results from single gene, RNAi-knockdown experiments show that several of the identified genes represent key nodes in the genetic network that determines the level of aggregation while feeding. Furthermore, we show that a single non-coding variant in the gene CG14205, a putative acyltransferase, is associated with both decreased mRNA expression and increased aggregate formation, which suggests that it has a specific role in inhibiting aggregation behavior. Our results identify, for the first time, the genetic components which interact to regulate naturally occurring levels of aggregation in D. melanogaster larvae.

Individual, but not population asymmetries, are modulated by social environment and genotype in Drosophila melanogaster

Theory predicts that social interactions can induce an alignment of behavioral asymmetries between individuals (i.e., population-level lateralization), but evidence for this effect is mixed. To understand how interaction with other individuals affects behavioral asymmetries, we systematically manipulated the social environment of Drosophila melanogaster, testing individual flies and dyads (female-male, female-female and male-male pairs). In these social contexts we measured individual and population asymmetries in individual behaviors (circling asymmetry, wing use) and dyadic behaviors (relative position and orientation between two flies) in five different genotypes. We reasoned that if coordination between individuals drives alignment of behavioral asymmetries, greater alignment at the population-level should be observed in social contexts compared to solitary individuals. We observed that the presence of other individuals influenced the behavior and position of flies but had unexpected effects on individual and population asymmetries: individual-level asymmetries were strong and modulated by the social context but population-level asymmetries were mild or absent. Moreover, the strength of individual-level asymmetries differed between strains, but this was not the case for population-level asymmetries. These findings suggest that the degree of social interaction found in Drosophila is insufficient to drive population-level behavioral asymmetries.

Chemosensation and Evolution of Drosophila Host Plant Selection

The ability to respond to chemosensory cues is critical for survival of most organisms. Among insects, Drosophila melanogaster has the best characterized olfactory system, and the availability of genome sequences of 30 Drosophila species provides an ideal scenario for studies on evolution of chemosensation. Gene duplications of chemoreceptor genes allow for functional diversification of the rapidly evolving chemoreceptor repertoire. Although some species of the genus Drosophila are generalists for host plant selection, rapid evolution of olfactory receptors, gustatory receptors, odorant-binding proteins, and cytochrome P450s has enabled diverse host specializations of different members of the genus. Here, I review diversification of the chemoreceptor repertoire among members of the genus Drosophila along with co-evolution of detoxification mechanisms that may have enabled occupation of diverse host plant ecological niches.

Evolution of Reproductive Behavior

Behaviors associated with reproduction are major contributors to the evolutionary success of organisms and are subject to many evolutionary forces, including natural and sexual selection, and sexual conflict. Successful reproduction involves a range of behaviors, from finding an appropriate mate, courting, and copulation, to the successful production and (in oviparous animals) deposition of eggs following mating. As a consequence, behaviors and genes associated with reproduction are often under strong selection and evolve rapidly. Courtship rituals in flies follow a multimodal pattern, mediated through visual, chemical, tactile, and auditory signals. Premating behaviors allow males and females to assess the species identity, reproductive state, and condition of their partners. Conflicts between the "interests" of individual males, and/or between the reproductive strategies of males and females, often drive the evolution of reproductive behaviors. For example, seminal proteins transmitted by males often show evidence of rapid evolution, mediated by positive selection. Postmating behaviors, including the selection of oviposition sites, are highly variable and Drosophila species span the spectrum from generalists to obligate specialists. Chemical recognition features prominently in adaptation to host plants for feeding and oviposition. Selection acting on variation in pre-, peri-, and postmating behaviors can lead to reproductive isolation and incipient speciation. Response to selection at the genetic level can include the expansion of gene families, such as those for detecting pheromonal cues for mating, or changes in the expression of genes leading to visual cues such as wing spots that are assessed during mating. Here, we consider the evolution of reproductive behavior in Drosophila at two distinct, yet complementary, scales. Some studies take a microevolutionary approach, identifying genes and networks involved in reproduction, and then dissecting the genetics underlying complex behaviors in D. melanogaster Other studies take a macroevolutionary approach, comparing reproductive behaviors across the genus Drosophila and how these might correlate with environmental cues. A full synthesis of this field will require unification across these levels.

What can a non-eusocial insect tell us about the neural basis of group behaviour?

Group behaviour has been extensively studied in canonically social swarming, shoaling and flocking vertebrates and invertebrates, providing great insight into the behavioural and ecological aspects of group living. However, the search for its neuronal basis is lagging behind. In the natural environment, Drosophila melanogaster, increasingly used as a model to study neuronal circuits and behaviour, spend their lives surrounded by several conspecifics of different stages, as well as heterospecifics. Despite their dynamic multi-organism natural environment, the neuronal basis of social behaviours has been typically studied in dyadic interactions, such as mating or aggression. This review will focus on recent studies regarding how the behaviour of fruit flies can be shaped by the nature of the surrounding group. We argue that the rich social environment of Drosophila melanogaster, its arsenal of neurogenetic tools and the ability to use large sample sizes for detailed quantitative behavioural analysis makes this species ideal for mechanistic studies of group behaviour.

A Plastic Visual Pathway Regulates Cooperative Behavior in Drosophila Larvae

Cooperative behavior emerges in biological systems through coordinated actions among individuals [1, 2]. Although widely observed across animal species, the cellular and molecular mechanisms underlying the establishment and maintenance of cooperative behaviors remain largely unknown [3]. To characterize the circuit mechanisms serving the needs of independent individuals and social groups, we investigated cooperative digging behavior in Drosophila larvae [4-6]. Although chemical and mechanical sensations are important for larval aggregation at specific sites [7-9], an individual larva's ability to participate in a cooperative burrowing cluster relies on direct visual input as well as visual and social experience during development. In addition, vision modulates cluster dynamics by promoting coordinated movements between pairs of larvae [5]. To determine the specific pathways within the larval visual circuit underlying cooperative social clustering, we examined larval photoreceptors (PRs) and the downstream local interneurons (lOLPs) using anatomical and functional studies [10, 11]. Our results indicate that rhodopsin-6-expressing-PRs (Rh6-PRs) and lOLPs are required for both cooperative clustering and movement detection. Remarkably, visual deprivation and social isolation strongly impact the structural and functional connectivity between Rh6-PRs and lOLPs, while at the same time having no effect on the adjacent rhodopsin-5-expressing PRs (Rh5-PRs). Together, our findings demonstrate that a specific larval visual pathway involved in social interactions undergoes experience-dependent modifications during development, suggesting that plasticity in sensory circuits could act as the cellular substrate for social learning, a possible mechanism allowing an animal to integrate into a malleable social environment and engage in complex social behaviors.

Larval Aggregation of Heortia vitessoides Moore (Lepidoptera: Crambidae) and Evidence of Horizontal Transfer of Avermectin

Aquilaria sinensis (Lour.) Gilg is an economically important tree species that produce the highly prized agarwood. In recent years, agarwood production has been seriously threatened by the outbreak of Heortia vitessoides Moore, a leaf-eating pest that shows gregariousness during the larval stage. However, little attention has been paid to the aggregation behavior of H. vitessoides larvae. In the present study, we collected 102 cohorts of H. vitessoides larvae (13,173 individuals in total) in the wild; 54 cohorts were comprised of the same-instar larvae, and 48 cohorts were comprised of larvae with different developmental stages (instars). In general, young larvae (<third instar) tended to form large aggregations, whereas older-instar larvae were either solitary or formed small aggregations. Laboratory studies showed a strong aggregation tendency in the newly hatched and second-instar larvae of H. vitessoides, whenever the individuals originated from the same or different sibling cohorts. In addition, all newly hatched larvae died within two days after they were isolated. When newly hatched larvae were initially assigned in 10-larvae cohorts (containing sibling individuals) or 20-larvae cohorts (either containing individuals originating from the same or different sibling cohorts), their larval survivorship, duration of larval stage, and adult emergence were not significantly different. Interestingly, combining avermectin-treated larvae (donors) with untreated ones (receptors) significantly decreased larval survivorship and adult emergence of receptors, indicating a horizontal transfer of avermectin among H. vitessoides larvae. This study enhances our understanding of the population ecology of H. vitessoides, and may bring novel insights into the management strategies against this pest.

Drosophila melanogaster cloak their eggs with pheromones, which prevents cannibalism

Oviparous animals across many taxa have evolved diverse strategies that deter egg predation, providing valuable tests of how natural selection mitigates direct fitness loss. Communal egg laying in nonsocial species minimizes egg predation. However, in cannibalistic species, this very behavior facilitates egg predation by conspecifics (cannibalism). Similarly, toxins and aposematic signaling that deter egg predators are often inefficient against resistant conspecifics. Egg cannibalism can be adaptive, wherein cannibals may benefit through reduced competition and added nutrition, but since it reduces Darwinian fitness, the evolution of anticannibalistic strategies is rife. However, such strategies are likely to be nontoxic because deploying toxins against related individuals would reduce inclusive fitness. Here, we report how D. melanogaster use specific hydrocarbons to chemically mask their eggs from cannibal larvae. Using an integrative approach combining behavioral, sensory, and mass spectrometry methods, we demonstrate that maternally provisioned pheromone 7,11-heptacosadiene (7,11-HD) in the eggshell’s wax layer deters egg cannibalism. Furthermore, we show that 7,11-HD is nontoxic, can mask underlying substrates (for example, yeast) when coated upon them, and its detection requires pickpocket 23 (ppk23) gene function. Finally, using light and electron microscopy, we demonstrate how maternal pheromones leak-proof the egg, consequently concealing it from conspecific larvae. Our data suggest that semiochemicals possibly subserve in deceptive functions across taxa, especially when predators rely on chemical cues to forage, and stimulate further research on deceptive strategies mediated through nonvisual sensory modules. This study thus highlights how integrative approaches can illuminate our understanding on the adaptive significance of deceptive defenses and the mechanisms through which they operate.

Egg-laying species that lack parental care often protect their eggs from predators by laying them in communal groups or by fortifying them with toxins. However, these strategies may backfire when the predators are from the same species (cannibals) since a) there are plenty of available eggs in these sites, b) the cannibals may be resistant to the toxins, and c) poisoning cannibals who may be related would reduce inclusive fitness. Under these circumstances, natural selection should favor anticannibalistic strategies that are likely to be nontoxic. Here, we investigate how fruit flies (Drosophila melanogaster), which oviposit communally, protect their eggs from cannibalism by their own larvae. We show that maternal hydrocarbons incorporated into the egg’s wax layer to make them waterproof interestingly also serve as a mask that conceals their identity from cannibal larvae. In particular, we identify one female sex pheromone that deters cannibalism by forming a layer around the egg to conceal it. We further demonstrate that this pheromone is nontoxic and can mask underlying substrates such as yeast when used as a coating. While deceptive strategies (such as camouflage) deployed to avoid predation are extensively studied from a visual perspective, our findings suggest that deceptive strategies operating through other nonvisual sensory systems might be equally common across taxa.

The behavior of adult Drosophila in the wild

Little is known about how Drosophila adults behave in the wild, including mating, allocation of food and space, and escape from predators. This lack of information has negative implications for our ability to understand the capabilities of the nervous system to integrate sensory cues necessary for the adaptation of organisms in natural conditions. We characterized a set of behavioral routines of D. melanogaster and D. simulans adults in three ecologically different orchards: grape, apple and prickly pear. We also investigated how the flies identify conspecifics and aliens in the wild to better understand relationships between group formation and adaptation of Drosophila to breeding sites. We characterized the locations by recording in each orchard humidity, temperature, illumination conditions, pH of fruits, the presence/absence of other Drosophila species and the predator ant Linepithema humile. Our findings suggest that the home range of these species of Drosophila includes decaying fruits and, principally, a variety of microhabitats that surround the fruits. The ecological heterogeneity of the orchards and odors emitted by adult D. melanogaster and D. simulans influence perch preferences, cluster formation, court and mating, egg-laying site selection, and use of space. This is one of the first large examinations of the association between changing, complex environments and a set of adult behaviors of Drosophila. Therefore, our results have implications for understanding the genetic differentiation and evolution of populations of species in the genus Drosophila.

Carabidae Semiochemistry: Current and Future Directions

Ground beetles (Carabidae) are recognized for their diverse, chemically-mediated defensive behaviors. Produced using a pair of pygidial glands, over 250 chemical constituents have been characterized across the family thus far, many of which are considered allomones. Over the past century, our knowledge of Carabidae exocrine chemistry has increased substantially, yet the role of these defensive compounds in mediating behavior other than repelling predators is largely unknown. It is also unclear whether non-defensive compounds produced by ground beetles mediate conspecific and heterospecific interactions, such as sex-aggregation pheromones or kairomones, respectively. Here we review the current state of non-exocrine Carabidae semiochemistry and behavioral research, discuss the importance of semiochemical research including but not limited to allomones, and describe next-generation methods for elucidating the underlying genetics and evolution of chemically-mediated behavior.

Composition and Control of a Deg/ENaC Channel during Presynaptic Homeostatic Plasticity

The homeostatic control of presynaptic neurotransmitter release stabilizes information transfer at synaptic connections in the nervous system of organisms ranging from insect to human. Presynaptic homeostatic signaling centers upon the regulated membrane insertion of an amiloride-sensitive degenerin/epithelial sodium (Deg/ENaC) channel. Elucidating the subunit composition of this channel is an essential step toward defining the underlying mechanisms of presynaptic homeostatic plasticity (PHP). Here, we demonstrate that the ppk1 gene encodes an essential subunit of this Deg/ENaC channel, functioning in motoneurons for the rapid induction and maintenance of PHP. We provide genetic and biochemical evidence that PPK1 functions together with PPK11 and PPK16 as a presynaptic, hetero-trimeric Deg/ENaC channel. Finally, we highlight tight control of Deg/ENaC channel expression and activity, showing increased PPK1 protein expression during PHP and evidence for signaling mechanisms that fine tune the level of Deg/ENaC activity during PHP.

Prothoracicotropic hormone modulates environmental adaptive plasticity through the control of developmental timing

Adult size and fitness are controlled by a combination of genetics and environmental cues. In Drosophila, growth is confined to the larval phase and final body size is impacted by the duration of this phase, which is under neuroendocrine control. The neuropeptide prothoracicotropic hormone (PTTH) has been proposed to play a central role in controlling the length of the larval phase through regulation of ecdysone production, a steroid hormone that initiates larval molting and metamorphosis. Here, we test this by examining the consequences of null mutations in the Ptth gene for Drosophila development. Loss of Ptth causes several developmental defects, including a delay in developmental timing, increase in critical weight, loss of coordination between body and imaginal disc growth, and reduced adult survival in suboptimal environmental conditions such as nutritional deprivation or high population density. These defects are caused by a decrease in ecdysone production associated with altered transcription of ecdysone biosynthetic genes. Therefore, the PTTH signal contributes to coordination between environmental cues and the developmental program to ensure individual fitness and survival.

Drosophila as a holistic model for insect pheromone signaling and processing

In recent years, research into the chemical ecology of the vinegar fly, Drosophila melanogaster, has yielded a wealth of information on the neural substrates that detect and process pheromones and control behavior. The studies reveal at the cellular and molecular level how behavioral responses to pheromones are initiated and modulated by social, environmental, and physiological factors. By taking into account both the complexity of the chemical world and the intricacies of the animal's physiological state, the emerging holistic perspective provides insight not only into chemical communication but more generally, how organisms balance internal and external signals when making behavioral decisions.

Species-specific modulation of food-search behavior by respiration and chemosensation in Drosophila larvae

Animals explore their environment to encounter suitable food resources. Despite its vital importance, this behavior puts individuals at risk by consuming limited internal energy during locomotion. We have developed a novel assay to investigate how food-search behavior is organized in Drosophila melanogaster larvae dwelling in hydrogels mimicking their natural habitat. We define three main behavioral modes: resting at the gel's surface, digging while feeding near the surface, and apneic dives. In unstimulated conditions, larvae spend most of their time digging. By contrast, deep and long exploratory dives are promoted by olfactory stimulations. Hypoxia and chemical repellents impair diving. We report remarkable differences in the dig-and-dive behavior of D. melanogaster and the fruit-pest D. suzukii. The present paradigm offers an opportunity to study how sensory and physiological cues are integrated to balance the limitations of dwelling in imperfect environmental conditions and the risks associated with searching for potentially more favorable conditions.

Fine scale mapping of genomic introgressions within the Drosophila yakuba clade

The process of speciation involves populations diverging over time until they are genetically and reproductively isolated. Hybridization between nascent species was long thought to directly oppose speciation. However, the amount of interspecific genetic exchange (introgression) mediated by hybridization remains largely unknown, although recent progress in genome sequencing has made measuring introgression more tractable. A natural place to look for individuals with admixed ancestry (indicative of introgression) is in regions where species co-occur. In west Africa, D. santomea and D. yakuba hybridize on the island of São Tomé, while D. yakuba and D. teissieri hybridize on the nearby island of Bioko. In this report, we quantify the genomic extent of introgression between the three species of the Drosophila yakuba clade (D. yakuba, D. santomea), D. teissieri). We sequenced the genomes of 86 individuals from all three species. We also developed and applied a new statistical framework, using a hidden Markov approach, to identify introgression. We found that introgression has occurred between both species pairs but most introgressed segments are small (on the order of a few kilobases). After ruling out the retention of ancestral polymorphism as an explanation for these similar regions, we find that the sizes of introgressed haplotypes indicate that genetic exchange is not recent (>1,000 generations ago). We additionally show that in both cases, introgression was rarer on X chromosomes than on autosomes which is consistent with sex chromosomes playing a large role in reproductive isolation. Even though the two species pairs have stable contemporary hybrid zones, providing the opportunity for ongoing gene flow, our results indicate that genetic exchange between these species is currently rare.

The Drosophila Postsynaptic DEG/ENaC Channel ppk29 Contributes to Excitatory Neurotransmission

The protein family of degenerin/epithelial sodium channels (DEG/ENaCs) is composed of diverse animal-specific, non-voltage-gated ion channels that play important roles in regulating cationic gradients across epithelial barriers. Some family members are also enriched in neural tissues in both vertebrates and invertebrates. However, the specific neurophysiological functions of most DEG/ENaC-encoding genes remain poorly understood. The fruit fly Drosophila melanogaster is an excellent model for deciphering the functions of DEG/ENaC genes because its genome encodes an exceptionally large number of DEG/ENaC subunits termed pickpocket (ppk) 1-31 Here we demonstrate that ppk29 contributes specifically to the postsynaptic modulation of excitatory synaptic transmission at the larval neuromuscular junction. Electrophysiological data indicate that the function of ppk29 in muscle is necessary for normal postsynaptic responsivity to neurotransmitter release and for normal coordinated larval movement. The ppk29 mutation does not affect gross synaptic morphology and ultrastructure, which indicates that the observed phenotypes are likely due to defects in glutamate receptor function. Together, our data indicate that DEG/ENaC ion channels play a fundamental role in the postsynaptic regulation of excitatory neurotransmission.SIGNIFICANCE STATEMENT Members of the degenerin/epithelial sodium channel (DEG/ENaC) family are broadly expressed in epithelial and neuronal tissues. To date, the neurophysiological functions of most family members remain unknown. Here, by using the power of Drosophila genetics in combination with electrophysiological and behavioral approaches, we demonstrate that the DEG/ENaC-encoding gene pickpocket 29 contributes to baseline neurotransmission, possibly via the modulation of postsynaptic glutamate receptor functionality.

The ability of Drosophila hybrids to locate food declines with parental divergence

Hybrids are generally less fit than their parental species, and the mechanisms underlying their fitness reductions can manifest through different traits. For example, hybrids can have physiological, behavioral, or ecological defects, and these defects can generate reproductive isolation between their parental species. However, the rate that mechanisms of postzygotic isolation other than hybrid sterility and inviability evolve has remained largely uninvestigated, despite isolated studies showing that behavioral defects in hybrids are not only possible but might be widespread. Here, we study a fundamental animal behavior-the ability of individuals to find food-and test the rate at which it breaks down in hybrids. We measured the ability of hybrids from 94 pairs of Drosophila species to find food and show that this ability decreases with increasing genetic divergence between the parental species and that male hybrids are more strongly (and negatively) affected than females. Our findings quantify the rate that hybrid dysfunction evolves across the diverse radiation of Drosophila and highlights the need for future investigations of the genetic and neurological mechanisms that affect a hybrid's ability to find a suitable substrate on which to feed and breed.

The neurogenetics of group behavior in Drosophila melanogaster

Organisms rarely act in isolation. Their decisions and movements are often heavily influenced by direct and indirect interactions with conspecifics. For example, we each represent a single node within a social network of family and friends, and an even larger network of strangers. This group membership can affect our opinions and actions. Similarly, when in a crowd, we often coordinate our movements with others like fish in a school, or birds in a flock. Contributions of the group to individual behaviors are observed across a wide variety of taxa but their biological mechanisms remain largely unknown. With the advent of powerful computational tools as well as the unparalleled genetic accessibility and surprisingly rich social life of Drosophila melanogaster, researchers now have a unique opportunity to investigate molecular and neuronal determinants of group behavior. Conserved mechanisms and/or selective pressures in D. melanogaster can likely inform a much wider phylogenetic scale. Here, we highlight two examples to illustrate how quantitative and genetic tools can be combined to uncover mechanisms of two group behaviors in D. melanogaster: social network formation and collective behavior. Lastly, we discuss future challenges towards a full understanding how coordinated brain activity across many individuals gives rise to the behavioral patterns of animal societies.

Neural Circuits Underlying Fly Larval Locomotion

Locomotion is a complex motor behavior that may be expressed in different ways using a variety of strategies depending upon species and pathological or environmental conditions. Quadrupedal or bipedal walking, running, swimming, flying and gliding constitute some of the locomotor modes enabling the body, in all cases, to move from one place to another. Despite these apparent differences in modes of locomotion, both vertebrate and invertebrate species share, at least in part, comparable neural control mechanisms for locomotor rhythm and pattern generation and modulation. Significant advances have been made in recent years in studies of the genetic aspects of these control systems. Findings made specifically using Drosophila (fruit fly) models and preparations have contributed to further understanding of the key role of genes in locomotion. This review focuses on some of the main findings made in larval fruit flies while briefly summarizing the basic advantages of using this powerful animal model for studying the neural locomotor system.

Interactions among Drosophila larvae before and during collision

In populations of Drosophila larvae, both, an aggregation and a dispersal behavior can be observed. However, the mechanisms coordinating larval locomotion in respect to other animals, especially in close proximity and during/after physical contacts are currently only little understood. Here we test whether relevant information is perceived before or during larva-larva contacts, analyze its influence on behavior and ask whether larvae avoid or pursue collisions. Employing frustrated total internal reflection-based imaging (FIM) we first found that larvae visually detect other moving larvae in a narrow perceptive field and respond with characteristic escape reactions. To decipher larval locomotion not only before but also during the collision we utilized a two color FIM approach (FIM^2c), which allowed to faithfully extract the posture and motion of colliding animals. We show that during collision, larval locomotion freezes and sensory information is sampled during a KISS phase (german: Kollisions Induziertes Stopp Syndrom or english: collision induced stop syndrome). Interestingly, larvae react differently to living, dead or artificial larvae, discriminate other Drosophila species and have an increased bending probability for a short period after the collision terminates. Thus, Drosophila larvae evolved means to specify behaviors in response to other larvae.

Extensive local adaptation within the chemosensory system following Drosophila melanogaster's global expansion

How organisms adapt to new environments is of fundamental biological interest, but poorly understood at the genetic level. Chemosensory systems provide attractive models to address this problem, because they lie between external environmental signals and internal physiological responses. To investigate how selection has shaped the well-characterized chemosensory system of Drosophila melanogaster, we have analysed genome-wide data from five diverse populations. By couching population genomic analyses of chemosensory protein families within parallel analyses of other large families, we demonstrate that chemosensory proteins are not outliers for adaptive divergence between species. However, chemosensory families often display the strongest genome-wide signals of recent selection within D. melanogaster. We show that recent adaptation has operated almost exclusively on standing variation, and that patterns of adaptive mutations predict diverse effects on protein function. Finally, we provide evidence that chemosensory proteins have experienced relaxed constraint, and argue that this has been important for their rapid adaptation over short timescales.

Fruit flies gain valuable information about their environment by sensing chemicals. Here, Arguello et al. show strong signals of recent selection on the chemosensory system of the fruit fly Drosophila melanogaster, consistent with the adaptation of populations to their local chemical environment.

Drosophila Food-Associated Pheromones: Effect of Experience, Genotype and Antibiotics on Larval Behavior

Animals ubiquitously use chemical signals to communicate many aspects of their social life. These chemical signals often consist of environmental cues mixed with species-specific signals-pheromones-emitted by conspecifics. During their life, insects can use pheromones to aggregate, disperse, choose a mate, or find the most suitable food source on which to lay eggs. Before pupariation, larvae of several Drosophila species migrate to food sources depending on their composition and the presence of pheromones. Some pheromones derive from microbiota gut activity and these food-associated cues can enhance larval attraction or repulsion. To explore the mechanisms underlying the preference (attraction/repulsion) to these cues and clarify their effect, we manipulated factors potentially involved in larval response. In particular, we found that the (i) early exposure to conspecifics, (ii) genotype, and (iii) antibiotic treatment changed D. melanogaster larval behavior. Generally, larvae-tested either individually or in groups-strongly avoided food processed by other larvae. Compared to previous reports on larval attractive pheromones, our data suggest that such attractive effects are largely masked by food-associated compounds eliciting larval aversion. The antagonistic effect of attractive vs. aversive compounds could modulate larval choice of a pupariation site and impact the dispersion of individuals in nature.

Drosophila Chemoreceptors: A Molecular Interface Between the Chemical World and the Brain

Chemoreception is essential for survival. Feeding, mating, and avoidance of predators depend on detection of sensory cues. Drosophila contains diverse families of chemoreceptors that detect odors, tastants, pheromones, and noxious stimuli, including receptors of the odor receptor (Or), gustatory receptor (Gr), ionotropic receptor (IR), Pickpocket (Ppk), and Trp families. We consider recent progress in understanding chemoreception in the fly, including the identification of new receptors, the discovery of novel biological functions for receptors, and the localization of receptors in unexpected places. We discuss major unsolved problems and suggest areas that may be particularly ripe for future discoveries, including the roles of these receptors in driving the circuits and behaviors that are essential to the survival and reproduction of the animal.

Taste processing in Drosophila larvae

The sense of taste allows animals to detect chemical substances in their environment to initiate appropriate behaviors: to find food or a mate, to avoid hostile environments and predators. Drosophila larvae are a promising model organism to study gustation. Their simple nervous system triggers stereotypic behavioral responses, and the coding of taste can be studied by genetic tools at the single cell level. This review briefly summarizes recent progress on how taste information is sensed and processed by larval cephalic and pharyngeal sense organs. The focus lies on several studies, which revealed cellular and molecular mechanisms required to process sugar, salt, and bitter substances.

A Wolbachia-Sensitive Communication between Male and Female Pupae Controls Gamete Compatibility in Drosophila

Gamete compatibility is fundamental to sexual reproduction. Wolbachia are maternally inherited endosymbiotic bacteria that manipulate gamete compatibility in many arthropod species. In Drosophila, the fertilization of uninfected eggs by sperm from Wolbachia-infected males often results in early developmental arrest. This gamete incompatibility is called cytoplasmic incompatibility (CI). CI is highest in young males, suggesting that Wolbachia affect sperm properties during male development. Here, we show that Wolbachia modulate testis development. Unexpectedly, this effect was associated with Wolbachia infection in females, not males. This raised the possibility that females influenced testis development by communicating with males prior to adulthood. Using a combinatorial rearing protocol, we provide evidence for such a female-to-male communication during metamorphosis. This communication involves the perception of female pheromones by male olfactory receptors. We found that this communication determines the compatibility range of sperm. Wolbachia interfere with this female-to-male communication through changes in female pheromone production. Strikingly, restoring this communication partially suppressed CI in Wolbachia-infected males. We further identified a reciprocal male-to-female communication at metamorphosis that restricts the compatibility range of female gametes. Wolbachia also perturb this communication by feminizing male pheromone production. Thus, Wolbachia broaden the compatibility range of eggs, promoting thereby the reproductive success of Wolbachia-infected females. We conclude that pheromone communication between pupae regulates gamete compatibility and is sensitive to Wolbachia in Drosophila.

The Genetics of Resistance to Morinda Fruit Toxin During the Postembryonic Stages in Drosophila sechellia

Although a great deal has been learned regarding the genetic changes that give rise to adaptation in bacteria and yeast, an understanding of how new complex traits arise in multicellular organisms is far less complete. Many phytophagous insect species are ecological specialists that have adapted to utilize a single host plant. Drosophila sechellia is a specialist that utilizes the ripe fruit of Morinda citrifolia, which is toxic to its sibling species, D. simulans. Here we apply multiplexed shotgun genotyping and QTL analysis to examine the genetic basis of resistance to M. citrifolia fruit toxin in interspecific hybrids. We identify a locus of large effect on the third chromosome (QTL-III_sima) in the D. simulans backcross that was not detected in previous analyses. We also identify a highly significant QTL of large effect on the X chromosome, QTL-X_sim. Additional smaller-effect loci were also identified in the D. simulans and D. sechellia backcrosses. We did not detect significant epistasis between loci. Instead, our analysis reveals large and smaller-effect loci that contribute to M. citrifolia resistance additively. The additive effect of each locus suggests that partial resistance to lower levels of M. citrifolia toxin could be passed through introgression from D. sechellia to D. simulans in nature. The identification of the major effect loci, QTL-III_sima and QTL-X_sim, is an important step toward identifying the molecular basis of adaptation in a multicellular organism.

Pheromone processing in Drosophila

Understanding how sensory stimuli are processed in the brain to instruct appropriate behavior is a fundamental question in neuroscience. Drosophila has become a powerful model system to address this problem. Recent advances in characterizing the circuits underlying pheromone processing have put the field in a position to follow the transformation of these chemical signals all the way from the sensory periphery to decision making and motor output. Here we describe the latest advances, outline emerging principles of pheromone processing and discuss future questions.

The neuropeptide tachykinin is essential for pheromone detection in a gustatory neural circuit

Gustatory pheromones play an essential role in shaping the behavior of many organisms. However, little is known about the processing of taste pheromones in higher order brain centers. Here, we describe a male-specific gustatory circuit in Drosophila that underlies the detection of the anti-aphrodisiac pheromone (3R,11Z,19Z)-3-acetoxy-11,19-octacosadien-1-ol (CH503). Using behavioral analysis, genetic manipulation, and live calcium imaging, we show that Gr68a-expressing neurons on the forelegs of male flies exhibit a sexually dimorphic physiological response to the pheromone and relay information to the central brain via peptidergic neurons. The release of tachykinin from 8 to 10 cells within the subesophageal zone is required for the pheromone-triggered courtship suppression. Taken together, this work describes a neuropeptide-modulated central brain circuit that underlies the programmed behavioral response to a gustatory sex pheromone. These results will allow further examination of the molecular basis by which innate behaviors are modulated by gustatory cues and physiological state.

An automated system for quantitative analysis of Drosophila larval locomotion

BACKGROUND

Drosophila larvae have been used as a model to study to genetic and cellular circuitries modulating behaviors. One of the challenges in behavioral study is the quantification of complex phenotypes such as locomotive behaviors. Experimental capability can be greatly enhanced by an automatic single-animal tracker that records an animal at a high resolution for an extended period, and analyzes multiple behavioral parameters.

RESULTS

Here we present MaggotTracker, a single-animal tracking system for Drosophila larval locomotion analysis. This system controls the motorized microscope stage while taking a video, so that the animal remains in the viewing center. It then reduces the animal to 13 evenly distributed points along the midline, and computes over 20 parameters evaluating the shape, peristalsis movement, stamina, and track of the animal. To demonstrate its utility, we applied MaggotTracker to analyze both wild-type and mutant animals to identify factors affecting locomotive behaviors. Each animal was tracked for four minutes. Our analysis on Canton-S third-instar larvae revealed that the distance an animal travelled was correlated to its striding speed rather than the percentage of time the animal spent striding, and that the striding speed was correlated to both the distance and the duration of one stride. Sexual dimorphism was observed in body length but not in locomotive parameters such as speed. Locomotive parameters were affected by animal developmental stage and the crawling surface. No significant changes in movement speed were detected in mutants of circadian genes such as period (per), timeout, and timeless (tim). The MaggotTracker analysis showed that ether a go-go (eag), Shaker (Sh), slowpoke (slo), and dunce (dnc) mutant larvae had severe phenotypes in multiple locomotive parameters such as stride distance and speed, consistent with their function in neuromuscular junctions. Further, the phenotypic patterns of the K(+) channel genes eag, Sh and slo are highly similar.

CONCLUSIONS

These results showed that MaggotTracker is an efficient tool for automatic phenotyping. The MaggotTracker software as well as the data presented here can be downloaded from our open-access site www.WormLoco.org/Mag.