A genetic screen for olfactory habituation mutations in Drosophila: analysis of novel foraging alleles and an underlying neural circuit

Comparing statistical learning methods for complex trait prediction from gene expression

Accurate prediction of complex traits is an important task in quantitative genetics. Genotypes have been used for trait prediction using a variety of methods such as mixed models, Bayesian methods, penalized regression methods, dimension reduction methods, and machine learning methods. Recent studies have shown that gene expression levels can produce higher prediction accuracy than genotypes. However, only a few prediction methods were tested in these studies. Thus, a comprehensive assessment of methods is needed to fully evaluate the potential of gene expression as a predictor of complex trait phenotypes. Here, we used data from the Drosophila Genetic Reference Panel (DGRP) to compare the ability of several existing statistical learning methods to predict starvation resistance and startle response from gene expression in the two sexes separately. The methods considered differ in assumptions about the distribution of gene effects-ranging from models that assume that every gene affects the trait to more sparse models-and their ability to capture gene-gene interactions. We also used functional annotation (i.e., Gene Ontology (GO)) as a source of biological information to inform prediction models. The results show that differences in prediction accuracy exist. For example, methods performing variable selection achieved higher prediction accuracy for starvation resistance in females, while they generally had lower accuracy for startle response in both sexes. Incorporating GO annotations further improved prediction accuracy for a few GO terms of biological significance. Biological significance extended to the genes underlying highly predictive GO terms. Notably, the Insulin-like Receptor (InR) was prevalent across methods and sexes for starvation resistance. For startle response, crumbs (crb) and imaginal disc growth factor 2 (Idgf2) were found for females and males, respectively. Our results confirmed the potential of transcriptomic prediction and highlighted the importance of selecting appropriate methods and strategies in order to achieve accurate predictions.

The Rpfor gene modulates the locomotory activity and host-seeking behaviour of Rhodnius prolixus

The molecular bases of animal behaviour are intricate due to the pleiotropic nature of behaviour-modulating genes, which are often expressed across multiple tissues. The foraging gene (for) encodes a cGMP-dependent protein kinase (PKG), pivotal in regulating downstream target proteins through phosphorylation. In insects, for has been implicated in various behavioural contexts and physiological processes regarding searching for food. Rhodnius prolixus, a hematophagous bug that transmits Trypanosoma cruzi, the causative agent of Chagas disease, exhibits specific activity patterns associated with its hematophagous behaviour. Our previous work demonstrated a correlation between locomotor activity profiles and the expression of Rpfor, suggesting its involvement in modulating triatomine locomotion. In this study, we investigated the impact of Rpfor knockdown on locomotory activity, host-seeking behaviour, feeding performance and lipid metabolism in R. prolixus nymphs. Using RNA interference, we achieved a significant reduction of Rpfor expression in both the brain and fat body of R. prolixus nymphs. Knocked-down nymphs exhibited diminished non-oriented locomotory activity compared with controls, without altering the characteristic bimodal pattern of activity. Additionally, they displayed an increased tendency to approach a host, suggesting a role for Rpfor in modulating host-seeking behaviour. Feeding performance and lipid metabolism remained unaffected by Rpfor knockdown. Our findings underscore the multifaceted role of Rpfor in modulating locomotor activity and host-seeking behaviour in R. prolixus nymphs, shedding light on the molecular mechanisms underlying their hematophagous behaviour and potential implications for disease transmission. Further research is necessary to elucidate the intricate interplay between Rpfor expression, behaviour and physiological processes in triatomine bugs.

Adult expression of the cell adhesion protein Fasciclin 3 is required for the maintenance of adult olfactory interneurons

The proper functioning of the nervous system is dependent on the establishment and maintenance of intricate networks of neurons that form functional neural circuits. Once neural circuits are assembled during development, a distinct set of molecular programs is likely required to maintain their connectivity throughout the lifetime of the organism. Here, we demonstrate that Fasciclin 3 (Fas3), an axon guidance cell adhesion protein, is necessary for the maintenance of the olfactory circuit in adult Drosophila. We utilized the TARGET system to spatiotemporally knockdown Fas3 in selected populations of adult neurons. Our findings show that Fas3 knockdown results in the death of olfactory circuit neurons and reduced survival of adults. We also demonstrated that Fas3 knockdown activates caspase-3-mediated cell death in olfactory local interneurons, which can be rescued by overexpressing baculovirus p35, an anti-apoptotic protein. This work adds to the growing set of evidence indicating a crucial role for axon guidance proteins in the maintenance of neuronal circuits in adults.

Screening for Differentially Expressed Memory Genes on a Diabetes Model Induced by High-Sugar Diet in Drosophila melanogaster: Potential Markers for Memory Deficits

Type 2 diabetes mellitus (T2DM) has been shown to affect a series of cognitive processes including memory, increasing the risk for dementia, particularly Alzheimer's disease (AD). Although increasing evidence has supported that both diseases share common features, the pathophysiological mechanisms connecting these two disorders remain to be fully elucidated. Herein, we used Drosophila melanogaster fed on a high-sugar diet (HSD) to mimic T2DM, and investigate its effects on memory as well as identify potential molecular players associated with the memory deficits induced by HSD. Flies hatched from and reared on HSD for 7 days had a substantial decrease in short-term memory (STM). The screening for memory-related genes using transcriptome data revealed that HSD altered the expression of 33% of memory genes in relation to the control. Among the differentially expressed genes (DEGs) with a fold change (FC) higher than two, we found five genes, related to synapse and memory trace formation, that could be considered strong candidates to underlie the STM deficits in HSD flies: Abl tyrosine kinase (Abl), bruchpilot (Brp), minibrain (Mnb), shaker (Sh), and gilgamesh (Gish). We also analyzed genes from the dopamine system, one of the most relevant signaling pathways for olfactory memory. Interestingly, the flies fed on HSD presented a decreased expression of the Tyrosine hydroxylase (Ple) and Dopa decarboxylase (Ddc) genes, signals of a possible dopamine deficiency. In this work, we present promising biomarkers to investigate molecular networks shared between T2DM and AD.

Mechanical vibration patterns elicit behavioral transitions and habituation in crawling Drosophila larvae

How animals respond to repeatedly applied stimuli, and how animals respond to mechanical stimuli in particular, are important questions in behavioral neuroscience. We study adaptation to repeated mechanical agitation using the Drosophila larva. Vertical vibration stimuli elicit a discrete set of responses in crawling larvae: continuation, pause, turn, and reversal. Through high-throughput larva tracking, we characterize how the likelihood of each response depends on vibration intensity and on the timing of repeated vibration pulses. By examining transitions between behavioral states at the population and individual levels, we investigate how the animals habituate to the stimulus patterns. We identify time constants associated with desensitization to prolonged vibration, with re-sensitization during removal of a stimulus, and additional layers of habituation that operate in the overall response. Known memory-deficient mutants exhibit distinct behavior profiles and habituation time constants. An analogous simple electrical circuit suggests possible neural and molecular processes behind adaptive behavior.

Modulation of the NO-cGMP pathway has no effect on olfactory responses in the Drosophila antenna

Olfaction is a crucial sensory modality in insects and is underpinned by odor-sensitive sensory neurons expressing odorant receptors that function in the dendrites as odorant-gated ion channels. Along with expression, trafficking, and receptor complexing, the regulation of odorant receptor function is paramount to ensure the extraordinary sensory abilities of insects. However, the full extent of regulation of sensory neuron activity remains to be elucidated. For instance, our understanding of the intracellular effectors that mediate signaling pathways within antennal cells is incomplete within the context of olfaction in vivo. Here, with the use of optical and electrophysiological techniques in live antennal tissue, we investigate whether nitric oxide signaling occurs in the sensory periphery of Drosophila. To answer this, we first query antennal transcriptomic datasets to demonstrate the presence of nitric oxide signaling machinery in antennal tissue. Next, by applying various modulators of the NO-cGMP pathway in open antennal preparations, we show that olfactory responses are unaffected by a wide panel of NO-cGMP pathway inhibitors and activators over short and long timescales. We further examine the action of cAMP and cGMP, cyclic nucleotides previously linked to olfactory processes as intracellular potentiators of receptor functioning, and find that both long-term and short-term applications or microinjections of cGMP have no effect on olfactory responses in vivo as measured by calcium imaging and single sensillum recording. The absence of the effect of cGMP is shown in contrast to cAMP, which elicits increased responses when perfused shortly before olfactory responses in OSNs. Taken together, the apparent absence of nitric oxide signaling in olfactory neurons indicates that this gaseous messenger may play no role as a regulator of olfactory transduction in insects, though may play other physiological roles at the sensory periphery of the antenna.

Long-Term Memory Formation in Drosophila Depends on the 3'UTR of CPEB Gene orb2

Activation of local translation in neurites in response to stimulation is an important step in the formation of long-term memory (LTM). CPEB proteins are a family of translation factors involved in LTM formation. The Drosophila CPEB protein Orb2 plays an important role in the development and function of the nervous system. Mutations of the coding region of the orb2 gene have previously been shown to impair LTM formation. We found that a deletion of the 3'UTR of the orb2 gene similarly results in loss of LTM in Drosophila. As a result of the deletion, the content of the Orb2 protein remained the same in the neuron soma, but significantly decreased in synapses. Using RNA immunoprecipitation followed by high-throughput sequencing, we detected more than 6000 potential Orb2 mRNA targets expressed in the Drosophila brain. Importantly, deletion of the 3'UTR of orb2 mRNA also affected the localization of the Csp, Pyd, and Eya proteins, which are encoded by putative mRNA targets of Orb2. Therefore, the 3'UTR of the orb2 mRNA is important for the proper localization of Orb2 and other proteins in synapses of neurons and the brain as a whole, providing a molecular basis for LTM formation.

Genetics, molecular control and clinical relevance of habituation learning

Habituation is the most fundamental form of learning. As a firewall that protects our brain from sensory overload, it is indispensable for cognitive processes. Studies in humans and animal models provide increasing evidence that habituation is affected in autism and related monogenic neurodevelopmental disorders (NDDs). An integrated application of habituation assessment in NDDs and their animal models has unexploited potential for neuroscience and medical care. With the aim to gain mechanistic insights, we systematically retrieved genes that have been demonstrated in the literature to underlie habituation. We identified 258 evolutionarily conserved genes across species, describe the biological processes they converge on, and highlight regulatory pathways and drugs that may alleviate habituation deficits. We also summarize current habituation paradigms and extract the most decisive arguments that support the crucial role of habituation for cognition in health and disease. We conclude that habituation is a conserved, quantitative, cognition- and disease-relevant process that can connect preclinical and clinical work, and hence is a powerful tool to advance research, diagnostics, and treatment of NDDs.

Does Habituation Affect the Efficacy of Semiochemical Oviposition Repellents Developed Against Drosophila suzukii?

The application of feeding and oviposition repellents is limited to arthropod systems in which habituation does not occur. Although several compounds appear to reduce Drosophila suzukii Matsumura (Dipetra: Drosophilidae) oviposition in berries, previous studies have yet to address whether habituation is a significant risk following preexposure. We tested the response of adult female D. suzukii to three previously identified semiochemical oviposition repellents, 1-octen-3-ol (octenol), ±-geosmin, and 2-n-pentylfuran, following adult and larval preexposure. Using a two-choice gated trap capture assay, we assessed captures in repellent-treated versus blank traps, female survival, and oviposition frequency in the selected trap. We did not find evidence of habituation to octenol or 2-pentylfuran in adult flies preexposed for 24, 48, or 72 hr. When exposed to each of the repellents as larvae, D. suzukii showed similar deterrence as those exposed as adults alone. However, mortality did decrease in F1 octenol treated flies. In contrast with previous investigations we did not observe repellent effects in response to geosmin. Our results suggest that neither exposure during the adult life stage nor during larval development inhibited the effectiveness of octenol and 2-pentylfuran. However, greater survivorship on octenol treated baits in F1 flies, combined with apparent neurotoxic effects of this compound, indicate that octenol may be less suited for field applications. For this reason, 2-pentylfuran appears to be a better candidate for ongoing research aimed at developing an effective push-pull system of behavioral management.

The foraging gene affects alcohol sensitivity, metabolism and memory in Drosophila

The genetic basis of alcohol use disorder (AUD) is complex. Understanding how natural genetic variation contributes to alcohol phenotypes can help us identify and understand the genetic basis of AUD. Recently, a single nucleotide polymorphism in the human foraging (for) gene ortholog, Protein Kinase cGMP-Dependent 1 (PRKG1), was found to be associated with stress-induced risk for alcohol abuse. However, the mechanistic role that PRKG1 plays in AUD is not well understood. We use natural variation in the Drosophila for gene to describe how variation of cGMP-dependent protein kinase (PKG) activity modifies ethanol-induced phenotypes. We found that variation in for affects ethanol-induced increases in locomotion and memory of the appetitive properties of ethanol intoxication. Further, these differences may stem from the ability to metabolize ethanol. Together, this data suggests that natural variation in PKG modulates cue reactivity for alcohol, and thus could influence alcohol cravings by differentially modulating metabolic and behavioral sensitivities to alcohol.

Expression of the foraging gene in adult Drosophila melanogaster

The foraging gene in Drosophila melanogaster, which encodes a cGMP-dependent protein kinase, is a highly conserved, complex gene with multiple pleiotropic behavioral and physiological functions in both the larval and adult fly. Adult foraging expression is less well characterized than in the larva. We characterized foraging expression in the brain, gastric system, and reproductive systems using a T2A-Gal4 gene-trap allele. In the brain, foraging expression appears to be restricted to multiple sub-types of glia. This glial-specific cellular localization of foraging was supported by single-cell transcriptomic atlases of the adult brain. foraging is extensively expressed in most cell types in the gastric and reproductive systems. We then mapped multiple cis-regulatory elements responsible for parts of the observed expression patterns by a nested cloned promoter-Gal4 analysis. The mapped cis-regulatory elements were consistently modular when comparing the larval and adult expression patterns. These new data using the T2A-Gal4 gene-trap and cloned foraging promoter fusion GAL4's are discussed with respect to previous work using an anti-FOR antibody, which we show here to be non-specific. Future studies of foraging's function will consider roles for glial subtypes and peripheral tissues (gastric and reproductive systems) in foraging's pleiotropic behavioral and physiological effects.

The foraging gene as a modulator of division of labour in social insects

The social ants, bees, wasps, and termites include some of the most ecologically-successful groups of animal species. Their dominance in most terrestrial environments is attributed to their social lifestyle, which enable their colonies to exploit environmental resources with remarkable efficiency. One key attribute of social insect colonies is the division of labour that emerges among the sterile workers, which represent the majority of colony members. Studies of the mechanisms that drive division of labour systems across diverse social species have provided fundamental insights into the developmental, physiological, molecular, and genomic processes that regulate sociality, and the possible genetic routes that may have led to its evolution from a solitary ancestor. Here we specifically discuss the conserved role of the foraging gene, which encodes a cGMP-dependent protein kinase (PKG). Originally identified as a behaviourally polymorphic gene that drives alternative foraging strategies in the fruit fly Drosophila melanogaster, changes in foraging expression and kinase activity were later shown to play a key role in the division of labour in diverse social insect species as well. In particular, foraging appears to regulate worker transitions between behavioural tasks and specific behavioural traits associated with morphological castes. Although the specific neuroethological role of foraging in the insect brain remains mostly unknown, studies in genetically tractable insect species indicate that PKG signalling plays a conserved role in the neuronal plasticity of sensory, cognitive and motor functions, which underlie behaviours relevant to division of labour, including appetitive learning, aggression, stress response, phototaxis, and the response to pheromones.

Gaining an understanding of behavioral genetics through studies of foraging in Drosophila and learning in C. elegans

The pursuit of understanding behavior has led to investigations of how genes, the environment, and the nervous system all work together to produce and influence behavior, giving rise to a field of research known as behavioral neurogenetics. This review focuses on the research journeys of two pioneers of aspects of behavioral neurogenetic research: Dr. Marla Sokolowski and Dr. Catharine Rankin as examples of how different approaches have been used to understand relationships between genes and behavior. Marla Sokolowski's research is centered around the discovery and analysis of foraging, a gene responsible for the natural behavioral polymorphism of Drosophila melanogaster larvae foraging behavior. Catharine Rankin's work began with demonstrating the ability to learn in Caenorhabditis elegans and then setting out to investigate the mechanisms underlying the "simplest" form of learning, habituation. Using these simple invertebrate organisms both investigators were able to perform in-depth dissections of behavior at genetic and molecular levels. By exploring their research and highlighting their findings we present ways their work has furthered our understanding of behavior and contributed to the field of behavioral neurogenetics.

The Drosophila melanogaster foraging gene affects social networks

Drosophila melanogaster displays social behaviors including courtship, mating, aggression, and group foraging. Recent studies employed social network analyses (SNAs) to show that D. melanogaster strains differ in their group behavior, suggesting that genes influence social network phenotypes. Aside from genes associated with sensory function, few studies address the genetic underpinnings of these networks. The foraging gene (for) is a well-established example of a pleiotropic gene that regulates multiple behavioral phenotypes and their plasticity. In D. melanogaster, there are two naturally occurring alleles of for called rover and sitter that differ in their larval and adult food-search behavior as well as other behavioral phenotypes. Here, we hypothesize that for affects behavioral elements required to form social networks and the social networks themselves. These effects are evident when we manipulate gene dosage. We found that flies of the rover and sitter strains exhibit differences in duration, frequency, and reciprocity of pairwise interactions, and they form social networks with differences in assortativity and global efficiency. Consistent with other adult phenotypes influenced by for, rover-sitter heterozygotes show intermediate patterns of dominance in many of these characteristics. Multiple generations of backcrossing a rover allele into a sitter strain showed that many but not all of these rover-sitter differences may be attributed to allelic variation at for. Our findings reveal the significant role that for plays in affecting social network properties and their behavioral elements in Drosophila melanogaster.

A cGMP-dependent protein kinase, encoded by the Drosophila foraging gene, regulates neurotransmission through changes in synaptic structure and function

A cGMP-dependent protein kinase (PKG) encoded by the Drosophila foraging (for) gene regulates both synaptic structure (nerve terminal growth) and function (neurotransmission) through independent mechanisms at the Drosophila larval neuromuscular junction (nmj). Glial for is known to restrict nerve terminal growth, whereas presynaptic for inhibits synaptic vesicle (SV) exocytosis during low frequency stimulation. Presynaptic for also facilitates SV endocytosis during high frequency stimulation. for's effects on neurotransmission can occur independent of any changes in nerve terminal growth. However, it remains unclear if for's effects on neurotransmission affect nerve terminal growth. Furthermore, it's possible that for's effects on synaptic structure contribute to changes in neurotransmission. In the present study, we examined these questions using RNA interference to selectively knockdown for in presynaptic neurons or glia at the Drosophila larval nmj. Consistent with our previous findings, presynaptic knockdown of for impaired SV endocytosis, whereas knockdown of glial for had no effect on SV endocytosis. Surprisingly, we found that knockdown of either presynaptic or glial for increased neurotransmitter release in response to low frequency stimulation. Knockdown of presynaptic for did not affect nerve terminal growth, demonstrating that for's effects on neurotransmission does not alter nerve terminal growth. In contrast, knockdown of glial for enhanced nerve terminal growth. This enhanced nerve terminal growth was likely the cause of the enhanced neurotransmitter release seen following knockdown of glial for. Overall, we show that for can affect neurotransmitter release by regulating both synaptic structure and function.

The Drosophila foraging gene plays a vital role at the start of metamorphosis for subsequent adult emergence

The foraging (for) gene has been extensively studied in many species for its functions in development, physiology, and behavior. It is common for genes that influence behavior and development to be essential genes, and for has been found to be an essential gene in both fruit flies and mammals, with for mutants dying before reaching the adult stage. However, the biological process underlying the lethality associated with this gene is not known. Here, we show that in Drosophila melanogaster, some but not all gene products of for are essential for survival. Specifically, we show that promoter 3 of for, but not promoters 1, 2, and 4 are required for survival past pupal stage. We use full and partial genetic deletions of for, and temperature-restricted knock-down of the gene to further investigate the stage of lethality. While deletion analysis shows that flies lacking for die at the end of pupal development, as pharate adults, temperature-restricted knock-down shows that for is only required at the start of pupal development, for normal adult emergence (AE) and viability. We further show that the inability of these mutants to emerge from their pupal cases is linked to deficiencies in emergence behaviors, caused by a possible energy deficiency, and finally, that the lethality of for mutants seems to be linked to protein isoform P3, transcribed from for promoter 3.

Acute Regulation of Habituation Learning via Posttranslational Palmitoylation

Habituation is an adaptive learning process that enables animals to adjust innate behaviors to changes in their environment. Despite its well-documented implications for a wide diversity of behaviors, the molecular and cellular basis of habituation learning is not well understood. Using whole-genome sequencing of zebrafish mutants isolated in an unbiased genetic screen, we identified the palmitoyltransferase Huntingtin interacting protein 14 (Hip14) as a critical regulator of habituation learning. We demonstrate that Hip14 regulates depression of sensory inputs onto an identified hindbrain neuron and provide evidence that Hip14 palmitoylates the Shaker-like K+ voltage-gated channel subunit (Kv1.1), thereby regulating Kv1.1 subcellular localization. Furthermore, we show that, like for Hip14, loss of Kv1.1 leads to habituation deficits and that Hip14 is dispensable in development and instead acts acutely to promote habituation. Combined, these results uncover a previously unappreciated role for acute posttranslational palmitoylation at defined circuit components to regulate learning.

The foraging Gene and Its Behavioral Effects: Pleiotropy and Plasticity

The Drosophila melanogaster foraging (for) gene is a well-established example of a gene with major effects on behavior and natural variation. This gene is best known for underlying the behavioral strategies of rover and sitter foraging larvae, having been mapped and named for this phenotype. Nevertheless, in the last three decades an extensive array of studies describing for's role as a modifier of behavior in a wide range of phenotypes, in both Drosophila and other organisms, has emerged. Furthermore, recent work reveals new insights into the genetic and molecular underpinnings of how for affects these phenotypes. In this article, we discuss the history of the for gene and its role in natural variation in behavior, plasticity, and behavioral pleiotropy, with special attention to recent findings on the molecular structure and transcriptional regulation of this gene.

The Mechanosensitive Ion Channel Piezo Inhibits Axon Regeneration

Neurons exhibit a limited ability of repair. Given that mechanical forces affect neuronal outgrowth, it is important to investigate whether mechanosensitive ion channels may regulate axon regeneration. Here, we show that DmPiezo, a Ca²⁺-permeable non-selective cation channel, functions as an intrinsic inhibitor for axon regeneration in Drosophila. DmPiezo activation during axon regeneration induces local Ca²⁺ transients at the growth cone, leading to activation of nitric oxide synthase and the downstream cGMP kinase Foraging or PKG to restrict axon regrowth. Loss of DmPiezo enhances axon regeneration of sensory neurons in the peripheral and CNS. Conditional knockout of its mammalian homolog Piezo1 in vivo accelerates regeneration, while its pharmacological activation in vitro modestly reduces regeneration, suggesting the role of Piezo in inhibiting regeneration may be evolutionarily conserved. These findings provide a precedent for the involvement of mechanosensitive channels in axon regeneration and add a potential target for modulating nervous system repair.

Casting a genome-wide net for learning mutants

Wolman et al. (2015) report a forward genetic screen in zebrafish that implicated pregnancy-associated plasma protein-aa in habituation of the acoustic startle response. PAPP-AA is expressed in the underlying circuit, including Mauthner cells, and regulates habituation via IGF signaling.

A genome-wide screen identifies PAPP-AA-mediated IGFR signaling as a novel regulator of habituation learning

Habituation represents a fundamental form of learning, yet the underlying molecular genetic mechanisms are not well defined. Here we report on a genome-wide genetic screen, coupled with whole-genome sequencing, that identified 14 zebrafish startle habituation mutants including mutants of the vertebrate-specific gene pregnancy-associated plasma protein-aa (pappaa). PAPP-AA encodes an extracellular metalloprotease known to increase IGF bioavailability, thereby enhancing IGF receptor signaling. We find that pappaa is expressed by startle circuit neurons, and expression of wild-type but not a metalloprotease-inactive version of pappaa restores habituation in pappaa mutants. Furthermore, acutely inhibiting IGF1R function in wild-type reduces habituation, while activation of IGF1R downstream effectors in pappaa mutants restores habituation, demonstrating that pappaa promotes learning by acutely and locally increasing IGF bioavailability. In sum, our results define the first functional gene set for habituation learning in a vertebrate and identify PAPPAA-regulated IGF signaling as a novel mechanism regulating habituation learning.

Spatial analysis of gene regulation reveals new insights into the molecular basis of upper thermal limits

The cellular stress response has long been the primary model for studying the molecular basis of thermal adaptation, yet the link between gene expression, RNA metabolism and physiological responses to thermal stress remains largely unexplored. We address this by comparing the transcriptional and physiological responses of three geographically distinct populations of Drosophila melanogaster from eastern Australia in response to, and recovery from, a severe heat stress with and without a prestress hardening treatment. We focus on starvin (stv), recently identified as an important thermally responsive gene. Intriguingly, stv encodes seven transcripts from alternative transcription sites and alternative splicing, yet appears to be rapidly heat inducible. First, we show genetic differences in upper thermal limits of the populations tested. We then demonstrate that the stv locus does not ubiquitously respond to thermal stress but is expressed as three distinct thermal and temporal RNA phenotypes (isoforms). The shorter transcript isoforms are rapidly upregulated under stress in all populations and show similar molecular signatures to heat-shock proteins. Multiple stress exposures seem to generate a reserve of pre-mRNAs, effectively 'priming' the cells for subsequent stress. Remarkably, we demonstrate a bypass in the splicing blockade in these isoforms, suggesting an essential role for these transcripts under heat stress. Temporal profiles for the weakly heat responsive stv isoform subset show opposing patterns in the two most divergent populations. Innate and induced transcriptome responses to hyperthermia are complex, and warrant moving beyond gene-level analyses.

Olfactory habituation in Drosophila-odor encoding and its plasticity in the antennal lobe

A ubiquitous feature of an animal's response to an odorant is that it declines when the odorant is frequently or continuously encountered. This decline in olfactory response, termed olfactory habituation, can have temporally or mechanistically different forms. The neural circuitry of the fruit fly Drosophila melanogaster's olfactory system is well defined in terms of component cells, which are readily accessible to functional studies and genetic manipulation. This makes it a particularly useful preparation for the investigation of olfactory habituation. In addition, the insect olfactory system shares many architectural and functional similarities with mammalian olfactory systems, suggesting that olfactory mechanisms in insects may be broadly relevant. In this chapter, we discuss the likely mechanisms of olfactory habituation in context of the participating cell types, their connectivity, and their roles in sensory processing. We overview the structure and function of key cell types, the mechanisms that stimulate them, and how they transduce and process odor signals. We then consider how each stage of olfactory processing could potentially contribute to behavioral habituation. After this, we overview a variety of recent mechanistic studies that point to an important role for potentiation of inhibitory synapses in the primary olfactory processing center, the antennal lobe, in driving the reduced response to familiar odorants. Following the discussion of mechanisms for short- and long-term olfactory habituation, we end by considering how these mechanisms may be regulated by neuromodulators, which likely play key roles in the induction, gating, or suppression of habituated behavior, and speculate on the relevance of these processes for other forms of learning and memory.