Fruit flies learn to avoid odours associated with virulent infection

While learning to avoid toxic food is common in mammals and occurs in some insects, learning to avoid cues associated with infectious pathogens has received little attention. We demonstrate that Drosophila melanogaster show olfactory learning in response to infection with their virulent intestinal pathogen Pseudomonas entomophila. This pathogen was not aversive to taste when added to food. Nonetheless, flies exposed for 3 h to food laced with P. entomophila, and scented with an odorant, became subsequently less likely to choose this odorant than flies exposed to pathogen-laced food scented with another odorant. No such effect occurred after an otherwise identical treatment with an avirulent mutant of P. entomophila, indicating that the response is mediated by pathogen virulence. These results demonstrate that a virulent pathogen infection can act as an aversive unconditioned stimulus which flies can associate with food odours, and thus become less attracted to pathogen-contaminated food.     

Exposure to cold: aversive Pavlovian conditioning in individual Drosophila melanogaster

Abstract. Temperature changes can be especially threatening for ectotherms, such as Drosophila melanogaster (Diptera: Drosophilidea Meigen, 1830 ), and in this study we tested whether flies can associate olfactory stimuli with a sudden drop in temperature. Such Pavlovian conditioning would allow them to make appropriate behavioural and/or physiological responses in the future. We found that exposing individual flies to one of two odours in the presence of a sudden drop in temperature resulted in Pavlovian conditioning with flies subsequently avoiding the odour paired with cold. The characteristics of Pavlovian conditioning in flies were comparable to those observed for mammalian species. Specifically, the strength of conditioning increased with increasing intensity of the cold and decreased as the time interval between the olfactory stimulus (CS) and cold (US) was lengthened. Finally, the order in which CS and US were presented affected the strength of conditioning. Learning was observed when the CS preceded US and when the US immediately preceded the CS, but not when the CS preceded the US by 30 s or more. These results provide further evidence for learning in individual flies, and confirm that Pavlovian conditioning is a general mechanism used by organisms to obtain information about their environment.     

Occasion setting in Pavlovian ambiguous target discriminations

Rats were trained in a Pavlovian serial ambiguous target discrimination, in which a target cue was reinforced if it was preceded by one stimulus (P-->T+) but was not reinforced if it was preceded by another stimulus (N-->T-). Test performance indicated that stimulus control by these features was weaker than that acquired by features trained within separate serial feature positive (P-->T+, T-) and serial feature negative (N-->W-, W+) discriminations. The form of conditioned responding and the patterns of transfer observed suggested that the serial ambiguous target discrimination was solved by occasion setting. The data are discussed in terms of the use of retrospective coding strategies when solving Pavlovian serial conditional discriminations, and the acquisition of special properties by both feature and target stimuli.     

Male fruit flies learn to avoid interspecific courtship

Experimental data suggest, and theoretical models typicallyassume, that males of many fruit flies (Drosophila spp) areat least partially indiscriminate while searching for mates,and that it is mostly the females who exert selective mate choice,which can lead to incipient speciation. Evidence on learningby male D. melanogaster in the context of courtship, however,raises the possibility that the initially indiscriminate malesbecome more selective with experience. I tested this possibilityby comparing the courtship behavior of male D. melanogasterexperienced at courting females of the closely related species,D. simulans, and inexperienced males. I found that comparedwith the inexperienced males, the males experienced with courtingD. simulans females showed significantly lower courtship towardfemale D. simulans. Both male treatments, however, showed virtuallyidentical courtship durations with female D. melanogaster. Theseresults indicate that male fruit flies adaptively refine theircourtship behavior with experience and suggest that the malescontribute more to assortative mating and incipient speciationthan is commonly assumed.     

Dacus dorsalis flies can learn to find and accept host fruit

Mature oriental fruit fly females, Dacus dorsalis, from a population cultured on host fruit in the laboratory for one generation responded positively to visual and olfactory stimuli of individual natural kumquat (Fortunella japonica) and apple (Malus pumila) host fruit or models of these fruit hung from branches of potted trees in field enclosures. When females were exposed for 3 days to natural kumquats or apples on trees and subsequently released individually onto trees harboring one or the other of these fruit types, a significantly greater proportion of those exposed to kumquats than those exposed to apples or those not exposed to any fruit visited kumquats. Females exposed to kimquats for at least 3 days followed by at least 3 days of exposure to apples retained ability to find kumquats. Compared with females exposed to apples for 3 days or with naive females, females exposed to kumquats for 3 days exhibitied no less ability to find apples but did significantly refrain from accepting apples for oviposition. A final test was conducted in which females were exposed to natural kumquats or apples for 3 days and tested for response to inanimate models of either the same color and size as natural kumquats (orange, 20-mm diameter) or apples (green, 75-mm diameter) or models of the same color but opposite size. Results suggest that fruit size is the principal character learned and used in finding kumquats, which apparently are somewhat inconspicuous to an inexperienced foraging D. dorsalisfemale.     

Differentiation and diversification of the follicular cells in flies: insight from the studies of the lower brachycerans' ovaries

Although all dipteran species have ovaries of the same meroistic-polytrophic type, the structure of individual ovarian follicles (egg chambers) as well as the course of oogenesis in major dipteran taxa are highly diversified and often significantly different from the widely known Drosophila model. In this report we present results of the morphological studies of the ovary structure in the representatives of three families of lower brachycerans (Orthorrhapha) and compare them with the present knowledge of the processes that lead to the formation of a mature egg cell in the model dipteran, the cyclorrhaphan fruit fly, Drosophila melanogaster. The most conspicuous and developmentally significant differences between Drosophila and lower brachycerans were found in the events that accompany the differentiation and diversification of somatic follicular cells. Our observations indicate that the directed migrations of some follicular cells within the egg chamber and the ability of border cells to invade the nurse cell compartment can be considered as evolutionary novelties that evolved in the ancestors of higher brachycerans.     

Disentangling the nature of the nicotine stimulus

Learning involving interoceptive stimuli likely plays an important role in many diseases and psychopathologies. Within this area, there has been extensive research investigating the interoceptive stimulus effects of abused drugs. In this pursuit, behavioral pharmacologists have taken advantage of what is known about learning processes and adapted the techniques to investigate the behavioral and receptor mechanisms of drug stimuli. Of particular interest is the nicotine stimulus and the use of the two-lever operant drug discrimination task and the Pavlovian drug discriminated goal-tracking task. There is strong concordance between the two methods when using "standard" testing protocols that minimize learning on test days. For example, ABT-418, nornicotine, and varenicline all fully evoked nicotine-appropriate responding. Notably, research from our laboratory with the discriminated goal-tracking task has used an alternative testing protocol. This protocol assesses stimulus substitution based on how well extinction learning using a non-nicotine ligand transfers back to the nicotine stimulus. These findings challenge conclusions based on more "standard" testing procedures (e.g., ABT-418 is not nicotine-like). As a starting point, we propose Thurstone scaling as a quantitative method for more precisely comparing transfer of extinction across doses, experiments, and investigators. We close with a discussion of future research directions and potential implications of the research for understanding interoceptive stimuli.     

Lateral axonal modulation is required for stimulus-specific olfactory conditioning in Drosophila

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The attractiveness of different odour sources from the fruit–host complex on Leptopilina boulardi, a larval parasitoid of frugivorous Drosophila spp.

In parasitoid insects, successful offspring development depends on the female’s ability to find a suitable host. Specific recognition is often based on responses to olfactory cues, but their source and nature have rarely been determined. –This paper deals with the recognition of odours involved in host location by Leptopilina boulardi[Barbotin, Carton & Kelner-Pillault] (Hymenoptera: Eucoilidae), a larval parasitoid of Drosophila species that develops in mature fruits. The nature and origin of volatile stimuli recognized among odours of the host–fruit complex, and the effect of learning on this recognition, were investigated. Oriented responses to these odours were observed in a four-armed olfactometer and were analysed with the observer software (Noldus Information Technology). Fruit odours alone (banana and pear) were not spontaneously attractive to naive parasitoids, whereas naturally-infested bananas were highly attractive. The attraction was related to the odour that adult Drosophila left on the substrate but not to Drosophila oviposition activity or larval development. A synergism between some fruit odours (banana and pear) and the odour left by adult Drosophila on damp filter paper was observed. However, when testing a non-fruit substrate (mushroom), no synergism was observed. Thus, female L. boulardi may innately recognize host–food substrate odours associated with odours from the adult stage of their host. In addition, an oviposition experience on an infested banana allows L. boulardi females to memorise the fruit odour itself through associative learning. The adaptive significance of this process is discussed.     

Drosophila Adult Olfactory Shock Learning

Drosophila have been used in classical conditioning experiments for over 40 years, thus greatly facilitating our understanding of memory, including the elucidation of the molecular mechanisms involved in cognitive diseases^1-7. Learning and memory can be assayed in larvae to study the effect of neurodevelopmental genes^8-10 and in flies to measure the contribution of adult plasticity genes^1-7. Furthermore, the short lifespan of Drosophila facilitates the analysis of genes mediating age-related memory impairment^5,11-13. The availability of many inducible promoters that subdivide the Drosophila nervous system makes it possible to determine when and where a gene of interest is required for normal memory as well as relay of different aspects of the reinforcement signal^3,4,14,16.Studying memory in adult Drosophila allows for a detailed analysis of the behavior and circuitry involved and a measurement of long-term memory^15-17. The length of the adult stage accommodates longer-term genetic, behavioral, dietary and pharmacological manipulations of memory, in addition to determining the effect of aging and neurodegenerative disease on memory^{3-6,11-13,15-21}.Classical conditioning is induced by the simultaneous presentation of a neutral odor cue (conditioned stimulus, CS⁺) and a reinforcement stimulus, e.g., an electric shock or sucrose, (unconditioned stimulus, US), that become associated with one another by the animal^1,16. A second conditioned stimulus (CS^-) is subsequently presented without the US. During the testing phase, Drosophila are simultaneously presented with CS+ and CS- odors. After the Drosophila are provided time to choose between the odors, the distribution of the animals is recorded. This procedure allows associative aversive or appetitive conditioning to be reliably measured without a bias introduced by the innate preference for either of the conditioned stimuli. Various control experiments are also performed to test whether all genotypes respond normally to odor and reinforcement alone.     

Operant Learning of Drosophila at the Torque Meter

For experiments at the torque meter, flies are kept on standard fly medium at 25°C and 60% humidity with a 12hr light/12hr dark regime. A standardized breeding regime assures proper larval density and age-matched cohorts. Cold-anesthetized flies are glued with head and thorax to a triangle-shaped hook the day before the experiment. Attached to the torque meter via a clamp, the fly''s intended flight maneuvers are measured as the angular momentum around its vertical body axis. The fly is placed in the center of a cylindrical panorama to accomplish stationary flight. An analog to digital converter card feeds the yaw torque signal into a computer which stores the trace for later analysis. The computer also controls a variety of stimuli which can be brought under the fly''s control by closing the feedback loop between these stimuli and the yaw torque trace. Punishment is achieved by applying heat from an adjustable infrared laser.     

Social behavior and aging: A fly model

The field of behavioral genetics has recently begun to explore the effect of age on social behaviors. Such studies are particularly important, as certain neuropsychiatric disorders with abnormal social interactions, like autism and schizophrenia, have been linked to older parents. Appropriate social interaction can also have a positive impact on longevity, and is associated with successful aging in humans. Currently, there are few genetic models for understanding the effect of aging on social behavior and its potential transgenerational inheritance. The fly is emerging as a powerful model for identifying the basic molecular mechanisms underlying neurological and neuropsychiatric disorders. In this review, we discuss these recent advancements, with a focus on how studies in Drosophila melanogaster have provided insight into the effect of aging on aspects of social behavior, including across generations.