Matthew Cobb

Drosophila Cuticular Hydrocarbons Revisited: Mating Status Alters Cuticular Profiles

Most living organisms use pheromones for inter-individual communication. In Drosophila melanogaster flies, several pheromones perceived either by contact/at a short distance (cuticular hydrocarbons, CHs), or at a longer distance (cis-vaccenyl acetate, cVA), affect courtship and mating behaviours. However, it has not previously been possible to precisely identify all potential pheromonal compounds and simultaneously monitor their variation on a time scale. To overcome this limitation, we combined Solid Phase Micro-Extraction with gas-chromatography coupled with mass-spectrometry. This allowed us (i) to identify 59 cuticular compounds, including 17 new CHs; (ii) to precisely quantify the amount of each compound that could be detected by another fly, and (iii) to measure the variation of these substances as a function of aging and mating. Sex-specific variation appeared with age, while mating affected cuticular compounds in both sexes with three possible patterns: variation was (i) reciprocal in the two sexes, suggesting a passive mechanical transfer during mating, (ii) parallel in both sexes, such as for cVA which strikingly appeared during mating, or (iii) unilateral, presumably as a result of sexual interaction. We provide a complete reassessment of all Drosophila CHs and suggest that the chemical conversation between male and female flies is far more complex than is generally accepted. We conclude that focusing on individual compounds will not provide a satisfactory understanding of the evolution and function of chemical communication in Drosophila.

The red flour beetle's large nose: An expanded odorant receptor gene family in Tribolium castaneum

The Tribolium castaneum genome sequence reveals a large number of odorant receptor (Or) genes compared to those found in other insects whose olfactory genomes have been studied-341 Or genes and pseudogenes, encoding 259 intact odorant receptor proteins. An RT-PCR study of larvae and adults revealed that only 145 (64%) of 233 genes with successful genomic DNA amplifications were expressed. No expression of the other 87 genes was detected at any age, suggesting either that these genes are not expressed in this particular strain, or that they are induced only in certain environmental or developmental conditions. TcOR1, the ortholog of the Drosophila Or83b (DmOr83b) gene, which is required for the function of olfactory receptor proteins in Drosophila, was expressed in extracts from adult and larval heads and in extracts from adult bodies. Expression of 41 TcOr genes was detected in extracts from larval head tissue and 111 in extracts from adult head tissue (both figures exclude TcOr1). Twenty-eight TcOrs were detected only in adult bodies. Beetle pupae were injected with TcOr1 dsRNA; unlike sham-injected and control beetles, these knock-down beetles showed no significant response to the Tribolium aggregation pheromone, supporting the hypothesis that TcOr1 plays a similar decisive role in olfaction to DmOr83b. The substantial number of Ors poses the question of why Tribolium has such a large olfactory receptor repertoire, and underlines the need for more studies of the natural history of this species.

An Inhibitory Sex Pheromone Tastes Bitter for Drosophila Males

Sexual behavior requires animals to distinguish between the sexes and to respond appropriately to each of them. In Drosophila melanogaster, as in many insects, cuticular hydrocarbons are thought to be involved in sex recognition and in mating behavior, but there is no direct neuronal evidence of their pheromonal effect. Using behavioral and electrophysiological measures of responses to natural and synthetic compounds, we show that Z-7-tricosene, a Drosophila male cuticular hydrocarbon, acts as a sex pheromone and inhibits male-male courtship. These data provide the first direct demonstration that an insect cuticular hydrocarbon is detected as a sex pheromone. Intriguingly, we show that a particular type of gustatory neurons of the labial palps respond both to Z-7-tricosene and to bitter stimuli. Cross-adaptation between Z-7-tricosene and bitter stimuli further indicates that these two very different substances are processed by the same neural pathways. Furthermore, the two substances induced similar behavioral responses both in courtship and feeding tests. We conclude that the inhibitory pheromone tastes bitter to the fly.

Regulation of reproduction in a queenless ant: aggression, pheromones and reduction in conflict

In the monogynous queenless ant Diacamma ceylonense, the future reproductive (future gamergate) is very aggressive towards infertile workers during the first days of her adult life. Overt aggression disappears at about three weeks, when the future gamergate begins to lay male–destined eggs and is ready to mate. Over the same period, her cuticular hydrocarbon profile alters, changing from a chemical signature similar to that of a sterile worker towards that of a gamergate. In nature, these behavioural and chemical changes will coincide with a reduction in conflict within the nest: faced with a virgin future gamergate, infertile workers have an interest in producing male–destined eggs; however, once the gamergate produces female eggs, they have an interest in rearing her offspring. This demonstration of a shift from physical inhibition to chemical signalling is interpreted in terms of sociogenetic theory, the role of cuticular hydrocarbons as an indicator of fertility in insects and the fact that the regulation of reproduction in Diacamma involves mechanisms redolent of both queenless and queenright ant species.

Species-specific effects of single sensillum ablation on mating position in Drosophila

SUMMARY Dipteran insects show a wide range of species-specific mating positions. Interspecific transitions from one position to another may reflect sexual or natural selection, or be pleiotropic consequences of other genetic changes. Like many cyclorrhaphan flies, Drosophila species mate with the male on the back of the female, positioned centrally. Mechanosensory sensilla on the male genitalia of three species of the melanogaster species sub-group of Drosophila have species-specific effects on mating position and on courtship success: ablation of a single pair of bristles on the genital claspers of D. melanogaster males halved homotypic mating success, and unilateral ablation produced a contralateral asymmetry in the males mating posture. Ablation of mechanoreceptors on the male genital lateral plate affected mating posture less radically and had no effect on mating frequency. Surprisingly, ablation of sensilla on the claspers of D. simulans and D. sechellia males showed no effect on homotypic mating. A similar result was found for D. melanogaster × D. simulans hybrid males. The existence of major differences in the sensory bases of mating position and copulation success in closely related species shows how differing mating positions may have evolved and underlines the need for detailed functional studies in studying the evolution of insect genitalia: homologous structures may serve different functions in different species.

Dog paw preference shows lability and sex differences

Paw preferences in domestic dogs were studied using three different behavioural tests, recording frequency, duration and latency of paw use. No overall population tendency to right- or left-paw preference was seen on any of the tests, nor could a sub-population of handed dogs be detected. This failure to replicate previous reports that male dogs tend to use their left paws while females use their right was counterbalanced by a significant tendency for male dogs to use their left paw when initially presented with one test, and for the latency of left paw use to be significantly shorter than that for right paw use on these initial presentations. This significant effect disappeared with repeated presentation of the test, and was not present in females. We conclude that behavioural lateralisation appears to be a labile category in dogs, and may be related to brain hemispheric effects in responding to novel stimuli.

Olfactory coding in Drosophila larvae investigated by cross-adaptation

SUMMARY In order to reveal aspects of olfactory coding, the effects of sensory adaptation on the olfactory responses of first-instar Drosophila melanogaster larvae were tested. Larvae were pre-stimulated with a homologous series of acetic esters (C3-C9), and their responses to each of these odours were then measured. The overall patterns suggested that methyl acetate has no specific pathway but was detected by all the sensory pathways studied here, that butyl and pentyl acetate tended to have similar effects to each other and that hexyl acetate was processed separately from the other odours. In a number of cases, cross-adaptation transformed a control attractive response into a repulsive response; in no case was an increase in attractiveness observed. This was investigated by studying changes in dose-response curves following pre-stimulation. These findings are discussed in light of the possible intra- and intercellular mechanisms of adaptation and the advantage of altered sensitivity for the larva.

Precise and fuzzy coding by olfactory sensory neurons.

The exact nature of the olfactory signals that arrive in the brain from the periphery, and their reproducibility, remain essentially unknown. In most organisms, the sheer number of olfactory sensory neurons (OSNs) makes it impossible to measure the individual responses of the entire population. We measured the individual in situ electrophysiological activity of OSNs in Drosophila larvae, in response to stimulation with 10 aliphatic odors (alcohols and esters). We studied control larvae (a total of 296 OSNs) and larvae with a single functional OSN, created using the Gal4-upstream activator sequence system. Most OSNs showed consistent, precise responses (either excitation or inhibition) in response to a given odor. Some OSNs also showed qualitatively variable responses (“fuzzy coding”). This robust variability was an intrinsic property of these neurons: it was not attributable to odor type, concentration, stimulus duration, genotype, or interindividual differences, and was seen in control larvae and in larvae with one and two functional OSNs. We conclude that in Drosophila larvae the peripheral code combines precise coding with fuzzy, stochastic responses in which neurons show qualitative variability in their responses to a given odor. We hypothesize that fuzzy coding occurs in other organisms, is translated into differing degrees of activation of the glomeruli, and forms a key component of response variability in the first stages of olfactory processing.

Chemical stimuli induce courtship dominance in Drosophila.

Courtship dominance in male Drosophila occurs when a male directs high levels of courtship towards another male, who remains passive [1]. We investigated the cues that shape this effect and report here that it is induced by the perception of adult male cuticular hydrocarbons during a critical period.

Social mutilation in the Ponerine ant Diacamma: cues originate in the victims

Summary.In the queenless ponerine ant genus Diacamma, all workers eclose with a pair of innervated thoracic appendages termed gemmae. The gamergate (= mated egg laying worker) maintains reproductive monopoly by mutilating the gemmae of all eclosing individuals. Such mutilation leads to irreversible behavioural and neurological changes such that the individual lacking gemmae becomes incapable of appropriate sexual calling and mating. In one population related to Diacamma ceylonense from India, Diacamma sp. from Nilgiri (hereafter referred to as ‘nilgiri’), gamergates do not mutilate their nestmates and yet maintain reproductive monopoly. To understand what triggers mutilation, we exchanged cocoons between the mutilating D. ceylonense colonies and the non mutilating ‘nilgiri’ colonies. ‘nilgiri’ callows were not mutilated even in D. ceylonense colonies while D. ceylonense callows were mutilated even in ‘nilgiri’ colonies, suggesting that the cues for mutilation originate in the victims (callows), presumably in the gemmae themselves. This finding should facilitate understanding the proximate mechanism and evolutionary significance of mutilation of gemmae as a method of resolution of reproductive conflicts in the genus Diacamma.