Frédéric Marion-Poll

High Genetic Variability of Herbivore-Induced Volatile Emission within a Broad Range of Maize Inbred Lines

Maize plants (Zea mays) attacked by caterpillars release a mixture of odorous compounds that attract parasitic wasps, natural enemies of the herbivores. We assessed the genetic variability of these induced volatile emissions among 31 maize inbred lines representing a broad range of genetic diversity used by breeders in Europe and North America. Odors were collected from young plants that had been induced by injecting them with caterpillar regurgitant. Significant variation among lines was found for all 23 volatile compounds included in the analysis: the lines differed enormously in the total amount of volatiles emitted and showed highly variable odor profiles distinctive of each genotype. Principal component analysis performed on the relative quantities of particular compounds within the blend revealed clusters of highly correlated volatiles, which may share common metabolic pathways. European and American lines belonging to established heterotic groups were loosely separated from each other, with the most clear-cut difference in the typical release of (E)-β-caryophyllene by European lines. There was no correlation between the distances among the lines based on their odor profiles and their respective genetic distances previously assessed by neutral RFLP markers. This most comprehensive study to date on intraspecific variation in induced odor emission by maize plants provides a further example of the remarkably high genetic diversity conserved within this important crop plant. A better understanding of the genetic control of induced odor emissions may help in the development of maize varieties particularly attractive to parasitoids and other biological control agents and perhaps more repellent for herbivores.

Differentiated response to sugars among labellar chemosensilla in Drosophila.

Abstract Recent findings have indicated that the Gr genes for putative gustatory receptors of Drosophila melanogaster are expressed in a spatially restricted pattern among chemosensilla on the labellum. However, evidence for a functional segregation among the chemosensilla is lacking. In this work, labellar chemosensilla were classified and numbered into three groups, L-, I- and S-type, based on their morphology. Electrophysiological responses to sugars and salt were recorded from all the accessible labellar chemosensilla by the tip-recording method. All the L-type sensilla gave good responses to sugars in terms of action potential firing rates, while the probability for successful recordings from the I-type and S-type sensilla was lower. No differences were found in the responses to sugars between chemosensilla belonging to the same type; however, dose-response curves for several different sugars varied among the sensilla types. The L-type sensilla gave the highest frequency of nerve responses to all the sugars. The I-type sensilla also responded to all the sugars but with a lower magnitude of firing rate than the L-type sensilla. The S-type sensilla gave a good response to sucrose, and lower responses to the other sugars. These results suggest that there might be variations in the expression level or pattern of multiple receptors for sugars among the three types of chemosensilla. The expression pattern of six Gr genes was examined using the Gal4/UAS-GFP system, and sensilla were identified according to the innervation pattern of each GFP-expressing taste cell. None of the spatial expression patterns of the six Gr genes corresponded to the sugar sensitivity differences we observed.

Parallel Reinforcement Pathways for Conditioned Food Aversions in the Honeybee

Summary Avoiding toxins in food is as important as obtaining nutrition. Conditioned food aversions have been studied in animals as diverse as nematodes and humans [1, 2], but the neural signaling mechanisms underlying this form of learning have been difficult to pinpoint. Honeybees quickly learn to associate floral cues with food [3], a trait that makes them an excellent model organism for studying the neural mechanisms of learning and memory. Here we show that honeybees not only detect toxins but can also learn to associate odors with both the taste of toxins and the postingestive consequences of consuming them. We found that two distinct monoaminergic pathways mediate learned food aversions in the honeybee. As for other insect species conditioned with salt or electric shock reinforcers [4–7], learned avoidances of odors paired with bad-tasting toxins are mediated by dopamine. Our experiments are the first to identify a second, postingestive pathway for learned olfactory aversions that involves serotonin. This second pathway may represent an ancient mechanism for food aversion learning conserved across animal lineages.

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.

Identification of Floral Volatiles Involved in Recognition of Oilseed Rape Flowers, Brassica napus by Honeybees, Apis mellifera

Volatiles from oilseed rape, Brassica napus, flowers were sampled by air entrainment and their relevance to the natural odor profile of the flowers was confirmed by conditioned proboscis extension (CPE) assays with honeybee, Apis mellifera L., foragers. Coupled gas chromatography (GC)-CPE analysis of the air entrainment samples was used to locate key compounds involved in the recognition of B. napus flowers, and the compounds were then identified using coupled gas chromatography-mass spectrometry and comparison with authentic samples. Six regions of the gas chromatograms elicited CPE responses from bees previously conditioned to the total extract, and from these areas 16 compounds were identified that elicited CPE activity from conditioned bees when tested with synthetic samples. Eight of the 16, α-pinene, phenylacetaldehyde, p-cymene, α-terpinene, linalool, 2-phenyl-ethanol, (E,E)-α-farnesene, and 3-carene, gave the highest responses. When the bees were conditioned to the total extract of flower volatiles, a mixture of the eight components elicited responses from 83% of the individuals, suggesting that the eight-component mixture accounted for a major part of the CPE activity of the total extract. In addition, a mixture of the three most active compounds, phenylacetaldehyde, linalool, and (E,E,)-α-farnesene, evoked responses from 85% of the bees after the latter had been conditioned to the eight-component mixture. Thus, these three compounds appear to play a key role in the recognition of the eight component mixture and, by inference, of oilseed rape flowers.

Temporal coding of pheromone pulses and trains in Manduca sexta

SummaryWe investigated the ability of pheromone-sensitive olfactory receptors of male Manduca sexta to respond to 20-ms pulses of bombykal, the major component of the conspecific pheromonal blend. Isolated pulses of bombykal elicited a burst of activity which decreased exponentially with a time constant of 160–250 ms. Trains of pulses delivered at increasing frequencies (0.5–10 Hz) elicited temporally modulated responses at up to 3 Hz. Concentration of the stimulus (1, 10, 100 ng per odor source) had a marginal effect on the temporal resolution of the receptors. Within a train, the responses to individual pulses remained constant, except for 10-Hz trains (short-term adaptation). A dose-dependent decline of responsiveness was observed during experiments (long-term adaptation). Although individual neurons may not respond faithfully to each pulse of a train, the population of receptors sampled in this study appears to be capable of encoding the onset of odor pulses at frequencies of up to at least 3 Hz.

Sex-specific non-pheromonal taste receptors in Drosophila

Taste receptors have recently been reported in Drosophila [1,2], but little is known of the relation between receptor and response. Morphological studies of the distribution of chemosensory sensilla indicate that the fruit fly has two major sites of gustation: the proboscis and the legs [3]. The taste sensilla on both these sites are similar in structure and each sensillum generally houses four gustatory neurons [4]. Early anatomical observations have demonstrated a sexual dimorphism in the number of tarsal sensilla [5] and in their central projections [6]. We measured the electrophysiological responses of the prothoracic taste sensilla to non-pheromonal substances--salts, sugars and water--and found a clear sexual dimorphism. From the response profile of individual sensilla, we were able to distinguish three types of tarsal sensilla in females as against only two types in males. The female-specific type, which responded specifically to sugar, was absent in males except when male gustatory neurons were genetically feminised. The fact that tarsal gustatory hairs exhibit a sexual dimorphism that affects the perception of non-pheromonal compounds suggests that sexual identity is more complex than has previously been thought [7,8].

Un-filtered recordings from insect taste sensilla

Electrophysiological recording from insect taste receptors has become a classical technique since the work of Hodgson et al. (1955). It consists of recording the electrical activity of sensilla as soon as they contact an electrolyte. Although such recordings lack information about the unstimulated and poststimulated state of the taste receptors, no alternative technique has yet emerged. Side-wall recordings (Morita & Yamashita, 1956) or tungsten recordings from the base of the hairs have been obtained but they necessitate both an adequate insect preparation and delicate manual skills; in addition, the preparations tend to deteriorate rapidly. Amplifiers designed for Hodgson type recordings must cope with two technical constraints. First, action potentials recorded extra-cellularly from insect sensilla have a low amplitude level and must be amplified to allow good recordings and reliable analysis. Secondly, taste sensilla exhibit a large DC offset relative to the ground, which would saturate the amplifier at such amplification factors. This problem is usually approached by using a high-pass filter (de Kramer & van der Molen, 1980; Frazier & Hanson, 1986) or by manually compensating for the DC signal (G6dde & Krefting, 1989; Schnuch & Hansen, 1990, 1992). Both approaches have limitations. In the first case, strong filtering modifies the spike waveforms and smoothes out baseline variations. This makes spike separation more difficult. The second approach requires recording the DC offset before measuring the response. We have designed a new amplifier, TastePROBE, that permits reliable DC recordings with the Hodgson technique. This new amplifier performs three functions: contact detection, DC compensation and amplification.

Taste detection of phytoecdysteroids in larvae of Bombyx mori, Spodoptera littoralis and Ostrinia nubilalis.

A number of plants produce significant amounts of phytoecdysteroids that can disrupt the hormonal levels of insects feeding upon them. Insects equipped with taste receptors sensitive to phytoecdysteroids are able to avoid such plants. How common is this strategy? By recording from the lateral and medial sensilla styloconica in two polyphagous species (Ostrinia nubilalis and Spodoptera littoralis) and in a monophagous species (Bombyx mori), we tested whether the receptors could detect three commonly occurring phytoecdysteroids 20-hydroxyecdysone (20E), ponasterone A (ponA) and ecdysone (E). In B. mori, 20E and ponA elicited dose-dependent responses with a threshold of 1 &mgr;M only in the medial sensilla. In O. nubilalis, 20E, E and ponA elicited responses at threshold of 1 &mgr;M in both sensilla. In S. littoralis, 20E elicited responses with a threshold of 10 &mgr;M in both sensilla. By means of behavioural choice tests, we show that 20E is an effective feeding deterrent for O. nubilalis and S. littoralis first instar larvae. This suggests that the perception of phytoecdysteroids is more common among phytophagous lepidoptera than previously thought, although their toxicity or antifeedancy varies between species.

Object-oriented approach to fast display of electrophysiological data under MS-Windows™

Microcomputers provide neuroscientists an alternative to a host of laboratory equipment to record and analyze electrophysiological data. Object-oriented programming tools bring an essential link between custom needs for data acquisition and analysis with general software packages. In this paper, we outline the layout of basic objects that display and manipulate electrophysiological data files. Visual inspection of the recordings is a basic requirement of any data analysis software. We present an approach that allows flexible and fast display of large data sets. This approach involves constructing an intermediate representation of the data in order to lower the number of actual points displayed while preserving the aspect of the data. The second group of objects is related to the management of lists of data files. Typical experiments designed to test the biological activity of pharmacological products include scores of files. Data manipulation and analysis are facilitated by creating multi-document objects that include the names of all experiment files. Implementation steps of both objects are described for an MS-Windows hosted application.