Christophe Gadenne

Plasticity of olfactory-guided behaviour and its neurobiological basis: lessons from moths and locusts

The sense of smell plays an important role in guiding the behaviour of many animals including insects. The attractiveness of a volatile is not only dependent on the nature of the chemical, but might change with the physiological status (e.g., age/hormone or mating status) or environmental conditions (e.g., photoperiod or temperature) of the individual. Here we summarize our studies focused on the plasticity of olfactory‐guided behaviour and its neurobiological basis linked with the physiological status in Lepidoptera and migratory locusts. In moths and locusts, age and juvenile hormone changed the behavioural responses to pheromones. In moths, mating had an effect on pheromone responses in males and plant odour responses in females. In all cases of behavioural plasticity studied, we found changes in the sensitivity of olfactory interneurons in the antennal lobe, whereas the peripheral system does not seem to show any plasticity in that context. Changes in the central nervous system were slow under the influence of juvenile hormone (days) or fast after mating (minutes). The olfactory system seems thus to adapt to the physiological or environmental situation of an animal to avoid a waste of energy. We discuss possible mechanisms underlying the observed plasticity.

Hormonal control of pheromone responsiveness in the male black cutworm Agrotis ipsilon

In Lepidoptera, reproduction is linked to chemical communication between conspecific partners. When exposed to the female sex pheromone, males respond by exhibiting typical sexual behaviour which leads to mating. Here we show that presence of the juvenile hormone producing gland (corpora allata) of the male black cutworm,Agrotis ipsilon, is necessary for pheromone responsiveness. Allatectomized males do not show any sexual behaviour, although their antennal olfactory system is functional. Allatectomized males implanted with active corpora allata recover full pheromone receptivity. It is suggested that reproductive processes are synchronized in males and females through endocrine control; timing of the mating activity could serve as an adaptive strategy linked to the migratory behaviour of this species.

Transient post-mating inhibition of behavioural and central nervous responses to sex pheromone in an insect

Mating is costly for both male and female insects and should therefore only occur if it is likely to be successful. Within one scotophase, which is the dark period of the light cycle, male moths can only produce one single spermatophore, which is transferred to the female during mating. Remating within the same scotophase would thus be unsuccessful. We tested the hypothesis that newly mated males of the moth Agrotis ipsilon have developed an energy-saving strategy based on the transient inhibition of their sexual behaviour, thus avoiding unsuccessful remating. Agrotis ipsilon males do not copulate more than once during the same scotophase. Moreover, newly mated males do not respond behaviourally to the female sex pheromone although electroantennograms showed that their peripheral olfactory system is fully functional. However, intracellular recordings of antennal lobe neurons showed that the sensitivity for the synthetic sex pheromone blend decreased as compared with that of unmated males. Both the sexual behaviour and the sensitivity of the antennal lobe neurons were restored when tested during the next scotophase. Our results show a fast, transient neuronal plasticity that ‘switches off’ the olfactory system, which could prevent males from mating unsuccessfully.

Central processing of sex pheromone stimuli is differentially regulated by juvenile hormone in a male moth

In the male moth, Agrotis ipsilon, the neuronal basis for juvenile hormone (JH)-linked modulation of sex pheromone responsiveness was investigated following stimulation of the antenna with i) an extract of female pheromone gland, ii) the synthetic pheromone blends from A. ipsilon and a closely related species, A. segetum, and iii) single components of the A. ipsilon blend. Response characteristics of olfactory interneurons were studied in the antennal lobe (AL) at different ages and with manipulated JH levels using intracellular recording techniques. Blend-specific, generalist and component-specific neurons were identified and described according to their response pattern. The proportion of low threshold AL interneurons increased significantly with age for all stimuli tested. Changes were, however, less pronounced for the minor single components. The proportion of low threshold AL interneurons in allatectomized (JH-deprived) mature males was significantly lower for all stimuli than in intact mature males. A large proportion of low threshold AL interneurons responding to the pheromone blends, but not as pronounced for single pheromone components, could be restored/induced by injecting JH either into JH-deprived mature males or into young immature males. The specificity for the species-specific blend compared to the A. segetum blend increased with age and JH injections.

Switching attraction to inhibition: mating-induced reversed role of sex pheromone in an insect.

SUMMARY In the moth, Agrotis ipsilon, newly mated males cease to be attracted to the female-produced sex pheromone, preventing them from re-mating until the next night, by which time they would have refilled their reproductive glands for a potential new ejaculate. The behavioural plasticity is accompanied by a decrease in neuron sensitivity within the primary olfactory centre, the antennal lobe (AL). However, it was not clear whether the lack of the sexually guided behaviour results from the absence of sex pheromone detection in the ALs, or if they ignore it in spite of detection, or if the sex pheromone itself inhibits attraction behaviour after mating. To test these hypotheses, we performed behavioural tests and intracellular recordings of AL neurons to non-pheromonal odours (flower volatiles), different doses of sex pheromone and their mixtures in virgin and newly mated males. Our results show that, although the behavioural and AL neuron responses to flower volatiles alone were similar between virgin and mated males, the behavioural response of mated males to flower odours was inhibited by adding pheromone doses above the detection threshold of central neurons. Moreover, we show that the sex pheromone becomes inhibitory by differential central processing: below a specific threshold, it is not detected within the AL; above this threshold, it becomes inhibitory, preventing newly mated males from responding even to plant odours. Mated male moths have thus evolved a strategy based on transient odour-selective central processing, which allows them to avoid the risk-taking, energy-consuming search for females and delay re-mating until the next night for a potential new ejaculate.

Evolution of noctuid pheromone binding proteins: identification of PBP in the black cutworm moth, Agrotis ipsilon

Male black cutworm moths (Agrotis ipsilon, Lepidoptera, Noctuoidea, Noctuidae), which are attracted by a three-component pheromone blend ((Z)-7-dodecenyl acetate, Z7-12:Ac; (Z)-9-tetradecenyl acetate, Z9-14:Ac; (Z)-11-hexadecenyl acetate, Z11-16:Ac), express diverse antennal pheromone binding proteins (PBPs). Two PBP isoforms (Aips-1 and Aips-2) that show 46% identity were cloned from antennal cDNA of male A. ipsilon. The protein Aips-1 displays a high degree of identity (70-95%) with PBPs of other noctuiids, but shows only 42-65% identity with the PBPs of more phylogenetically distant species. The other protein, Aips-2, represents a distinct group of PBP that includes proteins from Sphingidae and Yponomeutidae. These differences observed suggest that each of the two PBPs may be tuned to a specific pheromone ligand.

Sex Pheromone of the French Black Cutworm Moth, Agrotis ipsilon (Lepidoptera: Noctuidae): Identification and Regulation of a Multicomponent Blend

The sex pheromone blend of a European strain of the black cutworm moth, Agrotis ipsilon (Hufnagel), was investigated. Chemical analyses of pheromone gland extracts from 4- to 8-day-old females showed that individual isolated glands contained only very small amounts of pheromone. High-resolution gas chromatography combined with mass spectrometry (GC–MS) analyses showed the presence of cis-7-dodecenyl acetate (Z7-12:Ac), cis-9-tetradecenyl acetate (Z9-14:Ac), cis-11-tetradecenyl acetate (Z11-14:Ac), cis-11-hexadecenyl acetate (Z11-16:Ac), and cis-11-hexadecenol (Z11-16:OH) in biologically active pheromone gland extracts. Removing 27-12: Ac, Z9-14:Ac, or Z11-16:Ac from the complete gland extract by GC trapping techniques strongly reduced the attractiveness of the pheromone blend tested in a wind tunnel. Lack of cis-5-decenyl acetate (Z5-10:Ac) or Z11-16:OH did not affect the blend attractiveness. Chemical and behavioral analyses showed that pheromone components are produced during photophase, at least 2 hr before lights off. Quantitative data showed that decapitation inhibited the production of Z7-12:Ac, Z9-14:Ac, Z11-14:Ac, Z11-16:Ac, and Z11-16:OH, but production in decapitated females was stimulated in response to injection of synthetic Heliothis zea PBAN (pheromone biosynthesis activating neuropeptide) or A. ipsilon brain–subesophageal (Br-SEG) homogenates. Moreover, upon injecting BR-SEG homogenates, other minor components were detected, which were tentatively identified as cis-8-dodecenyl actetate (Z8-12:Ac) and Z5-10:Ac. Our study demonstrated that Z11-l6:Ac is one of the main active components produced by the pheromone gland of this European population of A. ipsilon, in addition to Z7-12:Ac/Z9-14:Ac, which were investigated in a previous behavioral analysis. All these data strongly suggest that some polymorphism is present in pheromone communication in different strains of A. ipsilon.

Effects of fenoxycarb, juvenile hormone mimetic, on female sexual behaviour of the black cutworm, Agrotis ipsilon (Lepidoptera: Noctuidae)

Abstract Topical applications of fenoxycarb on virgin females of Agrotis ipsilon induced a precocious onset of calling and mating behaviour, accelerated ovarian development and oviposition. Allatectomy performed on the day of emergence inhibited ovarian maturation, calling and mating. Allatectomized females treated on day 1 with a topical application of fenoxycarb reinitiated calling and mating. Ovariectomy performed on last-instar larvae did not disturb mating. Ovarian development was induced in decapitated females treated with fenoxycarb on day 0 after emergence. Females treated with fenoxycarb laid fertile eggs in equal number as control females. The role of juvenile hormone in the control of sexual behaviour and reproduction of A. ipsilon is discussed

Attraction of the grapevine moth to host and non-host plant parts in the wind tunnel: effects of plant phenology, sex, and mating status

In the European grapevine moth, Lobesia botrana (Denis and Schiffermüller) (Lepidoptera: Tortricidae), it has recently been shown that volatiles emitted from the main host plant, grapevine, are attractive to adult females. Here, using wind tunnel experiments, we tested the attractiveness of various grapevine, Vitis vinifera L. (Vitaceae), plant parts at different phenological stages, including ripe berries infested with the pathogenic fungus Botrytis cinerea (Persoon: Fries) (Sclerotiniaceae) to laboratory‐reared virgin and mated male and female moths. We also tested the attractiveness of the non‐host plant, tansy [Tanacetum vulgare (L.) (Asteraceae)], as flowers and flower volatiles were previously shown to be attractive to L. botrana females in the field. Mated female moths were the only adults to exhibit upwind orientation to grape plant parts in the flight tunnel. The most attractive parts of the grapevine plants were leaves, flower buds, and ripe berries. No attraction was observed to flowers. This corresponds to the phenological stages available when females of the 2–3 generations during a year are present in the field. No attraction was observed to leaves and flowers of tansy.

Adult neurogenesis in a moth brain

In both vertebrates and invertebrates, neurogenesis not only occurs during development but also persists throughout adult life. So far, adult neurogenesis has been detected in a few insect orders but not in Lepidoptera. In the moth, Agrotis ipsilon, the sensitivity of antennal lobe interneurons of males to sex pheromone is age‐ and juvenile hormone‐dependent in accordance with changes in the behavioral response. As a first step to understand this neuronal plasticity, we tested the hypothesis that adult neurogenesis could occur in the moth brain using 5‐bromo‐2′‐deoxyuridine injections on newly born and sexually mature moths. Our results show that persistent neurogenesis occurs in mushroom bodies of adult males and females of A. ipsilon. Two clusters of one to four neuroblasts in each brain hemisphere continue to divide in adult moths and give rise to small clusters of Kenyon cells in the cortex of the mushroom body calyces. Neurogenesis was observed in both newly born and sexually mature males. There was a clear increase in the number of newly born cells in brains as the time increased after the treatment that was performed soon after emergence. When treatments were performed in mature 3‐day‐old adults, neurogenesis was still detected in brains dissected 3 hours after treatment but was hardly visible 2 days later. Adult neurogenesis was also detected in the optic lobes but not in the antennal lobes. We hypothesize that the newly born neurons could play a role in the central nervous plasticity of olfactory processing in the adult moth A. ipsilon. J. Comp. Neurol. 495:635–643, 2006.