Juvenile-hormone-mediated plasticity of aggregation behaviour and olfactory processing in adult desert locusts

Pheromone representation in the ant antennal lobe changes with age

While the neural basis of age-related decline has been extensively studied,1,2,3 less is known about changes in neural function during the pre-senescent stages of adulthood. Adult neural plasticity is likely a key factor in social insect age polyethism, where individuals perform different tasks as they age and divide labor in an age-dependent manner.4,5,6,7,8,9 Primarily, workers transition from nursing to foraging tasks,5,10 become more aggressive, and more readily display alarm behavior11,12,13,14,15,16 as they get older. While it is unknown how these behavioral dynamics are neurally regulated, they could partially be generated by altered salience of behaviorally relevant stimuli.4,6,7 Here, we investigated how odor coding in the antennal lobe (AL) changes with age in the context of alarm pheromone communication in the clonal raider ant (Ooceraea biroi).17 Similar to other social insects,11,12,16 older ants responded more rapidly to alarm pheromones, the chemical signals for danger. Using whole-AL calcium imaging,18 we then mapped odor representations for five general odorants and two alarm pheromones in young and old ants. Alarm pheromones were represented sparsely at all ages. However, alarm pheromone responses within individual glomeruli changed with age, either increasing or decreasing. Only two glomeruli became sensitized to alarm pheromones with age, while at the same time becoming desensitized to general odorants. Our results suggest that the heightened response to alarm pheromones in older ants occurs via increased sensitivity in these two core glomeruli, illustrating the importance of sensory modulation in social insect division of labor and age-associated behavioral plasticity.

Pheromone representation in the ant antennal lobe changes with age

While the neural basis of age-related decline has been extensively studied (1-3), less is known about changes in neural function during the pre-senescent stages of adulthood. Adult neural plasticity is likely a key factor in social insect age polyethism, where individuals perform different tasks as they age and divide labor in an age-dependent manner (4-9). Primarily, workers transition from nursing to foraging tasks (5, 10), become more aggressive, and more readily display alarm behavior (11-16) as they get older. While it is unknown how these behavioral dynamics are neurally regulated, they could partially be generated by altered salience of behaviorally relevant stimuli (4, 6, 7). Here, we investigated how odor coding in the antennal lobe (AL) changes with age in the context of alarm pheromone communication in the clonal raider ant (Ooceraea biroi) (17). Similar to other social insects (11, 12, 16), older ants responded more rapidly to alarm pheromones, the chemical signals for danger. Using whole-AL calcium imaging (18), we then mapped odor representations for five general odorants and two alarm pheromones in young and old ants. Alarm pheromones were represented sparsely at all ages. However, alarm pheromone responses within individual glomeruli changed with age, either increasing or decreasing. Only two glomeruli became sensitized to alarm pheromones with age, while at the same time becoming desensitized to general odorants. Our results suggest that the heightened response to alarm pheromones in older ants occurs via increased sensitivity in these two core glomeruli, illustrating the importance of sensory modulation in social insect division of labor and age-associated behavioral plasticity.

Innate attraction and aversion to odors in locusts

Many animals display innate preferences for some odors, but the physiological mechanisms underlying these preferences are poorly understood. Here, with behavioral tests, we establish a model system well suited to investigating olfactory mechanisms, the locust Schistocerca americana. We conducted open field tests in an arena designed to provide only olfactory cues to guide navigation choices. We found that newly hatched locusts navigated toward, and spent more time near, the odor of wheat grass than humidified air. In similar tests, we found that hatchlings avoided moderate concentrations of major individual components of the food blend odor, 1-hexanol (1% v/v) and hexanal (0.9% v/v) diluted in mineral oil relative to control presentations of unscented mineral oil. Hatchlings were neither attracted nor repelled by a lower concentration (0.1% v/v) of 1-hexanol but were moderately attracted to a low concentration (0.225% v/v) of hexanal. We quantified the behavior of the animals by tracking their positions with the Argos software toolkit. Our results establish that hatchlings have a strong, innate preference for food odor blend, but the valence of the blend's individual components may be different and may change depending on the concentration. Our results provide a useful entry point for an analysis of physiological mechanisms underlying innate sensory preferences.

Sex- and maturity-dependent antennal detection of host plant volatiles in the cabbage root fly, Delia radicum

Adult insect behaviour in response to plant-emitted volatile compounds varies between the sexes and as a function of maturity. These differences in behavioural responses can be due to modulation in the peripheral or central nervous system. In the cabbage root fly, Delia radicum, behavioural effects of certain host plant volatiles on mature female behaviour have been evaluated, and a large number of compounds emitted by brassicaceous host plants have been identified. We recorded here dose-dependent electroantennogram responses to all tested compounds and investigated if the antennal detection of individual volatile compounds emitted by intact and damaged host plants differs between male and female, as well as immature and mature flies. Our results showed dose-dependent responses in mature and immature males and females. Mean response amplitudes varied significantly between sexes for three compounds, and between maturity states for six compounds. For some additional compounds significant differences occurred only for high stimulus doses (interaction between dose and sex and/or dose and maturity status). Multivariate analysis revealed a significant global effect of maturity on electroantennogram response amplitudes and for one experimental session also a significant global effect of the sex. Interestingly, allyl isothiocyanate, a compound stimulating oviposition behaviour, elicited stronger responses in mature than in immature flies, whereas ethylacetophenone, an attractive flower volatile, elicited stronger responses in immature than in mature flies, which correlates with the behavioural role of these compounds. Several host-derived compounds elicited stronger responses in females than in males and, at least at high doses, stronger responses in mature than in immature flies, indicating differential antennal sensitivity to behaviourally active compounds. Six compounds did not cause any significant differences in responses between the different groups of flies. Our results thus confirm peripheral plasticity in plant volatile detection in the cabbage root fly and provide a basis for future behavioural investigations on the function of individual plant compounds.

Phenotypic and Differential Gene Expression Analyses of Phase Transition in Oedaleus asiaticus under High-Density Population Stress

The high-density-dependent phase change from solitary to gregarious individuals in locusts is a typical example of phenotypic plasticity. However, the underlying molecular mechanism is not clear. In this study, first, Oedaleus asiaticus were treated with high-density population stress and then analyzed by Illumina sequencing on days 1, 3, 5, and 7 of the body color change to identify the stage-specific differentially expressed genes (DEGs). The KEGG pathway enrichment analysis of the identified DEGs revealed their role in metabolic pathways. Furthermore, the expression patterns of the nine key DEGs were studied in detail; this showed that the material change in locusts began on the third day of the high-density treatment, with the number of DEGs being the largest, indicating the importance of this period in the phase transition. In addition, the phenotypic change involved several key genes of important regulatory pathways, possibly working in a complex network. Phenotypic plasticity in locusts is multifactorial, involving multilevel material network interactions. This study improves the mechanistic understanding of phenotypic variation in insects at the genetic level.

Effects of Age, Phase Variation and Pheromones on Male Sperm Storage in the Desert Locust, Schistocerca gregaria

Simple Summary

We investigated male sperm storage in the desert locust Schistocerca gregaria. Phase (solitary or gregarious) did not affect sperm distribution in the vas deferens and seminal vesicle, whereas sperm accumulation of the seminal vesicle in gregarious locusts was promoted more than in solitary ones. Pheromones received from neither mature adults nor nymphs affected sperm distribution in the vas deferens and seminal vesicle. However, sperm accumulation in the seminal vesicle was more promoted in the gregarious locusts which received pheromones from mature adults than those obtained from nymphs at early adult stage, especially seven days after adult emergence.

Abstract

In general, sperm produced in the testis are moved into the seminal vesicle via the vas deferens in insects, where they are stored. How this sperm movement is controlled is less well understood in locusts or grasshoppers. In this study, the effects of age, phase variation and pheromones on male sperm storage were investigated in the desert locust, Schistocerca gregaria (Forskål). In this locust, a pair of ducts, the vasa deferentia, connect the testes to a pair of the long, slender seminal vesicles that are folded approximately thirty times, and where the sperm are stored. We found that phase variation affected the level of sperm storage in the seminal vesicle. Moreover, adult males that detected pheromones emitted by mature adult males showed enhanced sperm storage compared with males that received the pheromones emitted from nymphs: The former, adult male pheromones are known to promote sexual maturation of immature adults of both sexes, whereas the latter, nymphal pheromones delay sexual maturation. Most mature adult males had much sperm in the vasa deferentia at all times examined, suggesting daily sperm movement from the testes to the seminal vesicles via the vasa deferentia. As adult males aged, sperm were accumulated from the proximal part to the distal end of the seminal vesicle. Many sperm remained in the seminal vesicle after mating. These results suggest that young or new sperm located near the proximal part of the seminal vesicle could be used for mating, whereas old sperm not used for mating are stored in the distal part of the seminal vesicle.

OBP14 (Odorant-Binding Protein) Sensing in Adelphocoris lineolatus Based on Peptide Nucleic Acid and Graphene Oxide

OBPs play a crucial role in the recognition of ligands and are involved in the initial steps of semiochemical perception. The diverse expression of OBP genes allows them to participate in different physiological functions in insects. In contrast to classic OBPs with typical olfactory roles in A. lineolatus, the physiological functions of Plus-C OBPs remain largely unknown. In addition, detection of the expression of insect OBP genes by conventional methods is difficult in vitro. Here, we focused on AlinOBP14, a Plus-C OBP from A. lineolatus, and we developed a PNA-GO-based mRNA biosensor to detect the expression of AlinOBP14. The results demonstrated that AlinOBP14 plays dual roles in A. lineolatus. The AlinOBP14 is expressed beneath the epidermis of the vertex and gena in heads of A. lineolatus, and it functions as a carrier for three terpenoids, while AlinOBP14 is also expressed in the peripheral antennal lobe and functions as a carrier for endogenous compounds such as precursors for juvenile hormone (JH) and JHⅢ. Our investigation provides a new method to detect the expression of OBP genes in insects, and the technique will facilitate the use of these genes as potential targets for novel insect behavioral regulation strategies against the pest.

Development of the Neurochemical Architecture of the Central Complex

The central complex represents one of the most conspicuous neuroarchitectures to be found in the insect brain and regulates a wide repertoire of behaviors including locomotion, stridulation, spatial orientation and spatial memory. In this review article, we show that in the grasshopper, a model insect system, the intricate wiring of the fan-shaped body (FB) begins early in embryogenesis when axons from the first progeny of four protocerebral stem cells (called W, X, Y, Z, respectively) in each brain hemisphere establish a set of tracts to the primary commissural system. Decussation of subsets of commissural neurons at stereotypic locations across the brain midline then establishes a columnar neuroarchitecture in the FB which is completed during embryogenesis. Examination of the expression patterns of various neurochemicals in the central complex including neuropeptides, a neurotransmitter and the gas nitric oxide (NO), show that these appear progressively and in a substance-specific manner during embryogenesis. Each neuroactive substance is expressed by neurons located at stereotypic locations in a given central complex lineage, confirming that the stem cells are biochemically multipotent. The organization of axons expressing the various neurochemicals within the central complex is topologically related to the location, and hence birthdate, of the neurons within the lineages. The neurochemical expression patterns within the FB are layered, and so reflect the temporal topology present in the lineages. This principle relates the neuroanatomical to the neurochemical architecture of the central complex and so may provide insights into the development of adaptive behaviors.

Plasticity in Insect Olfaction: To Smell or Not to Smell?

In insects, olfaction plays a crucial role in many behavioral contexts, such as locating food, sexual partners, and oviposition sites. To successfully perform such behaviors, insects must respond to chemical stimuli at the right moment. Insects modulate their olfactory system according to their physiological state upon interaction with their environment. Here, we review the plasticity of behavioral responses to different odor types according to age, feeding state, circadian rhythm, and mating status. We also summarize what is known about the underlying neural and endocrinological mechanisms, from peripheral detection to central nervous integration, and cover neuromodulation from the molecular to the behavioral level. We describe forms of olfactory plasticity that have contributed to the evolutionary success of insects and have provided them with remarkable tools to adapt to their ever-changing environment.

Proteomic analysis reveals that COP9 signalosome complex subunit 7A (CSN7A) is essential for the phase transition of migratory locust

The migratory locust displays a reversible, density-dependent transition between the two phases of gregaria and solitaria. This phenomenon is a typical kind of behavior plasticity. Here, we report that COP9 signalosome complex subunit 7A (CSN7A) is involved in the regulation of locust phase transition. Firstly, 90 proteins were identified to express differentially between the two phases by quantitative proteomic analysis. Gregaria revealed higher levels in proteins related to structure formation, melanism and energy metabolism, whereas solitaria had more abundant proteins related to digestion, absorption and chemical sensing. Subsequently, ten proteins including CSN7A were found to reveal differential mRNA expression profiles between the two phases. The CSN7A had higher mRNA level in the gregaria as compared with the solitaria, and the mRNA amount in the gregaria decreased remarkably during the 32 h-isolation. However, the mRNA level in the solitaria kept constant during the crowding rearing. Finally and importantly, RNA interference of CSN7A in gregaria resulted in obvious phase transition towards solitaria within 24 h. It suggests that CSN7A plays an essential role in the transition of gregaria towards solitaria in the migratory locust. To our knowledge, it's the first time to report the role of CSN in behavior plasticity of animals.

Phenotypic plasticity in a willow leaf beetle depends on host plant species: release and recognition of beetle odors

Aggregation behavior of herbivorous insects is mediated by a wide range of biotic and abiotic factors. It has been suggested that aggregation behavior of the blue willow leaf beetle Phratora vulgatissima is mediated by both host plant odor and by odor released by the beetles. Previous studies show that the beetles respond to plant odors according to their prior host plant experiences. Here, we analyzed the effect of the host plant species on odor released and perceived by adult P. vulgatissima. The major difference between the odor of beetles feeding on salicin-rich and salicin-poor host plants was the presence of salicylaldehyde in the odor of the former, where both males and females released this compound. Electrophysiological studies showed that the intensity of responses to single components of odor released by beetles was sex specific and dependent on the host plant species with which the beetles were fed. Finally, behavioral studies revealed that males feeding on salicin-rich willows were attracted by salicylaldehyde, whereas females did not respond behaviorally to this compound, despite showing clear antennal responses to it. Finally, the ecological relevance of the influence of a host plant species on the plasticity of beetle odor chemistry, perception, and behavior is discussed.

Electrophysiological responses of the rice leaffolder, Cnaphalocrocis medinalis, to rice plant volatiles

The rice leaffolder, Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae), is one of the most destructive pests of rice. Electrophysiological responses of this species to 38 synthetic volatiles known to be released from rice plants (Poaceae: Oryza spp.) were studied using the electroantennogram (EAG) method. Compounds that elicited the strongest EAG responses for each physiological condition were selected for EAG dose-response tests at five concentrations. These compounds included: methyl salicylate, heptanol, linalool, cyclohexanol, and 2-heptanone for one-day-old male moths; heptanol, hexanal, (Z)-2-hexen-1-ol, and nonadecane for one-day- old females; methyl salicylate, heptanol, (E)-2-hexen-1-ol, and (Z)-2-hexen-1-ol for three-day- old males; linalool, heptanol, (E)-2-hexen-1-ol, 2-heptanone, and hexanal for three-day-old females; 2-heptanone, cyclohexanol, linalool, heptanol, and methyl salicylate for five-day-old virgin females; and methyl benzoate, (Z)-2-hexen-1-ol, heptanol, linalool, and hexanal for five- day-old mated females. Female and male C. medinalis exhibited broad overlap in their EAG responses, and there was no clear difference between male and female EAG responses to different compounds. Statistical analyses revealed that both volatile compound chemical structure and C. medinalis physiological condition (age, sex, and mating condition) had an effect on EAG response.

Molecular mechanisms of phase change in locusts

Phase change in locusts is an ideal model for studying the genetic architectures and regulatory mechanisms associated with phenotypic plasticity. The recent development of genomic and metabolomic tools and resources has furthered our understanding of the molecular basis of phase change in locusts. Thousands of phase-related genes and metabolites have been highlighted using large-scale expressed sequence tags, microarrays, high-throughput transcriptomic sequences, or metabolomic approaches. However, only several key factors, including genes, metabolites, and pathways, have a critical role in phase transition in locusts. For example, CSP (chemosensory protein) and takeout genes, the dopamine pathway, protein kinase A, and carnitines were found to be involved in the regulation of behavioral phase change and gram-negative bacteria-binding proteins in prophylaxical disease resistance of gregarious locusts. Epigenetic mechanisms including small noncoding RNAs and DNA methylation have been implicated. We review these new advances in the molecular basis of phase change in locusts and present some challenges that need to be addressed.

The transcription factor Krüppel homolog 1 is linked to the juvenile hormone-dependent maturation of sexual behavior in the male moth, Agrotis ipsilon

In the male moth, Agrotis ipsilon, the behavioral response and neuronal sensitivity in the primary olfactory center, the antennal lobe (AL), to sex pheromone increase with age and juvenile hormone (JH) biosynthesis. Although JH has been shown to control this age-dependent plasticity, the underlying signaling pathway remains obscure. In this context, we cloned a full cDNA encoding the Krüppel homolog 1 transcription factor (AipsKr-h1) of A. ipsilon, which was found to be predominantly expressed in ALs, where its amount increased concomitantly with age and sex pheromone responses. Conversely, the expression of AipsKr-h1 protein in the antenna was age-independent. Moreover, the administration of JH in immature males or fluvastatin, an inhibitor of JH biosynthesis, in mature males induced an increase or a decline of the AipsKr-h1 protein level in ALs, respectively. This effect was suppressed with a combined injection of fluvastatin and JH. Our results showed that Aipskr-h1 is a JH-upregulated gene that might mediate JH action on central pheromone processing, modulating sexual behavior in A. ipsilon.

Peripheral regulation by ecdysteroids of olfactory responsiveness in male Egyptian cotton leaf worms, Spodoptera littoralis

Physiological and behavioral plasticity allows animals to adapt to changes in external (environmental) and internal (physiological) factors. In insects, the physiological state modulates adult behavior in response to different odorant stimuli. Hormones have the potential to play a major role in the plasticity of the olfactory responses. To explore if peripheral olfactory processing could be regulated by steroid hormones, we characterized the molecular, electrophysiological, and behavioral response to changes in endogenous hormone levels in adult male Spodoptera littoralis. The expression of the receptor complex (EcR/USP) was localized by in situ hybridization in the olfactory sensilla of antennae. Injections of 20-hydroxyecdysone (20E) induced an ecdysteroid signaling pathway in antennae and increased expression of the nuclear receptors EcR, USP and E75. Diacylglycerol kinase (DGK) and CaM expression were also up-regulated by 20E. Taken together, these molecular, electrophysiological, and behavioral results suggest a hormonal regulation of the peripheral olfactory processing in S. littoralis.

Peripheral and behavioral plasticity of pheromone response and its hormonal control in a long-lived moth

Reproductive success in many animals depends on the efficient production of and response to sexual signals. In insects, plasticity in sexual communication is predicted in species that experience periods of reproductive inactivity when environmental conditions are unsuitable for reproduction. Here, we study a long-lived moth Caloptilia fraxinella (Ely) (Lepidoptera: Gracillariidae) that is reproductively inactive from eclosion in summer until the following spring. Male sex pheromone responsiveness is plastic and corresponds with female receptivity. Pheromone response plasticity has not been studied in a moth with an extended period of reproductive inactivity. In this study, we ask whether male antennal response and flight behavior are plastic during different stages of reproductive inactivity and whether these responses are regulated by juvenile hormone. Antennal response to the pheromone blend is significantly reduced in reproductively inactive males tested in the summer and autumn as compared with reproductively active males tested in the spring. Reproductively inactive autumn but not summer males show lower antennal responses to individual pheromone components compared with spring males. Treatment with methoprene enhances antennal response of autumn but not summer males to high doses of the pheromone blend. Behavioral response is induced by methoprene treatment in males treated in the autumn but not in the summer. Plasticity of pheromone response in C. fraxinella is regulated, at least in part, by the peripheral nervous system. Antennal and behavioral response to pheromone differed in reproductively active and inactive males and increased with methoprene treatment of inactive males.

Overexpression of Drosophila juvenile hormone esterase binding protein results in anti-JH effects and reduced pheromone abundance

The titer of juvenile hormone (JH), which has wide ranging physiological effects in insects, is regulated in part by JH esterase (JHE). We show that overexpression in Drosophila melanogaster of the JHE binding protein, DmP29 results in a series of apparent anti-JH effects. We hypothesize that DmP29 functions in transport of JHE such that over- or under-expression of DmP29 results in increased or decreased JH degradation at specific sites respectively. Overexpression of DmP29 during the first or second instar was lethal, while overexpression during the third instar resulted in eclosion of small adults. Overexpression of DmP29 in newly eclosed flies reduced ovarian development and fecundity in addition to reducing the abundance of aggregation pheromone (cis-vaccenyl acetate) in males and courtship pheromone (cis,cis-7,11-heptacosadiene) in females. Both sexes also had lower levels of 23 and 25 carbon monoenes. Females exhibited reduced receptivity to mating, and males exhibited male-male courtship behavior, with both sexes being hyperactive: Male flies covered 2.7 times the distance of control flies at 2.9 times the maximum velocity. Application of the JH analog methoprene reversed impaired ovarian development, supporting a role for reduced JH in production of this phenotype. Rather than increasing lifespan as expected from a JH deficiency, overexpression of DmP29 reduced the life span of adult flies which may result from the hyperactivity of these flies. Underexpression of DmP29 resulted in reduced longevity, increased fecundity and reduced titers of pupal JHE. An alternative hypothesis, that mitochondrial dysfunction rather than JHE results in the JH-mediated phenotypes, is discussed.

Effects of [His(7)]-corazonin on the phase state of isolated-reared (solitarious) desert locusts, Schistocerca gregaria

[His(7)]-corazonin is a neuropeptide produced in the pars lateralis of the brain. It is stored in the corpora cardiaca and probably released from there. The only well-documented effect in locusts is increased melanization of the cuticle. We investigated whether this hormone might also be causally related to changes in behavior and morphometrics that, like melanization, occur during crowding-induced gregarization. Solitary fourth-instar nymphs of Schistocerca gregaria (Forsk.) were injected thrice with 1 nmol [His(7)]-corazonin. After molting to the 5th stadium their behavioral phase state was measured in an arena assay and analyzed using multiple logistic regression analysis. The hormone was found not to induce behavioral phase changes. Upon reaching adulthood, morphometrics shifts occurred towards values typical for crowd-reared and regregarized animals. Our results thus indicate that [His(7)]-corazonin is not involved in behavioral gregarization but may participate in morphometrical phase change.