Neural correlates to flight-related density-dependent phase characteristics in locusts

Locust phase polymorphism is an extreme example of behavioral plasticity; in response to changes in population density, locusts dramatically alter their behavior. These changes in behavior facilitate the appearance of various morphological and physiological phase characteristics. One of the principal behavioral changes is the more intense flight behavior and improved flight performance of gregarious locusts compared to solitary ones. Surprisingly, the neurophysiological basis of the behavioral phase characteristics has received little attention. Here we present density-dependent differences in flight-related sensory and central neural elements in the desert locust. Using techniques already established for gregarious locusts, we compared the response of locusts of both phases to controlled wind stimuli. Gregarious locusts demonstrated a lower threshold for wind-induced flight initiation. Wind-induced spiking activity in the locust tritocerebral commissure giants (TCG, a pair of identified interneurons that relay input from head hair receptors to thoracic motor centers) was found to be weaker in solitary locusts compared to gregarious ones. The solitary locusts' TCG also demonstrated much stronger spike frequency adaptation in response to wind stimuli. Although the number of forehead wind sensitive hairs was found to be larger in solitary locusts, the stimuli conveyed to their flight motor centers were weaker. The tritocerebral commissure dwarf (TCD) is an inhibitory flight-related interneuron in the locust that responds to light stimuli. An increase in TCD spontaneous activity in dark conditions was significantly stronger in gregarious locusts than in solitary ones. Thus, phase-dependent differences in the activity of flight-related interneurons reflect behavioral phase characteristics.     

Phase-related differences in egg production of the migratory locust regulated by differential oosorption through microRNA-34 targeting activinβ

Outbreaks of locust plagues result from the long-term accumulation of high-density egg production. The migratory locust, Locusta migratoria, displays dramatic differences in the egg-laid number with dependence on population density, while solitarious locusts lay more eggs compared to gregarious ones. However, the regulatory mechanism for the egg-laid number difference is unclear. Herein, we confirm that oosorption plays a crucial role in the regulation of egg number through the comparison of physiological and molecular biological profiles in gregarious and solitarious locusts. We find that gregarious oocytes display a 15% higher oosorption ratio than solitarious ones. Activinβ (Actβ) is the most highly upregulated gene in the gregarious terminal oocyte (GTO) compared to solitarious terminal oocyte (STO). Meanwhile, Actβ increases sharply from the normal oocyte (N) to resorption body 1 (RB1) stage during oosorption. The knockdown of Actβ significantly reduces the oosorption ratio by 13% in gregarious locusts, resulting in an increase in the egg-laid number. Based on bioinformatic prediction and experimental verification, microRNA-34 with three isoforms can target Actβ. The microRNAs display higher expression levels in STO than those in GTO and contrasting expression patterns of Actβ from the N to RB1 transition. Overexpression of each miR-34 isoform leads to decreased Actβ levels and significantly reduces the oosorption ratio in gregarious locusts. In contrast, inhibition of the miR-34 isoforms results in increased Actβ levels and eventually elevates the oosorption ratio of solitarious locusts. Our study reports an undescribed mechanism of oosorption through miRNA targeting of a TGFβ ligand and provides new insights into the mechanism of density-dependent reproductive adaption in insects.     

Syntaxin 1A modulates the sexual maturity rate and progeny egg size related to phase changes in locusts

The migratory locust (Locusta migratoria) exhibits clear phenotypic plasticity depending on its population density. Previous studies have explored the molecular mechanisms of body colour, behavior, immunity, and metabolism between high population density gregarious (G) and low population density solitarious (S) locusts. However, the molecular mechanisms underlying differences in reproductive traits remain unknown. G locusts reach sexual maturation much faster and lay larger eggs compared with S locusts. The traits of G locusts decreased significantly with isolation, whereas those of S locusts increased with crowding. Analysis of gene expression in female adults indicated that syntaxin 1A (Syx1A) was expressed significantly higher in G locusts than in S locusts. After silencing Syx1A expression in G locusts by RNA interference (RNAi), their sexual maturity rate and progeny egg size changed towards those of S locusts. Similarly, increment in the traits of S locusts with crowding was blocked by Syx1A interference. Changes in the traits were also confirmed by decrease in the level of vitellogenin, which is regulated by Syx1A. In conclusion, plasticity of the sexual maturity rate and progeny egg size of G and S locusts, which is beneficial for locusts to adapt to environmental changes, is regulated by Syx1A.     

Neural correlates to flight‐related density‐dependent phase characteristics in locusts

Locust phase polymorphism is an extreme example of behavioral plasticity; in response to changes in population density, locusts dramatically alter their behavior. These changes in behavior facilitate the appearance of various morphological and physiological phase characteristics. One of the principal behavioral changes is the more intense flight behavior and improved flight performance of gregarious locusts compared to solitary ones. Surprisingly, the neurophysiological basis of the behavioral phase characteristics has received little attention. Here we present density‐dependent differences in flight‐related sensory and central neural elements in the desert locust. Using techniques already established for gregarious locusts, we compared the response of locusts of both phases to controlled wind stimuli. Gregarious locusts demonstrated a lower threshold for wind‐induced flight initiation. Wind‐induced spiking activity in the locust tritocerebral commissure giants (TCG, a pair of identified interneurons that relay input from head hair receptors to thoracic motor centers) was found to be weaker in solitary locusts compared to gregarious ones. The solitary locusts' TCG also demonstrated much stronger spike frequency adaptation in response to wind stimuli. Although the number of forehead wind sensitive hairs was found to be larger in solitary locusts, the stimuli conveyed to their flight motor centers were weaker. The tritocerebral commissure dwarf (TCD) is an inhibitory flight‐related interneuron in the locust that responds to light stimuli. An increase in TCD spontaneous activity in dark conditions was significantly stronger in gregarious locusts than in solitary ones. Thus, phase‐dependent differences in the activity of flight‐related interneurons reflect behavioral phase characteristics. © 2003 Wiley Periodicals, Inc. J Neurobiol 57: 152–162, 2003     

Adult female desert locusts require contact chemicals and light for progeny gregarization

Crowding causes many organisms to express phenotypic plasticity in various traits. Phase polyphenism in desert locusts represents one extreme example in which a solitary form (solitarious phase) turns into a gregarious form (gregarious phase) in response to crowding. Conspicuous differences in body size and colour occur even in hatchlings. The phase‐specific differences in hatchling characteristics are caused by the tactile stimuli perceived by the antennae of their mother. However, the nature of the tactile stimuli and the mechanism by which the perceived stimuli are processed as a gregarizing signal remain unknown. To explore this problem, the antennae of solitarious adult females of the desert locust Schistocerca gregaria are touched with the bodies of conspecific locusts at different physiological stages and those of other species. The results suggest that a cuticular chemical factor at a specific developmental stage of conspecific locusts causes the solitarious females to produce large eggs that give rise to black hatchlings characteristic of gregarious forms (progeny gregarization), and that this or a similar compound occurs in other acridids, crickets and cockroaches but not in beetles. The involvement of a chemical substance is also supported by hexane extracts of cuticular surfaces of locusts that induce the same effects. Interestingly, crowding induces such gregarizing effects only when the female receives the appropriate stimulus in the presence of light. Solitarious female S. gregaria with their head capsule coated with phosphorescent paint exhibit progeny gregarization in response to crowding and light pulses in darkness, whereas those treated in the same way without light pulses fail to do so.     

Characteristics and expression patterns of histone-modifying enzyme systems in the migratory locust

The density-dependent phase polyphenism in locusts offers an excellent model to investigate the epigenetic regulatory mechanisms underlying phenotypic plasticity. In this study, we identified histone-modifying enzymes mediating histone post-translational modifications, which serve as a major regulatory mechanism of epigenetic processes, on the basis of the whole genome sequence of the migratory locust, Locusta migratoria. We confirmed the existence of various functional histone modifications in the locusts. Compared with other sequenced insect genomes, the locust genome contains a richer repertoire of histone-modifying enzymes. Several locust histone-modifying enzymes display vertebrate-like characteristics, such as the presence of a Sirt3-like gene and multiple alternative splicing of GCN5 gene. Most histone-modifying enzymes are highly expressed in the eggs or in the testis tissues of male adults. Several histone deacetylases and H3K4-specific methyltransferases exhibit differential expression patterns in brain tissues between solitarious and gregarious locusts. These results reveal the main characteristics of histone-modifying enzymes and provide important cues for understanding the epigenetic mechanisms underlying phase polyphenism in locusts.     

Effect of phase status on responses to AKHI in the desert locust,Schistocerca gregaria gregaria

Hyperlipaemic response to adipokinetic hormone (AKH I) was demonstrated in both solitary and gregarious phases of the desert locust, Schistocerca gregaria gregaria. Time-course studies showed that the gregarious locusts had a faster response to the hormone than their solitary counterparts. At peak response time (90 min), the gregarious locusts were more sensitive to AKH I doses below 2 pmol while the solitary locusts had a higher response above this dose. Upon injection of the hormone, lipoprotein conversion occurred, resulting in the formation of the low density lipoprotein (LDLp). The LDLp formed in the gregarious locusts was much larger than that of the solitary locusts. The fat body lipid reserve (expressed as % fat body dry weight) was significantly (P < 0.01) higher in the gregarious (79.02 ± 2.77%) than in the solitary locusts (65.23 ± 2.55%). Triacylglycerol was the major lipid class representing 83.9 and 73.9% of the total lipids in gregarious and solitary locusts, respectively. The higher fat body lipid reserves and efficient LDLp formation in response to AKH in gregarious locusts compared to solitary locusts suggests a physiological adaptation for prolonged flights. © 1996 Wiley-Liss, Inc.     

Composition and emission dynamics of migratory locust volatiles in response to changes in developmental stages and population density

Chemical communication plays an important role in density‐dependent phase change in locusts. However, the volatile components and emission patterns of the migratory locust, Locusta migratoria, are largely unknown. In this study, we identified the chemical compositions and emission dynamics of locust volatiles from the body and feces and associated them with developmental stages, sexes and phase changes. The migratory locust shares a number of volatile components with the desert locust (Schistocerca gregaria), but the emission dynamics of the two locust species are significantly different. The body odors of the gregarious nymphs in the migratory locust consisted of phenylacetonitrile (PAN), benzaldehyde, guaiacol, phenol, aliphatic acids and 2,3‐butanediol, and PAN was the dominant volatile. Volatiles from the fecal pellets of the nymphs primarily consist of guaiacol and phenol. Principal component analysis (PCA) showed significant differences in the volatile profiles between gregarious and solitary locusts. PAN and 4‐vinylanisole concentrations were significantly higher in gregarious individuals than in solitary locusts. Gregarious mature males released significantly higher amounts of PAN and 4‐vinylanisole during adulthood than mature females and immature adults of both sexes. Furthermore, PAN and 4‐vinylanisole were completely lost in gregarious nymphs during the solitarization process, but were obtained by solitary nymphs during gregarization. The amounts of benzaldehyde, guaiacol and phenol only unidirectionally decreased from solitary to crowded treatment. Aliphatic aldehydes (C7 to C10), which were previously reported as locust volatiles, are now identified as environmental contaminants. Therefore, our results illustrate the precise odor profiles of migratory locusts during developmental stages, sexes and phase change. However, the function and role of PAN and other aromatic compounds during phase transition need further investigation.     

Adipokinetic Hormone and Flight Fuel Related Characteristics of Density-Dependent Locust Phase Polymorphism: A Review

Recent findings on differences between the gregarious and solitary phases of locusts are reviewed in relation to flight fuel utilization, adipokinetic responses, and adipokinetic hormones. Laboratory results obtained with Locusta migratoria migratorioides show that the amount of lipid reserves, resting levels of haemolymph lipids, and hyperlipaemic responses to flight and to injection of corpus cardiacum extract or of synthetic adipokinetic hormones, are higher in crowded than in isolated locusts. No major phase-dependent differences seem to exist in flight-related carbohydrate metabolism. The adipokinetic hormone content of the corpora cardiaca is higher in younger isolated locusts than in crowded ones. Adipokinetic hormone precursor-related peptide content of the corpora cardiaca is also higher in isolated than in crowded locusts. Crowded locusts have higher lipid reserves and higher hyperlipaemic responses to flight than isolated locusts also in Schistocerca gregaria and, following injection of synthetic adipokinetic hormone, the formation of low density lipophorin is higher in crowded than in isolated locusts of this species. The laboratory results obtained with isolated and crowded locusts are extrapolated to understand the ecophysiology of the migrations of solitary and gregarious field populations of L.m. migratorioides according to available information on the differences in the migration of the two phases. It is inferred that in this species solitary locusts have a rather coarse adipokinetic strategy focused on a single prereproductive long-distance migratory flight, whereas gregarious locusts possess a fine adipokinetic balance for reiterative, sometimes unpredictably long-distance, migrations in the prereproductive, as well as reproductive, periods. The differences between the adipokinetic strategies of solitary and gregarious S. gregaria seem to be less dramatic, nevertheless, they indicate a better adaptation of the gregarious phase to prolonged flights.     

Solitary and gregarious locusts differ in circadian rhythmicity of a visual output neuron

Locusts demonstrate remarkable phenotypic plasticity driven by changes in population density. This density dependent phase polyphenism is associated with many physiological, behavioral, and morphological changes, including observations that cryptic solitarious (solitary-reared) individuals start to fly at dusk, whereas gregarious (crowd-reared) individuals are day-active. We have recorded for 24-36 h, from an identified visual output neuron, the descending contralateral movement detector (DCMD) of Schistocerca gregaria in solitarious and gregarious animals. DCMD signals impending collision and participates in flight avoidance maneuvers. The strength of DCMD's response to looming stimuli, characterized by the number of evoked spikes and peak firing rate, varies approximately sinusoidally with a period close to 24 h under constant light in solitarious locusts. In gregarious individuals the 24-h pattern is more complex, being modified by secondary ultradian rhythms. DCMD's strongest responses occur around expected dusk in solitarious locusts but up to 6 h earlier in gregarious locusts, matching the times of day at which locusts of each type are most active. We thus demonstrate a neuronal correlate of a temporal shift in behavior that is observed in gregarious locusts. Our ability to alter the nature of a circadian rhythm by manipulating the rearing density of locusts under identical light-dark cycles may provide important tools to investigate further the mechanisms underlying diurnal rhythmicity.     

The role of egg pod foam and rearing conditions of the phase state of the Asian migratory locust Locusta migratoria migratoria (Orthoptera, Acrididae)

Coloration phase state, morphometrical ratios and the numbers of mature oocytes of Locusta migratoria migratoria were examined in a series of experiments to determine the means by which phase characteristics are passed to the next generation. Washing with distilled water of eggs from egg pods laid by gregarious crowd-reared females resulted in solitarization of the hatchlings after their isolation, indicating that a factor present in eggs encapsulated in foam is causal to gregarization. Such locusts showed a significant shift towards the typical solitarious body coloration, morphometry and number of mature oocytes as compared to locusts resulting from unwashed eggs. Gregarious coloration, morphometrical ratios and oocyte numbers could be partially restored when hatchlings from washed eggs were regrouped. When gregarious locusts were reared in isolation, they showed a solitary body color, whereas, morphometry and oocyte numbers were not affected by isolation.     

Effect of Paranosema locustae (Microsporidia) on the behavioural phases of Locusta migratoria (Orthoptera: Acrididae) in the laboratory

The ability of parasites to modify the behaviour of their hosts is a wide spread phenomenon, but the effects of microsporidian parasites on locust behaviour remain unexplored. Here the frequencies of directional changes (ND) and jumping (NJ) per minute of gregarious locusts infected with 2000 spores of the microsporidian parasite Paranosema locustae were significantly different from those of untreated locusts 10 and 16 days after infection, being similar to values for solitary nymphs. In contrast, the behaviour of locusts inoculated with the lower doses of 200 spores/locust was sometimes like that of solitary nymphs. At other times, behaviour was intermediate between solitary and gregarious, i.e. transitional. The rearing density did not affect the turning and jumping behaviour of infected locusts, and their behaviours were similar to those of solitary locusts at 10–16 days after infection. Our study demonstrates that infection with P. locustae may lead gregarious locusts to change some of their behaviour to that typical of solitary locusts.     

Neurophysiological studies of flight-related density-dependent phase characteristics in locusts

Locusts show an extreme example of density-dependent phase polymorphism, demonstrating within the species differences in morphology as well as biology, dependent on the population density. Behavior is the primary density-dependent change which facilitates the appearance of various morphological and physiological phase characteristics. We have studied density dependent differences in flight related sensory and central neural elements in the desert locust Schistocerca gregaria. Wind generated high frequency spiking activity in the tritocerebral commissure giant (TCG, an identified interneuron that relay inputs from head hair receptors to thoracic motor centers) that was much less intense in solitary locusts, compared to gregarious ones. In addition the solitary locusts' TCG demonstrated much stronger adaptation of its response. In cases when flight was initiated high frequency TCG activity was independent of the locust phase. The tritocerebral commissure dwarf (TCD) is a GABAergic flight related interneuron that is sensitive to ambient illumination intensity. An increase in the TCD spontaneous activity under dark vs. light conditions was significantly higher in gregarious locusts then in solitary ones, implying a flight-related inhibitory mechanism that is far more active in gregarious locusts under dark conditions. Thus, density-dependent phase differences in interneuron activity pattern and properties well reflect and may be at least partially responsible to behavioral flight-related characteristics.