Genetic and hormonal control of melanization in reddish–brown and albino mutants in the desert locust Schistocerca gregaria

The genetic and hormonal control of body colouration is investigated using two recessive genetic mutant strains, the reddish–brown (RB) mutant and an albino mutant, as well as a normal (pigmented) strain of the desert locust Schistocerca gregaria. The colour patterns of the RB nymphs are similar to those of a normal strain, although the intensity of the melanization is weaker in the former. Reciprocal crosses between the RB and albino mutants produce only normal phenotypes in the F₁ generation. In the F₂ generation, the normal, RB and albino phenotypes appear in a ratio of 9 : 3 : 4, indicating that two Mendelian units might determine the appearance of dark body colour and the intensity of melanization, respectively. In other words, at least two steps of regulation might be involved in the expression of body colour. Injections of [His⁷]‐corazonin, a neuropeptide inducing dark colour in this locust, fail to induce dark colour in albino nymphs but show a dose‐dependent darkening in RB nymphs in the range, 10 pmol to 1 nmol. Some RB nymphs become indistinguishable from normal individuals after injection of the peptide. Implantation of corpora cardiaca (CC) taken from RB mutants into other RB individuals induces darkening in the latter and CC from RB, albino and normal strains have similar dark colour‐inducing activity when implanted into albino Locusta migratoria. These results suggest the possibility that the RB mutant gene regulates the intensity of melanization, possibly through controlling the pathway of pigment biosynthesis associated with [His⁷]‐corazonin.     

Ability of the desert locust (Schistocerca gregaria) to compensate for visual deprivation during the final pre‐adult and adult stages

In the desert locust (Schistocerca gregaria Forsskål), vision is a seemingly indispensable prerequisite for many behaviour patterns. The question arises as to whether and to what extent other senses can compensate for the loss of vision, and whether this can take place in the adult stage. To answer this question, both of the compound eyes of nymphs in the final pre‐adult stage are blinded (but not the ocelli), resulting in permanent visual deprivation during adult life. The results are somewhat unexpected: under laboratory conditions, in comparison with sighted controls, the blinded locusts do not exhibit any noticeable change in overall agility relative to daily activity, nor any detriment in terms of the final moult, reproductive rate or longevity. In their search for a specific food source in an experimental arena with a narrow passage between the compartment where the animals are released and the compartment containing food, in the first trial, visually deprived females need significantly more time than the sighted controls. However with an increasing number of trials, each performed after 1 day of food deprivation, the food finding latency of blinded locusts (S. gregaria) approaches or even surpasses that of normally sighted locusts. The blinded locusts use their antennae, mouthparts and tarsi more extensively, suggesting a more frequent use of tactile and chemical cues, which they gradually learn to use more efficiently. The results indicate that pre‐adult and adult S. gregaria can respond to abrupt, permanent changes in their sensory inputs, and have a significant capacity for adaptation.     

Density-dependent accumulation of phase characteristics in a natural population of the desert locust Schistocerca gregaria

Abstract. Phase characteristics of locusts from parents that experienced different population densities were investigated under field conditions in Morocco. The density experienced by adults induced a marked phase change in colour, behaviour and morphometry of their offspring. A high-density subpopulation gave rise to a preponderance of black hatchlings that exhibited a high level of aggregation as later stage nymphs and showed gregarious morphometric features as adults, whereas a low-density subpopulation produced a majority of green hatchlings with a lesser tendency to group as final-instar nymphs and more solitarious morphometry as adults. The constrained isolation of insects from the low-density subpopulation, or crowding of insects from the high-density subpopulation, resulted in a behavioural and morphometric change towards even more solitarious characteristics in the former and more pronounced gregarious characteristics in the latter, relative to field-caught insects of the same age. These results from the field are consistent with those in the laboratory and provide more evidence for the dual roles of an individual locust's experience of crowding as well as that of its parents in density-dependent phase change.     

Distinct signalling pathways control Toxoplasma egress and host‐cell invasion

Calcium signalling coordinates motility, cell invasion, and egress by apicomplexan parasites, yet the key mediators that transduce these signals remain largely unknown. One underlying assumption is that invasion into and egress from the host cell depend on highly similar systems to initiate motility. Using a chemical‐genetic approach to specifically inhibit select calcium‐dependent kinases (CDPKs), we instead demonstrate that these pathways are controlled by different kinases: both TgCDPK1 and TgCDPK3 were required during ionophore‐induced egress, but only TgCDPK1 was required during invasion. Similarly, microneme secretion, which is necessary for motility during both invasion and egress, universally depended on TgCDPK1, but only exhibited TgCDPK3 dependence when triggered by certain stimuli. We also demonstrate that egress likely comes under a further level of control by cyclic GMP‐dependent protein kinase and that its activation can induce egress and partially compensate for the inhibition of TgCDPK3. These results demonstrate that separate signalling pathways are integrated to regulate motility in response to the different signals that promote invasion or egress during infection by Toxoplasma gondii.     

Changes in haemolymph octopamine levels associated with food deprivation in the locust, Schistocerca gregaria

ABSTRACT. Levels of octopamine in the haemolymph and locomotor activity have been measured over 24 h in fed and food-deprived adult male locusts, Schistocerca gregaria (Forskål). Locusts are predominantly diurnal, the locomotor activity of individuals with continuous access to food was higher during the photophase. The basal level of octopamine in haemolymph sampled during the first quarter of the light cycle was 7.8±1.1 pg/μl (51 nm). Depriving locusts of food at the start of the photophase for 9 h caused an increase in both the speed of movement and total activity compared to fed individuals. There was a corresponding increase in the haemolymph octopamine concentration which continued for as long as the insects were unable to feed, reaching a concentration of 20.7 ± 3.7 pg/μl (135 nM). While declining, the levels of octopamine remained higher in fed locusts for up to 4 h after replacing the food. The results are discussed in relation to the regulation of feeding behaviour and a proposed neurohormonal role of octopamine in insects.     

Short‐term assessment of dung beetle response to carbosulfan treatment against desert locust in Sudan

The beneficial role of dung beetles (Coleoptera: Scarabaeidae) is well known. Potential risks to these beetles from the widespread use of insecticides against the desert locust, a significant plant pest in Africa, the Near East and South West Asia, have not been studied previously. Short‐term responses of dung beetles to carbamate carbosulfan (Marshal®, ultra low‐volume formulation, 100 g active ingredient ha^?1) were assessed during desert locust control operations at five sites within two major biotopes: Acacia tortilis shrubland and cultivated wetland; on the Red Sea Coast of Sudan. The study took place during January–February 2004. At each site, fresh dung from Zebu cows was placed in areas targeted for desert locust control. Dung pats were placed in plots in two areas and left for 24 h, before and after insecticide application. Beetles were extracted by floatation. There was a significant decrease in abundance between the pre‐ and post‐spray period in treated areas for the Scarabaeinae species Onthophagus margaritifer (a dark colour morph). In contrast, it was found that Aphodius lucidus and beetles at the subfamily level of Aphodiinae increased in numbers after insecticide treatment. Mortality and sublethal impacts as well as a repellent effect of the insecticide may explain the decrease in Onthophagus margaritifer, while the increase in Aphodiinae beetles could be an indirect response to lower numbers of Scarabaeinae beetles in competing for the same resource. These organisms and the applied methodology may be useful for environmental monitoring of desert locust control, thus further studies are suggested. The assessment also revealed a marked difference between the two biotopes with high abundance and species richness of dung beetles in A. tortilis shrubland, while these measures were low in the cultivated wetland. Five new species of dung beetles for Sudan were found in this study.     

Yellowing,morphology and behaviour in sexually mature gynandromorphs of the desert locust Schistocerca gregaria

The morphology, colouration and sexual behaviour of two gynandromorph adults of the desert locust Schistocerca gregaria Forskål are described. In each of these specimens, the left half of the external genitalia has male characteristics and the right half has female characteristics. Yellowing of the epidermis occurs to different degrees in normal sexually mature male and female adult S. gregaria under crowded conditions. Two hypotheses are known to explain this phenomenon. One suggests involvement of a sex‐hormone/receptor complex, whereas the other proposes different sensitivities of the epidermis to the same hormonal environment. To examine which hypothesis is more likely, one gynandromorph adult is kept under crowded conditions and the other is maintained under isolated conditions. In the former, the left half of the body turns bright yellow, as in a sexually mature crowded male, and the right half is brownish with little yellowing, as in a sexually mature crowded female of the same age. The bilaterally divided body colouration may support the second hypothesis. In the second gynandromorph, which is isolated after adult emergence, the body colour remains brownish. Upon sexual maturity, this individual is kept together with normal male or female adults for behavioural observations. It attempts to mount a female but is approached and mounted by males.     

The influence of environmental microstructure on the behavioural phase state and distribution of the desert locust Schistocerca gregaria

Abstract. In previous studies we used logistic regression analysis to quantify the change in behavioural phase state of Schistocerca gregaria (Forskål) nymphs subjected to variations in population density. Such work involved restricting insects in small containers either alone or in a crowd. In the present paper we have shown that the fine-scale distribution of food plants, perches and favourable microclimatic sites influences the spatial distribution of locusts, both in the laboratory and under semi-field conditions. When multiple resource sites were provided, solitarious locusts tended to disperse and behavioural gregarization was inhibited. However, provision of only a single site promoted congregation, overcoming the tendency of solitarious insects to avoid each other, and led to behavioural gregarization. The time-course and extent of this response was fully consistent with our earlier experiments using enforced crowding. We suggest that such quantitative, experimental studies of the effects of environmental microstructure on behaviour may yield fundamental insights into the dynamics of plague formation in the desert locust.     

Purification of fat body glutathione S‐transferase from the desert locust Schistocerca gregaria: investigation of flavonoid inhibitory effects on enzyme activity

Focus on the development of botanical insecticides such as polyphenols may represent an alternative method to chemical control. In the present study, total glutathione concentration and its related antioxidant enzymes in foregut, midgut, hindgut and fat body homogenates of the desert locust Schistocerca gregaria are examined. Glutathione S‐transferase (GST) activity exhibits a significantly higher value in fat bodies compared with other tissues. A simple and reproducible procedure for the purification of S. gregaria fat body GST is established and the purified enzyme is shown to be homogenous. The purified GST displays a typical Michaelis behaviour with respect to its substrates. Characterization of the GST, including optimum pH, substrate specificity and inhibitor effects, is carried out. The ability of some flavonoids to inhibit S. gregaria fat body GST activity is examined. High‐performance liquid chromatography analysis indicates that the major components in Glycyrrhiza glabra roots are 18α‐glycyrrhetinic acid, quercetin and rutin, and the major components in Hibiscus sabdariffa calyx are cyanidin 3‐O‐glucoside chloride and delphinidin. Quercetin and delphinidin chloride exhibit strong GST inhibition and the inhibition type is determined for both. Rutin shows a smaller inhibitory effect, whereas 18α‐glycyrrhetinic acid and cyanidin have no effect. Inhibition of S. gregaria fat body GST activity would be expected to prevent, or at least delay, the development of resistance to chemical pesticides. Among the examined levels of the antioxidant enzymes, total glutathione concentration and its related enzymes in foregut, midgut, hindgut and fat body crude homogenates of S. gregaria GST activity exhibit a significantly higher value in fat bodies compared with other tissues. Some flavonoids that are detected in H. sabdariffa calyx and G. glabra root extracts are the most effective inhibitors of the purified S. gregaria fat body GST activity. Inhibition of S. gregaria fat body GST activity by quercetin and delphinidin (major compounds detected by HPLC) would be expected to prevent, or at least delay, the development of resistance to chemical pesticides.     

PCR primers for polymorphic microsatellite loci in the desert locust,Schistocerca gregaria (Orthoptera: Acrididae)

Nine pairs of polymerase chain reaction (PCR) primers that amplify polymorphic microsatellite loci in the desert locust, Schistocerca gregaria (Forskal), were developed using a magnetic bead‐based enrichment protocol. A sample of 48 locusts collected during the 1993 and 1995 upsurge periods in Eritrea, East Africa, were genotyped. The number of alleles per locus ranged from six to 20; the average was 12.67. Allelic distributions were significantly different between samples from different localities.     

Isolation and characterization of nine polymorphic microsatellite loci from the desert locust,Schistocerca gregaria

Because of the scarcity of polymorphic genetic markers, only a few genetic studies on the population structure of the desert locust, Schistocerca gregaria, have been carried out. We isolated and characterized nine polymorphic dinucleotide microsatellite loci. These markers were evaluated using individuals from Niger and Senegal. Seven of these microsatellite markers are also applicable to the nongregarious subspecies Schistocerca gregaria flaviventris. Cross‐species applicability was limited to one of the loci in the sister species S. americana and in the locust Locusta migratoria.     

A behavioural analysis of phase change in the desert locust

A programme of research into phase change in the desert locust, Schistocerca gregaria, is described. The ability to change phase between solitarious and gregarious forms in response to population density is a key feature of locusts and is central to their occasional yet catastrophic impact on humans. Phase polymorphism is an extreme form of phenotypic plasticity. The most labile phase characteristic is behaviour. It is argued that a fully integrated study of behavioural phase change provides a powerful tool for understanding both the mechanisms of phase change and locust population dynamics, both of which offer possibilities for improved management and control of desert locust plagues. An assay for measuring behavioural phase-state in individual locusts was derived, based on logistic regression analysis. Experiments are described that used the assay to quantify the time-course of behavioural change, both within the life of individual locusts and across generations. The locust-related stimuli that provoke behavioural gregarization were investigated. Complex interactions were found between tactile, visual and olfactory stimuli, with the former exerting the strongest effect. Behavioural analysis also directed a study of the mechanisms whereby adult females exert an epigenetic influence over the phase-state of their developing offspring. Female locusts use their experience of the extent and recency of being crowded to predict the probability that their offspring will emerge into a high-density population, and alter the development of their embryos accordingly through a gregarizing agent added to the foam that surrounds the eggs at laying. There is also a less pronounced paternal influence on hatchling phase-state. An understanding of the time-course of behavioural phase change led to a study of the effect of the fine-scale distribution of resources in the environment on interactions between individual locusts, and hence on phase change. This, in turn, stimulated an exploration of the implications of individual behavioural phase change for population dynamics. Cellular automata models were derived that explore the relationships between population density, density of food resources and the distribution of resources in the environment. The results of the simulation showed how the extent of gregarization within a population increases with rising population size relative to food abundance and increasing concentration of food resources. Of particular interest was the emergence of critical zones across particular combinations of resource abundance, resource distribution and population size, where a solitarious population would rapidly gregarize. The model provided the basis for further laboratory and field experiments, which are described.     

Overlapped and differential expression of cAMP‐dependent kinase‐inhibitor isoforms during avian organogenesis period

Although cAMP‐dependent kinase (PKA) has been known to regulate many biological systems, including patterning, cell differentiation and proliferation, it is not well understood how the spatial‐temporal specificity of the PKA signaling is generated. While the PKA signal activation is regulated in many ways, a direct visualization of PKA activity in situ is not possible. Thus, examinations of PKA regulators will provide indirect, but nonetheless important information to elucidate the distribution of PKA activity. In the present study, three isoforms of PKA‐inhibitor (PKI) genes were identified from avian genome, and their expression patterns were examined during the organogenesis period. PKI genes were strongly expressed in muscle, liver, and nervous system primordia, suggesting their inhibitory roles on the PKA signaling in the development of these tissues.     

Effects of hatchling body colour and rearing density on body colouration in last-stadium nymphs of the desert locust, Schistocerca gregaria

Abstract.  The influences of hatchling character and rearing density on body colour at the last-nymphal stadium are investigated for the desert locust, Schistocerca gregaria . Hatchlings are divided into five groups based on the darkness of the body colour and reared either under isolated or crowded conditions. Two types of body colour variation at the last-nymphal stadium are separately analysed (i.e. the background colour and black patterns). Under isolated conditions, the background body colour is either greenish or brownish. Most individuals are greenish and the highest percentage of brownish insects is obtained from hatchlings with the darkest body colour. Under crowded conditions, the background colour is yellow or orange and the percentage of yellowish nymphs tends to decrease when they are darker at hatching. The intensity of black patterns differs depending on the body colour at hatching and subsequent rearing density. Most isolated-reared nymphs exhibit few or no black patterns but nymphs with some black patterns also appear, particularly among those that had been dark at hatching. Under crowded conditions, the black patterns become more intense when they are darker at hatching. Therefore, last-stadium nymphs with typical solitarious or gregarious body colouration appear when they have the phase-specific body colouration at hatching as well. The present results demonstrate that both body colour at hatching and rearing density during nymphal development influence body colouration at the last-nymphal stadium.     

Up‐regulation of Tyrosinase Gene by Nitric Oxide in Human Melanocytes

Ultraviolet light (UV) radiation causes skin‐tanning, which is thought to be mediated by stimulating the release of melanogenic factors from keratinocytes as well as other cells. Nitric oxide (NO) has been reported to be generated after UV radiation and to stimulate melanocytes as one of the melanogens. In a previous experiment by another group on melanogenesis induced by NO, increases in both tyrosinase activity and tyrosinase protein levels were observed after daily stimulation of NO for 4 days. In the present study, we investigated tyrosinase gene expression within the first 24 hr of NO‐induced melanogenesis. Tyrosinase mRNA expression was found to be induced 2 hr after a single treatment with S‐nitroso‐N‐acetyl‐ l ‐arginine. An increase of tyrosinase activity was also detected time‐dependently within the 24‐hr period, accompanied by an increase of tyrosinase protein levels. The induction of mRNA expression was suppressed by a cyclic guanosine 3′,5′‐monophosphate (cGMP)‐dependent protein kinase (cGMP/PKG) inhibitor. These results suggest that the enhancement of tyrosinase gene expression via the cGMP pathway may be a primary mechanism for NO‐induced melanogenesis.     

Purification and sequence determination of a yellow protein from sexually mature males of the desert locust, Schistocerca gregaria

A yellow protein from abdominal cuticle of the desert locust, Schistocerca gregaria, has been purified and its amino acid sequence determined. The yellow color comes from bound carotene, the protein is only deposited in the epidermis and cuticle of male locusts during their sexual maturation, and the deposition is dependent upon a sufficiently high titer of juvenile hormone. The sequence of the protein is atypical for a cuticular protein, but it has some similarity to a putative juvenile hormone binding protein from Manduca sexta. It is suggested that the protein is involved in the transport of carotenes from internal tissues to epidermis and cuticle of the locust.     

Crosstalk between PKA and PKG controls pH‐dependent host cell egress of Toxoplasma gondii

Toxoplasma gondii encodes three protein kinase A catalytic (PKAc1‐3) and one regulatory (PKAr) subunits to integrate cAMP‐dependent signals. Here, we show that inactive PKAc1 is maintained at the parasite pellicle by interacting with acylated PKAr. Either a conditional knockdown of PKAr or the overexpression of PKAc1 blocks parasite division. Conversely, down‐regulation of PKAc1 or stabilisation of a dominant‐negative PKAr isoform that does not bind cAMP triggers premature parasite egress from infected cells followed by serial invasion attempts leading to host cell lysis. This untimely egress depends on host cell acidification. A phosphoproteome analysis suggested the interplay between cAMP and cGMP signalling as PKAc1 inactivation changes the phosphorylation profile of a putative cGMP‐phosphodiesterase. Concordantly, inhibition of the cGMP‐dependent protein kinase G (PKG) blocks egress induced by PKAc1 inactivation or environmental acidification, while a cGMP‐phosphodiesterase inhibitor circumvents egress repression by PKAc1 or pH neutralisation. This indicates that pH and PKAc1 act as balancing regulators of cGMP metabolism to control egress. These results reveal a crosstalk between PKA and PKG pathways to govern egress in T. gondii.