M. P. Pener

Locust Phase Polymorphism and its Endocrine Relations

Publisher Summary Locust phase polymorphism is environmentally regulated but it is regarded as a “continuous polymorphism.” This term means that a continuous range of intermediates exists between the two extreme phases. In environmentally controlled insect polymorphism (including physiological and behavioral polymorphism), extrinsic cues induce sensory and/or nutritional inputs. These are somehow coupled to the mechanisms that make a decision to prefer a certain morph to the other(s) and then substantiate this preference in the course of development. Components of the endocrine system are usually involved in these mechanisms and are often major factors in the control of polymorphism. Different locust species belong to several different subfamilies. This situation probably indicates that locust phase polymorphism evolved several times by convergent evolution within the family of acridids. The species dependent differences, both in certain phase characteristics and in the amplitude of their changes, may be well explained by this assumption.

Locust phase polyphenism: an update.

Publisher Summary This chapter updates former reviews on locust polyphenism and also discusses recent findings. Over 200 articles were published in scientific journals on various aspects of locust phase polyphenism, markedly advancing the knowledge of the subject. However, the chapter refers to the older literature when background information is necessary for complementary and better treatment, or because of historical importance. Some of the recent publications report contradictory findings, and such contradictions have been emphasized in the chapter. Substantial progress has been made in the study of locust phase polyphenism over the past several years. The topic has well and truly emerged from the realms of applied entomology to assume a prominent position in the modern study of phenotypic plasticity, whereby adaptive phenotypes arise during development as a result of plastic interactions between genes and the environment. The study of behavioral gregarization has seen some of the most far-reaching progress. The involvement of locust-emitted volatiles in aspects of phase biology has generated substantial research and its fair share of productive controversy. It has been especially heartening to see that some of the major areas of controversy, such as the role of phenylacetonitrile, appear to have been at least partially resolved during the past year. New genetic resources also offer a solution to a more prosaic problem in locust research. There has been a growing realization that some of the differences reported between laboratories in aspects of phase polyphenism most likely reflect effects of rearing locusts in long-term culture in the laboratory.

The physiology of locust phase polymorphism: an update.

The considerable progress made between 1990 and 1997 in locust phase-related research and in understanding the physiology of locust phase polymorphism is reviewed. The traits of locust phases are discussed and it is concluded that there are distinct strain-dependent differences in phase characteristics and their amplitudes even in the same species. Despite some advances, no major break-through was achieved in the putative endocrine control of locust phase polymorphism. Phase-dependent differences in adipokinesis, flight fuels and migration of adult locusts, as well as novel methods in studying aggregation behaviour and activity of hoppers and adults, opened new lines in research of the physiology of locust phase polymorphism. Marked advances were made in phase-related locust pheromone research, revealing, in Schistocerca gregaria, differences between the pheromonal system of the hoppers and that of the adults. These systems turned out to be more complex than previously assumed. Phenylacetonitrile, produced by sexually mature adult males, serving both as an attractant and a mutration-accelerating factor, was identified as the major compound of the adult pheromonal system in S. gregaria. A new aspect of transmission of phase characteristics from parent to progeny through the foam (froth) of the egg pod was revealed. Effects of some plant substances on locust phases were reported. However, no research has yet been published on the aspects of molecular biology of locust phase polymorphism.

Density-dependent phase polymorphism affects response to adipokinetic hormone in Locusta

Abstract 1. 1. Haemolymph lipid levels were measured before and after injection of graded doses of corpus cardiacum (CC) extracts, and of synthetic adipokinetic hormone (AKH) I, to solitary and gregarious adult males of Locusta migratoria migratorioides. 2. 2. Gregarious locusts showed higher resting lipid levels and a consistently higher increase of haemolymph lipids, 90–100 min after injection. 3. 3. Haemolymph volume was lower in solitary than in gregarious locusts; therefore, the phase-dependent differences in haemolymph lipid levels and in adipokinetic response were not caused by differences in haemolymph volume. 4. 4. The authors conclude that solitary locusts have lower haemolymph lipid levels and a less intense response to AKH than gregarious locusts.

Structure–activity relations of the dark-colour-inducing neurohormone of locusts

The dark-colour-inducing effect of several peptides in comparison to that of the dark-colour-inducing neurohormone (DCIN, [His(7)]-corazonin) of locusts was investigated by a bioassay based on nymphs of a DCIN-deficient albino mutant of Locusta migratoria. The study was aimed at elucidating the active part of the DCIN and to explore the contribution of its amino acids to the activity. Graded doses of all peptides were injected in oil. [Arg(7)]-corazonin and DCIN were equally effective. Certain arthropod neuropeptides having the -SXGW- partial sequence (a part of the DCIN and of [Arg(7)]-corazonin; X=His and X=Arg, respectively) yielded the following findings: Scg-AKH-II (adipokinetic hormone II of Schistocerca gregaria X=Thr), Grb-AKH ( adipokinetic hormone of Gryllus bimaculatus X=Thr) and RPCH (red pigment concentrating hormone of crustaceans X=Pro) evoked a moderate darkening response, but Lom-AKH-II (adipokinetic hormone II of L. migratoria X=Ala) was ineffective. Step by step shortening of the sequence of the DCIN at the N-terminal, from pGlu-3-11DCIN to pGlu-9-11DCIN, resulted in a decreasing activity, but even pGlu-9-11DCIN induced a weak response with high doses. Shortening of the DCIN from the C-terminal revealed a moderate activity of 1-7DCIN-NH(2) and a weak activity of 1-5DCIN-NH(2). An octadecapeptide which induces dark colour in moth larvae, having the pentamer FTPRL-NH(2) at its C-terminal, evoked no darkening in the albino locusts. We conclude that although the -SXGW- partial sequence has some role in induction of darkening, for obtaining maximal effect the whole sequence of the DCIN (or of [Arg(7)]-corazonin) is necessary.

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

Abstract 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.

Comparative study of neuropeptides from the corpora cardiaca of solitary and gregarious Locusta

Neuropeptide content of the corpora cardiaca (CC) was studied in crowded (gregarious phase) and isolated (solitary phase) Locusta migratoria migratorioides adults, using electrophoretic, chromatographic, and immunological techniques. Quantitative differences were found in the three neuropeptides investigated (neuroparsins, Lom-OMP, and APRP). The amount of neuroparsin A was higher in the CC of crowded locusts. Neuroparsin B content of the CC was quite similar in isolated and crowded locusts, or in some cases slightly higher in the latter. The comparative amounts of the ovary maturating parsin, Lom-OMP, in the CC were dependent on the sexual maturation of the locusts, being nearly similar in maturing isolated locusts and immature crowded locusts, but higher in crowded locusts when both phases were completely mature. The amount of AKH-precursor related peptides (APRP) was markedly and consistently higher in the CC of isolated locusts. These findings are discussed in relation to other physiological and ecological phase-dependent differences in locusts.

DUET SINGING AND FEMALE CHOICE IN THE BUSHCRICKET PHANEROPTERA NANA

Summary In the phaneropterine bushcricket Phaneroptera nana both males and females sing, producing a duet. The male’ s song is answered by a short female ‘ tick’ . A male that receives a response in a dee ned interval after the end of his chirp usually initiates phonotaxis toward the female. Measurements of temporal, spectral, and energy characters of male chirps indicate that individual males vary considerably in several temporal song characters. Females exhibited equivalent response rates to solitary singing males in laboratory trials regardless of their song characters. However, when simultaneously presented with two singing males, females showed consistent preferences: They responded preferentially to relatively longer chirps produced by larger males. Male chirp rate had no ine uence on female response, suggesting that females were not ine uenced by a summation of acoustic input integrated over multiple chirps. Rather,female ‘ decisions’ torespond were probably made following individual chirps. The selective responses exhibited by Ph. nana females contradict some previous expectations that females in duet signalling systems should be indiscriminate. Selectivity may ree ect

Chemical allatectomy of late Locusta embryos by a synthetic precocene and its effect on hopper morphogenesis

Abstract The anti-allatin substance, 7-ethoxy-precocene II (= “precocene III”) was topically applied to eggs of Locusta migratoria migratorioides with fully grown embryos in stage XX (about 64 ± 4% of the whole period of egg development). A day after precocene application the eggs were washed for 10–15 s in acetone and then transferred to clean containers for removing precocene residues and for preventing contamination at hatching. The treatment induced prothetelic morphogenetic disturbances which became overt in the subsequent hoppers; the effect was dose dependent and the ED 50 (= effective anti-allatin dose 50%) was low, 20.5 μg precocene III per g fresh egg weight (= 0.37 μg per egg). Quite similar results were obtained following application of precocene III to eggs with embryos in stage XXI (73 ± 4% of the egg development). These findings and direct examination of histological sections of the embryonic corpora allata demonstrated that precocene chemically allatectomizes late Locusta embryos. The lethal effect of precocene III was dependent on the washing. When the eggs were washed in acetone a day after application, mortality did not occur in a dose-dependent way; even the highest dose applied, 256 μg precocene III per egg (= 14405 μg per g fresh weight), was less than the LD 50 (lethal dose 50%). In contrast, without washing mortality was dose dependent, but it occurred later, at or after hatching; the LD 50 was 1334.9 μg per g (= 22.7 μg/egg). The results show that the late embryos are highly susceptible to the anti-allatin effect of the precocene, but are extremely insusceptible to its lethal effect; toward hatching, however, susceptibility to the lethal effect becomes marked. With doses between 45–14405 μg precocene III per g fresh egg weight, the anti-allatin effect became overt by a quite-uniform belated morphogenetic response. All hoppers which hatched from precocenetreated eggs were morphogenetically normal in the 1st instar and in the beginning of the 2nd instar, but the duration of the 2nd instar was almost doubled and at the end of this instar over 96% of the locusts died in the moult, being unable to shed the exuvia. Artificial removal of the apolyzed old cuticle revealed 3rd instar prothetelic adultiforms. These results and some data in the literature indicate that allatectomy of the embryo does not result in prothetelic morphogenetic disturbances in the 1st and early 2nd instar larvae and may impose the question what is the role of the juvenile hormone in late embryos and early larvae.

Effect of exogenous juvenile hormones on mating behaviour and yellow colour in allatectomized adult male desert locusts

ABSTRACT. A comprehensive dose–response study of exogenous juvenile hormones, JHI and JHIII, on mating behaviour and on yellowing was carried out in allatectomized, crowded adult Schistocerca americana gregaria males. Injections of either hormone induced mating behaviour, but topical applications were almost completely ineffective. In contrast, topical applications were at least as effective as injections for inducing yellow colour. Induction and/or intensity of mating behaviour was dependent on the number of (repeated) administrations (from one to eight) and on the cumulative dose (80 or 160/μg) of the hormone, but these factors, within the above ranges, showed no distinct dose‐response relationship to yellowing. Even 160/μg of cumulative dose, given in the relatively most effective way (eight repeated injections of 20 /μg each), did not fully restore mating behaviour; the level of maximum intensity reached about half of that obtained for non‐allatectomized controls. JHI and JHIII did not differ markedly in this respect, but a 1:1 mixture of the two hormones improved the effect, revealing a mild synergy. JHI induced maximum or near maximum yellowing, but JHIII, under any conditions tested, was unable to induce full yellowing. The effect was always temporary; after cessation of JH administrations mating behaviour rapidly declined and the yellow colour slowly faded. The results indicate that the response of the two physiological systems (male mating behaviour and yellowing) to exogenous JH is completely independent. Although, depending on conditions of administration and on the kind of hormone, exogenous JHs were able to restore mating behaviour and yellowing, effective doses were extremely high. These conclusions are further discussed.