Maternal effects on phase characteristics in the desert locust, Schistocerca gregaria: a review of current understanding

Egg size-dependent embryonic development in the desert locust, Schistocerca gregaria

Phenotypic plasticity in body size is a product of modification of the developmental pathway. Although hatchlings of the desert locust, Schistocerca gregaria, show egg size-dependent plasticity in body size, it remains unclear how embryogenesis during egg development regulates final embryonic body size. To determine the developmental pathway causing body size variation at hatching, we examined egg and embryonic development at the early, middle, and late egg developmental stages in S. gregaria by comparing small and large eggs. Crowd-reared females produced larger eggs than isolated-reared females. The daily egg developmental rate was similar between small and large eggs: eggs dramatically absorbed external water after days 3 to 7 and nearly doubled the initial egg weight at the late stage of day 12. Morphological measurements of eggs and embryos at different days after oviposition revealed that large eggs were longer than small eggs throughout developmental stages. However, embryo length was similar between small and large eggs at the early stage (anatrepsis). Embryos begin to absorb yolk into their bodies after blastokinesis. The size of large-egg embryos increased significantly from the middle stage (katatrepsis) due to absorption of more yolk than small eggs. Egg length and embryo length were conspicuously larger in large eggs than in small eggs on day 12 of late katatrepsis. These results suggest that egg size did not influence the egg developmental rate and initial embryo size. Large eggs had more yolk and space, resulting in larger final embryos than small eggs. The amount of yolk and size of eggshells during katatrepsis could play a key role in determining hatchling body size in S. gregaria.

A journey towards an integrated understanding of behavioural phase change in locusts

Behavioural phase change initiates and functionally couples the suite of traits that comprise density-dependent polyphenism in locusts. Here I provide a semi-expurgated account of my 25-year research journey studying behavioural phase transition in the desert locust. The journey spans continents, involves a cast of extraordinary colleagues, and travels across levels of biological organisation from deep within the nervous system of individual locusts to mass migration and the evolution and population dynamics of swarming.

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.

Who listens to mother? A whole-family perspective on the evolution of maternal hormone allocation

Maternal effects, or the influence of maternal environment and phenotype on offspring phenotype, may allow mothers to fine-tune their offspring's developmental trajectory and resulting phenotype sometimes long after the offspring has reached independence. However, maternal effects on offspring phenotype do not evolve in isolation, but rather within the context of a family unit, where the separate and often conflicting evolutionary interests of mothers, fathers and offspring are all at play. While intrafamilial conflicts are routinely invoked to explain other components of reproductive strategy, remarkably little is known about how intrafamilial conflicts influence maternal effects. We argue that much of the considerable variation in the relationship between maternally derived hormones, nutrients and other compounds and the resulting offspring phenotype might be explained by the presence of conflicting selection pressures on different family members. In this review, we examine the existing literature on maternal hormone allocation as a case study for maternal effects more broadly, and explore new hypotheses that arise when we consider current findings within a framework that explicitly incorporates the different evolutionary interests of the mother, her offspring and other family members. Specifically, we hypothesise that the relationship between maternal hormone allocation and offspring phenotype depends on a mother's ability to manipulate the signals she sends to offspring, the ability of family members to be plastic in their response to those signals and the capacity for the phenotypes and strategies of various family members to interact and influence one another on both behavioural and evolutionary timescales. We also provide suggestions for experimental, comparative and theoretical work that may be instrumental in testing these hypotheses. In particular, we highlight that manipulating the level of information available to different family members may reveal important insights into when and to what extent maternal hormones influence offspring development. We conclude that the evolution of maternal hormone allocation is likely to be shaped by the conflicting fitness optima of mothers, fathers and offspring, and that the outcome of this conflict depends on the relative balance of power between family members. Extending our hypotheses to incorporate interactions between family members, as well as more complex social groups and a wider range of taxa, may provide exciting new developments in the fields of endocrinology and maternal effects.

The desert locust, Schistocerca gregaria, plastically manipulates egg size by regulating both egg numbers and production rate according to population density

Egg-size adjustment is one of the important plastic life-history traits for animals living in heterogeneous environments. The adaptive investment hypothesis predicts that mothers should increase progeny size according to certain cues predicting adverse future conditions of their offspring. However, reproductive resources are limited, and females have to simultaneously reduce egg number to allocate more resources to increase size. It remains unclear how single individuals alter egg size and number according to temporally heterogeneous environments. In the present study, we examined how desert locusts, Schistocerca gregaria, plastically alter egg size and number according to population density. We also investigated the trans-generational maternal effects on progeny characteristics as well as their own maternal physiological response (oviposition interval). Females kept in crowded conditions laid significantly larger and heavier eggs by reducing clutch size (number of eggs per egg pod) compared to isolated females, suggesting the existence of a reproductive trade-off between the two traits. The crowding-forced isolated females induced concerted changes not only in egg size but also in egg number tending towards those characteristics of gregarious control, implying that single individuals showed trade-off when egg size was increased. Double-blind testing confirmed the rapid crowding effects on egg size. Females also responded to crowding by extending the oviposition interval. As the oviposition interval extended, egg size increased, but clutch size decreased. Eggs from crowding-forced isolated females began to produce gregarious-phase type hatchlings (large and black) instead of solitarious-phase type ones (small and green). These results suggested that S. gregaria plastically manipulate egg size by regulating egg numbers and egg production rate, and indicated the presence of trans-generational maternal effects on progeny phase.

Epigenetics and insect polyphenism: mechanisms and climate change impacts

Phenotypic plasticity is a ubiquitous process found in all living organisms. Polyphenism is an extreme case of phenotypic plasticity which shares a common scheme in insects such as honeybees, locusts or aphids: an initial perception of environmental stimuli, a neuroendocrine transmission of these signals to the target tissues, the activation of epigenetic mechanisms allowing the setup of alternative transcriptional programs responsible for the establishment of discrete phenotypes. Climate change can modulate the environmental stimuli triggering polyphenisms, and/or some epigenetics marks, thus modifying on the short and long terms the discrete phenotype proportions within populations. This might result in critical ecosystem changes.

Parental effect of diapause in relation to photoperiod and temperature in the cabbage beetle, Colaphellus bowringi (Coleoptera: Chrysomelidae)

Increasing evidence has demonstrated that the environmental conditions experienced by parents can shape offspring phenotypes. Here, we examined the effects of the photoperiod and temperature experienced by parents on the incidence of diapause in their progeny in the cabbage beetle, Colaphellus bowringi, using three experiments. The first experiment examined parental diapause incidence under different photoperiods at 25°C and the incidence of diapause in progeny from both non-diapausing and diapausing parents under the same rearing conditions. The results revealed that the incidence of diapause among progeny was exactly opposite to that of their parents, i.e., higher parental diapause incidence led to lower progeny diapause incidence, showing a negative relationship in diapause incidence between the parental generation and the progeny generation. The incidence of diapause among progeny produced by diapausing parents was higher than that in progeny produced by non-diapausing parents. The second experiment examined parental diapause incidence at different temperatures under LD 12:12 and the incidence of diapause in progeny from both non-diapausing and diapausing parents under the same rearing conditions. Similarly, the incidence of diapause in progeny was also opposite to that of their parents. However, the incidence of diapause in progeny produced by non-diapausing parents was different from that in progeny produced by diapausing parents. In the third experiment, naturally diapausing adults were maintained at a constant temperature of 9, 28°C or the mean daily summer temperature of 27.84°C under continuous darkness for 3 months of dormancy. After dormancy, the progeny of these post-diapause parents were reared under different photoperiods at 25°C. The results showed that the incidence of diapause among progeny was higher when their parents experienced high temperatures than when they experienced low temperatures. All results demonstrate that the photoperiod and temperature experienced by parents may significantly affect the diapause incidence among progeny.

Adult Influence on Juvenile Phenotypes by Stage-Specific Pheromone Production

Many animal and plant species respond to population density by phenotypic plasticity. To investigate if specific age classes and/or cross-generational signaling affect density-dependent plasticity, we developed a dye-based method to differentiate co-existing nematode populations. We applied this method to Pristionchus pacificus, which develops a predatory mouth form to exploit alternative resources and kill competitors in response to high population densities. Remarkably, adult, but not juvenile, crowding induces the predatory morph in other juveniles. High-performance liquid chromatography-mass spectrometry of secreted metabolites combined with genetic mutants traced this result to the production of stage-specific pheromones. In particular, the P. pacificus-specific di-ascaroside#1 that induces the predatory morph is induced in the last juvenile stage and young adults, even though mouth forms are no longer plastic in adults. Cross-generational signaling between adults and juveniles may serve as an indication of rapidly increasing population size, arguing that age classes are an important component of phenotypic plasticity.

Social density, but not sex ratio, drives ecdysteroid hormone provisioning to eggs by female house crickets (Acheta domesticus)

Social environment profoundly influences the fitness of animals, affecting their probability of survival to adulthood, longevity, and reproductive output. The social conditions experienced by parents at the time of reproduction can predict the social environments that offspring will face. Despite clear challenges in predicting future environmental conditions, adaptive maternal effects provide a mechanism of passing environmental information from parent to offspring and are now considered pervasive in natural systems. Maternal effects have been widely studied in vertebrates, especially in the context of social environment, and are often mediated by steroid hormone (SH) deposition to eggs. In insects, although many species dramatically alter phenotype and life-history traits in response to social density, the mechanisms of these alterations, and the role of hormone deposition by insect mothers into their eggs, remains unknown. In the experiments described here, we assess the effects of social environment on maternal hormone deposition to eggs in house crickets (Acheta domesticus). Specifically, we tested the hypotheses that variable deposition of ecdysteroid hormones (ESH) to eggs is affected by both maternal (a) social density and (b) social composition. We found that while maternal hormone deposition to eggs does not respond to social composition (sex ratio), it does reflect social density; females provision their eggs with higher ESH doses under low-density conditions. This finding is consistent with the interpretation that variable ESH provisioning is an adaptive maternal response to social environment and congruent with similar patterns of variable maternal provisioning across the tree of life. Moreover, our results confirm that maternal hormone provisioning may mediate delayed density dependence by introducing a time lag in the response of offspring phenotype to population size.

Epigenetics and developmental plasticity in orthopteroid insects

Developmental plasticity is a key driver of the extraordinary ecological success of insects. Epigenetic mechanisms provide an important link between the external stimuli that initiate polyphenisms, and the stable changes in gene expression that govern alternative insect morphs. We review the epigenetics of orthopteroid insects, focussing on recent research on locusts and termites, two groups which display high levels of phenotypic plasticity, and for which genome sequences have become available in recent years. We examine research on the potential role of DNA methylation, histone modifications, and non-coding RNAs in the regulation of gene expression in these insects. DNA methylation patterns in orthopteroids share a number of characteristics with those of hymenopteran insects, although methylation levels are much higher, and extend to introns and repeat elements. Future examinations of epigenetic mechanisms in these insects will benefit from comparison of tissues from aged-matched individuals from alternative morphs, and adequate biological replication.

Locust Collective Motion and Its Modeling

Over the past decade, technological advances in experimental and animal tracking techniques have motivated a renewed theoretical interest in animal collective motion and, in particular, locust swarming. This review offers a comprehensive biological background followed by comparative analysis of recent models of locust collective motion, in particular locust marching, their settings, and underlying assumptions. We describe a wide range of recent modeling and simulation approaches, from discrete agent-based models of self-propelled particles to continuous models of integro-differential equations, aimed at describing and analyzing the fascinating phenomenon of locust collective motion. These modeling efforts have a dual role: The first views locusts as a quintessential example of animal collective motion. As such, they aim at abstraction and coarse-graining, often utilizing the tools of statistical physics. The second, which originates from a more biological perspective, views locust swarming as a scientific problem of its own exceptional merit. The main goal should, thus, be the analysis and prediction of natural swarm dynamics. We discuss the properties of swarm dynamics using the tools of statistical physics, as well as the implications for laboratory experiments and natural swarms. Finally, we stress the importance of a combined-interdisciplinary, biological-theoretical effort in successfully confronting the challenges that locusts pose at both the theoretical and practical levels.

Epigenetics and locust life phase transitions

Insects are one of the most successful classes on Earth, reflected in an enormous species richness and diversity. Arguably, this success is partly due to the high degree to which polyphenism, where one genotype gives rise to more than one phenotype, is exploited by many of its species. In social insects, for instance, larval diet influences the development into distinct castes; and locust polyphenism has tricked researchers for years into believing that the drastically different solitarious and gregarious phases might be different species. Solitarious locusts behave much as common grasshoppers. However, they are notorious for forming vast, devastating swarms upon crowding. These gregarious animals are shorter lived, less fecund and transmit their phase characteristics to their offspring. The behavioural gregarisation occurs within hours, yet the full display of gregarious characters takes several generations, as does the reversal to the solitarious phase. Hormones, neuropeptides and neurotransmitters influence some of the phase traits; however, none of the suggested mechanisms can account for all the observed differences, notably imprinting effects on longevity and fecundity. This is why, more recently, epigenetics has caught the interest of the polyphenism field. Accumulating evidence points towards a role for epigenetic regulation in locust phase polyphenism. This is corroborated in the economically important locust species Locusta migratoria and Schistocerca gregaria. Here, we review the key elements involved in phase transition in locusts and possible epigenetic regulation. We discuss the relative role of DNA methylation, histone modification and small RNA molecules, and suggest future research directions.

The information value of non-genetic inheritance in plants and animals

Parents influence the development of their offspring in many ways beyond the transmission of DNA. This includes transfer of epigenetic states, nutrients, antibodies and hormones, and behavioural interactions after birth. While the evolutionary consequences of such non-genetic inheritance are increasingly well understood, less is known about how inheritance mechanisms evolve. Here, we present a simple but versatile model to explore the adaptive evolution of non-genetic inheritance. Our model is based on a switch mechanism that produces alternative phenotypes in response to different inputs, including genes and non-genetic factors transmitted from parents and the environment experienced during development. This framework shows how genetic and non-genetic inheritance mechanisms and environmental conditions can act as cues by carrying correlational information about future selective conditions. Differential use of these cues is manifested as different degrees of genetic, parental or environmental morph determination. We use this framework to evaluate the conditions favouring non-genetic inheritance, as opposed to genetic determination of phenotype or within-generation plasticity, by applying it to two putative examples of adaptive non-genetic inheritance: maternal effects on seed germination in plants and transgenerational phase shift in desert locusts. Our simulation models show how the adaptive value of non-genetic inheritance depends on its mechanism, the pace of environmental change, and life history characteristics.

Effects of temperature and moisture on Mormon cricket reproduction with implications for responses to climate change

During the last decade, populations of flightless Mormon crickets Anabrus simplex (Orthoptera: Tettigoniidae) increased suddenly over vast areas of the Western United States, suggesting that climate is an important factor driving outbreaks. Moreover summer temperatures are predicted to increase and precipitation is expected to decrease in most areas of the U.S. Great Basin, but little is known of the response of Mormon crickets to changes in temperature and soil moisture. In a laboratory study, we varied ambient temperature and lighting and measured the propensity of mating pairs to mate, and the proportion of eggs that developed into embryos. We found that reproduction was optimal when ambient temperature reached 30°C and the insects were beneath broad-spectrum lights such that maternal body and soil temperatures reached 35°C. Fewer eggs that developed fully were laid when maternal body and soil temperatures reached 30°C or 37-39°C. We also varied initial soil moisture from 0% to 100% saturated and found that more eggs reached embryonic diapause when initial soil moisture was 25% or 50% of saturated volume. However more of the developed eggs hatched when treated in summer soils with 0-25% of saturated moisture. We conclude that small changes in temperature had large effects on reproduction, whereas large changes in moisture had very small effects on reproduction. This is the first report of Mormon crickets mating in a laboratory setting and laying eggs that hatched, facilitating further research on the role of maternal and embryonic environments in changes in population size.

Non-swarming grasshoppers exhibit density-dependent phenotypic plasticity reminiscent of swarming locusts

Locusts are well known for exhibiting an extreme form of density-dependent phenotypic plasticity known as locust phase polyphenism. At low density, locust nymphs are cryptically colored and shy, but at high density they transform into conspicuously colored and gregarious individuals. Most of what we know about locust phase polyphenism come from the study of the desert locust Schistocerca gregaria (Forskål), which is a devastating pest species affecting many countries in North Africa and the Middle East. The desert locust belongs to the grasshopper genus Schistocerca Stål, which includes mostly non-swarming, sedentary species. Recent phylogenetic studies suggest that the desert locust is the earliest branching lineage within Schistocerca, which raises a possibility that the presence of density-dependent phenotypic plasticity may be a plesiomorphic trait for the whole genus. In order to test this idea, we have quantified the effect of rearing density in terms of the resulting behavior, color, and morphology in two non-swarming Schistocerca species native to Florida. When reared in both isolated and crowded conditions, the two non-swarming species, Schistocerca americana (Drury) and Schistocerca serialis cubense (Saussure) clearly exhibited plastic reaction norms in all traits measured, which were reminiscent of the desert locust. Specifically, we found that both species were more active and more attracted to each other when reared in a crowded condition than in isolation. They were mainly bright green in color when isolated, but developed strong black patterns and conspicuous background colors when crowded. We found a strong effect of rearing density in terms of size. There were also more mechanoreceptor hairs on the outer face of the hind femora in the crowded nymphs in both species. Although both species responded similarly, there were some clear species-specific differences in terms of color and behavior. Furthermore, we compare and contrast our findings with those on the desert locust and other relevant studies. We attribute the presence of density-dependent phenotypic plasticity in the non-swarming Schistocerca species to phylogenetic conservatism, but there may be a possible role of local adaptation in further shaping the ultimate expressions of plasticity.

Neuropeptidergic regulation of reproduction in insects

Successful animal reproduction depends on multiple physiological and behavioral processes that take place in a timely and orderly manner in both mating partners. It is not only necessary that all relevant processes are well coordinated, they also need to be adjusted to external factors of abiotic and biotic nature (e.g. population density, mating partner availability). Therefore, it is not surprising that several hormonal factors play a crucial role in the regulation of animal reproductive physiology. In insects (the largest class of animals on planet Earth), lipophilic hormones, such as ecdysteroids and juvenile hormones, as well as several neuropeptides take part in this complex regulation. While some peptides can affect reproduction via an indirect action (e.g. by influencing secretion of juvenile hormone), others exert their regulatory activity by directly targeting the reproductive system. In addition to insect peptides with proven activities, several others were suggested to also play a role in the regulation of reproductive physiology. Because of the long evolutionary history of many insect orders, it is not always clear to what extent functional data obtained in a given species can be extrapolated to other insect taxa. In this paper, we will review the current knowledge concerning the neuropeptidergic regulation of insect reproduction and situate it in a more general physiological context.

Behavioral phase shift in nymphs of the desert locust, Schistocerca gregaria: special attention to attraction/avoidance behaviors and the role of serotonin

Schistocerca gregaria exhibits a phase-specific behavior in response to crowding. Nymphs occurring at low population densities (solitarious phase) tend to avoid one another, whereas those occurring at high population densities (gregarious phase) are attracted to one another. This study examined how this attraction/avoidance behavior changed after isolation or crowding. The behavior of the test nymphs was assessed by determining their positioning with respect to a stimulus cup, which contained 12 gregarious nymphs, placed at one end of an elliptical arena and an empty cup placed at the opposite end. Gregarious (crowd-reared) nymphs were most frequently observed close to the stimulus cup, whereas solitarious (isolated-reared) nymphs tended to avoid it. This tendency was easily changed by exposing the nymphs to isolation or crowding. However, contrary to a previously reported conclusion that behavioral gregarization occurs in 4-8 h of crowding, the nymphs required at least 3 days to achieve a significant change in the attraction/avoidance behavior in either direction, from solitarious to gregarious or the converse. The discrepancies between the present study and previous studies appear to result from the different behaviors observed. The present study focused on the attraction/avoidance behavior that is most likely the most important element in the gregarization and solitarization processes, whereas other studies used a mixture of various activity-related behaviors. No evidence was obtained for a trans-generational accumulation in the attraction/avoidance behaviors. Serotonin, a biogenic amine that is suggested by others to be involved in behavioral phase changes in S. gregaria, had no influence on this behavior or body color when injected into the solitarious nymphs.

Phase-dependent locomotor activity in first-stadium nymphs of the desert locust, Schistocerca gregaria: effects of parental and progeny rearing density

This study examined the effects of parental and progeny rearing density on locomotor activity of 1st-stadium nymphs of the desert locust, Schistocerca gregaria, using an actograph. Progeny obtained from solitarious (isolated-reared) or gregarious (crowd-reared) locusts were reared in isolation or in a group of 30 nymphs. Crowding after hatching had a slight influence on mean activity shortly after the start of measurements, but no clear effect was detected until day 2, when maximum activity during the 6-24 h of observation was significantly higher than that of the nymphs kept in isolation. On the other hand, the effects of parental rearing density on locomotor activity manifested at all ages examined (0-2 days old). Progeny of gregarious locusts showed consistently higher activity than those of solitarious locusts. In newly hatched nymphs, the effect of parental rearing density was explained by variation in body size at hatching, one of the phase-dependent characteristics. Hatchling body color was also correlated with locomotor activity and body weight. Similar levels of locomotor activity were exhibited when green, solitarious and black, gregarious nymphs were similar in body weight. These results suggested that parental rearing density indirectly influences locomotor activity in the progeny shortly after hatching by affecting their body size as eggs or hatchlings.

Locust phase polyphenism: Does epigenetic precede endocrine regulation?

The morphological, physiological and behavioural differences between solitarious and gregarious desert locusts are so pronounced that one could easily mistake the two phases as belonging to different species, if one has no knowledge of the phenomenon of phenotypic plasticity. A number of phase-specific features are hormonally controlled. Juvenile hormone promotes several solitarious features, the green cuticular colour being the most obvious one. The neuropeptide corazonin elicits the dark cuticular colour that is typical for the gregarious phase, as well as particular gregarious behavioural characteristics. However, it had to be concluded, for multiple reasons, that the endocrine system is not the primary phase-determining system. Our observation that longevity gets imprinted in very early life by crowding of the young hatchlings, and that it cannot be changed thereafter, made us consider the possibility that, perhaps, epigenetic control of gene expression might be, if not the missing, a primary phase-determining mechanism. Imprinting is likely to involve DNA methylation and histone modification. Analysis of a Schistocerca EST database of nervous tissue identified the presence of several candidate genes that may be involved in epigenetic control, including two DNA methyltransferases (Dnmts). Dnmt1 and Dnmt2 are phase-specifically expressed in certain tissues. In the metathoracic ganglion, important in the serotonin pathway for sensing mechanostimulation, their expression is clearly affected by crowding. Our data urge for reconsidering the role of the endocrine system as being sandwiched in between genetics and epigenetics, involving complementary modes of action.

A theoretical model of the evolution of maternal effects under parent-offspring conflict

The evolution of maternal effects on offspring phenotype should depend on the extent of parent-offspring conflict and costs and constraints associated with maternal and offspring strategies. Here, we develop a model of maternal effects on offspring dispersal phenotype under parent-offspring conflict to evaluate such dependence. In the absence of evolutionary constraints and costs, offspring evolve dispersal rates from different patch types that reflect their own, rather than the maternal, optima. This result also holds true when offspring are unable to assess their own environment because the maternal phenotype provides an additional source of information. Consequently, maternal effects on offspring diapause, dispersal, and other traits that do not necessarily represent costly resource investment are more likely to maximize offspring than maternal fitness. However, when trait expression was costly, the evolutionarily stable dispersal rates tended to deviate from those under both maternal and offspring control. We use our results to (re)interpret some recent work on maternal effects and their adaptive value and provide suggestions for future work.

Gregarious desert locusts have substantially larger brains with altered proportions compared with the solitarious phase

The behavioural demands of group living and foraging have been implicated in both evolutionary and plastic changes in brain size. Desert locusts show extreme phenotypic plasticity, allowing brain morphology to be related to very different lifestyles in one species. At low population densities, locusts occur in a solitarious phase that avoids other locusts and is cryptic in appearance and behaviour. Crowding triggers the transformation into the highly active gregarious phase, which aggregates into dense migratory swarms. We found that the brains of gregarious locusts have very different proportions and are also 30 per cent larger overall than in solitarious locusts. To address whether brain proportions change with size through nonlinear scaling (allometry), we conducted the first comprehensive major axis regression analysis of scaling relations in an insect brain. This revealed that phase differences in brain proportions arise from a combination of allometric effects and deviations from the allometric expectation (grade shifts). In consequence, gregarious locusts had a larger midbrainoptic lobe ratio, a larger central complex and a 50 per cent larger ratio of the olfactory primary calyx to the first olfactory neuropile. Solitarious locusts invest more in low-level sensory processing, having disproportionally larger primary visual and olfactory neuropiles, possibly to gain sensitivity. The larger brains of gregarious locusts prioritize higher integration, which may support the behavioural demands of generalist foraging and living in dense and highly mobile swarms dominated by intense intraspecific competition.

Genetic variability in tolerance to copper contamination in a herbivorous marine invertebrate

Anthropogenic stresses such as metal contamination can have profound ecological impacts in a wide range of habitats. Reduced survival of organisms in contaminated habitats has the potential to result in the evolution of genotypes tolerant to deleterious contaminants. Local adaptation to contamination requires directional selection, genetic variation in traits relating to tolerance, spatial variability in exposure to the contaminant and limited gene flow between populations. This study assesses variation in tolerance in the herbivorous marine invertebrate Peramphithoe parmerong whose algal diets in Sydney Harbour readily accumulate the metal copper. A quantitative genetics approach (a full-sib, split family design) was used to quantify variation among families in survival on the contaminated diet. A significant genotype-by-environment interaction in offspring survival between the copper contaminated and uncontaminated diet treatments revealed variation in tolerance to copper by P. parmerong. Amphipods that survived 30 days of exposure to copper contaminated diets were slightly smaller and ate less algae than those reared on uncontaminated food. This reflects an additional sub-lethal effect associated with the consumption of contaminated algae. However, there was no evidence of acclimation to contaminated diets, nor a cost of reduced feeding for those genotypes with increased tolerance. This study provides strong evidence for the potential of a marine invertebrate to evolve tolerance to contaminants found in their diet.

A review of maternal and embryonic control of phase-dependent progeny characteristics in the desert locust

Hatchlings of the desert locust, Schistocerca gregaria, display phase polyphenism in body coloration and size. This phenomenon has been found to be maternally controlled and two different mechanisms have been proposed to explain the underlying process. One mechanism claims that a water-soluble pheromonal factor secreted by gregarious female adults into the foam plugs of egg pods induces darkening in their progeny. The other mechanism states that hatchling body coloration is pre-determined in the ovaries and that no foam factor is involved in this phenomenon. This mechanism was supported by the observation that hatchling melanization was not prevented by the early washing or separation of eggs, which should have removed the pheromonal factor from the eggs and produced green hatchlings according to the other mechanism. This paper reviews the latest findings related to this phenomenon with special reference to reproductive cycles and genetic differences. The close relationships between egg size and the degree of melanization in hatchlings may provide strong evidence against the possible involvement of the pheromonal factor, because egg size is determined in the ovarioles. Furthermore, the absence of "solitarizing" effects of early washing and separation on hatchling melanization was also confirmed in different genetic strains. A hypothesis proposed by others that such effects occur only in eggs from the first reproductive cycle was tested and rejected. Based on these and other results, a model to explain the mechanisms underlying the maternal control of progeny characteristics and embryonic control of melanization in the hatchling was proposed.

The trans-generational phase accumulation in the desert locust: morphometric changes and extra molting

To understand the underlying trans-generational phase accumulation, a classical morphometric characteristic, the F/C ratio (F, hind femur length; C, maximum head width), of adult desert locusts (Schistocerca gregaria) was monitored over eight consecutive generations. Adult F/C ratios, which are larger in solitarious locusts than in gregarious ones, were negatively correlated to the darkness of body color at hatching. Two successive generations were required for a complete shift from the gregarious (crowd-reared) to the solitarious (isolated-reared) phase and vice versa in the laboratory. That is (1) female adults needed to be exposed to crowded (or isolated) conditions so that their hatchlings would become large (or small) and dark (or green) in color, and (2) the hatchlings then needed to be exposed to crowded (or isolated) conditions for their entire nymphal stage. Solitarious locusts exhibited extra molting that influenced the F/C ratio in the adult stage, but did not exert significant influences on the trans-generational changes in this trait because the incidence was low. The incidence of extra molting was negatively correlated with nymphal survival rates. The morphometric trans-generational changes may be explained without assuming any accumulating internal factor.

Artificial miniaturization causes eggs laid by crowd-reared (gregarious) desert locusts to produce green (solitarious) offspring in the desert locust, Schistocerca gregaria

The mechanism underlying the phase-dependent polyphenism in hatchling body coloration was studied by testing for a possible causal relationship with egg size in the desert locust, Schistocerca gregaria. Crowd-reared (gregarious) females typically produce large, black offspring, whereas females reared in isolation (solitarious) deposit small, green offspring. We first tested for possible genetic differences in the role of egg foam by washing or separating eggs from two strains of locust. No solitarizing effect was found in either of the strains tested, supporting a previous finding, using another laboratory strain, to show that the hatchling body coloration and size are pre-determined in the ovary of the mother and no egg foam factor is involved in the control of the hatchling body coloration. Topical application of fenoxycarb, a juvenile hormone analog (JHA), and implantation of extra corpora allata (CA), taken from Locusta migratoria, caused gregarious female adults of S. gregaria to produce small eggs. Some eggs laid by CA-implanted females produced green hatchlings. All large eggs chosen among those deposited by gregarious females produced black hatchlings. When eggs were either kept on dry filter paper at nearly saturated relative humidity during embryogenesis or pricked with a needle so that some egg yolk was squeezed out, some produced small, green hatchlings. These results suggested that the amount of egg yolk or the availability of yolk material may determine the body coloration of hatchlings.

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.

Endocrinology of reproduction and phase transition in locusts

In the last decade, important progress has been made in the experimental analysis of the endocrine mechanisms controlling reproduction and phase transition in locusts. Phase transition is a very fascinating, but complex, phenomenon of phenotypic plasticity that is triggered by changes in population density and can lead to the formation of extremely devastating hopper bands and adult gregarious locust swarms. While some phase characteristics change within hours, others appear more gradually in the next stage(s), or even in the next generation(s). In adults, the phase status also has a major influence on the process of reproduction. A better understanding of how solitarious locusts become gregarious and how this switch affects reproductive physiology may result in novel strategies to fight locust plagues. In this paper, we will review the current knowledge concerning this close interaction between locust phase polyphenism and reproduction.

Development and evolution of insect pigmentation: genetic mechanisms and the potential consequences of pleiotropy

Insect pigmentation is a premier model system in evolutionary and developmental biology. It has been at the heart of classical studies as well as recent breakthroughs. In insects, pigments are produced by epidermal cells through a developmental process that includes pigment patterning and synthesis. Many aspects of this process also impact other phenotypes, including behavior and immunity. This review discusses recent work on the development and evolution of insect pigmentation, with a focus on pleiotropy and its effects on color pattern diversification.

Phase-specific developmental and reproductive strategies in the desert locust

Locusts modify developmental and reproductive traits over successive generations depending on the population density. A trade-off between developmental rate and body size and between progeny size and number is often observed in organisms. In this study, we present evidence that this rule is evaded by desert locusts, Schistocerca gregaria Forskål, which often undergo outbreaks. Under isolated conditions, large hatchlings, typical of the gregarious forms, grow faster but emerge as larger adults than do small hatchlings typical of the solitarious forms, except for some individuals of the latter group that undergo extra molting. Under crowded conditions, large and small hatchlings grow at a similar rate, but the former become larger adults than the latter. Small hatchlings show a trade-off between development time and body size at maturation, but this constraint is avoided by large hatchlings. Phase-specific, as well as body size-dependent, differences are also detected in reproductive performance. As adult body size increases, females of a solitarious line produce more but slightly smaller eggs, whereas those of a gregarious line produce more and larger eggs. Total egg mass per pod is larger in gregarious forms than in solitarious forms. A trade-off between egg size and number is shown by a solitarious line but not by a gregarious line that produces relatively large eggs with similar numbers of eggs per pod. These results suggest that phase transformation involves not just a shift of resource allocation but also an enhanced capability expressed in response to crowding.

Maternal effects on progeny size, number and body color in the desert locust, Schistocerca gregaria: Density- and reproductive cycle-dependent variation

The effects of rearing density and maternal age on the progeny size, number and coloration of the desert locust, Schistocerca gregaria, were investigated. Isolated-reared females deposited smaller, but more eggs than crowd-reared females. The former produced smaller and more eggs with age, whereas the latter showed a tendency to produce larger and fewer eggs over time. A similar tendency was also observed with virgin females, indicating that mating or the presence of males was not important. The first egg pod produced by each mated crowd-reared female contained significantly smaller and more eggs than did the subsequent egg pods. The former often produced many green hatchlings (0-100%) characteristic of solitarious forms, whereas the egg pods deposited after the first pod produced predominantly black hatchlings typical of gregarious forms. Adults were highly sensitive to a shift in rearing density and quickly modified the quality and quantity of their progeny depending on the density encountered. The number of eggs per pod was influenced not only by the mother's rearing density but also by rearing density of the grandmother. The present results demonstrated that the characteristics of progeny are influenced not only by the crowding conditions experienced by the mother and grandmother but also by the mother's reproductive cycle.

Quantitative RT-PCR analysis of pacifastin-related precursor transcripts during the reproductive cycle of solitarious and gregarious desert locusts

In locusts, little is known about the physiological and biochemical mechanisms regulating complex processes, such as reproduction and phase transition. The pacifastin family constitutes a family of peptidic inhibitors of serine proteases that are considered to be important regulators of several physiological processes in arthropods. We have performed a detailed transcript profiling analysis of two pacifastin-related peptide precursors, SGPP-2 and SGPP-4, during the reproductive cycle of adult desert locusts (Schistocerca gregaria). This quantitative real-time (RT)-PCR analysis revealed a temporal regulation of both transcripts, which is paralleled by several events that occur during the reproductive cycle of adult locusts. The observed temporal transcript profiles display a strong tissue-, gender- and phase-dependence. In addition, a partial regregarization experiment suggests that both transcript levels are regulated during phase transition and can be employed as molecular markers of the gregarization process.

Maternal effects on progeny body size and color in the desert locust, Schistocerca gregaria: examination of a current view

Hatchling body color and size of the desert locust, Schistocerca gregaria, are determined by the population density of the mothers during their reproductive period. Smaller green hatchlings are produced by adults at low population density (solitarious conditions) and larger dark hatchlings at high population density (gregarious conditions). One claim states that a pheromonal factor secreted by gregarious mothers into foam plugs of egg pods induces darkening in hatchlings. Previous research suggests that the foam factor can be removed by separating eggs individually within 1h of deposition, causing presumptive gregarious eggs to hatch without darkening. The present study re-examined this claim and possible factors that have been proposed which could account for the difference between our results and those reported earlier. Early separation was performed on eggs with a low mortality rate. The results showed that the egg separation did not increase the incidence of green hatchlings. Once chorionated in the ovary, eggs remained unchanged in size until the second day after oviposition in either isolated or crowded locusts. This and other results suggest that the phase-dependent differences in body size and color of hatchlings are established in the ovary and that modifications by the accessory gland factor either in the oviduct or after deposition are unlikely.

Swarm formation in the desert locust Schistocerca gregaria:isolation and NMR analysis of the primary maternal gregarizing agent

Depending on their rearing density, female desert locusts Schistocerca gregaria epigenetically endow their offspring with differing phenotypes. To identify the chemical basis for such maternal transmission, we compared solitarious and gregarious locust egg pod foam using high performance liquid chromatography (HPLC). We revealed causal relationships between foam chemistry and hatchling phenotype (phase state) by iteratively applying HPLC fractions from gregarious locust egg foam extracts to solitarious eggs and assessing resulting hatchlings with a behavioural bioassay involving logistic regression. Selection and application of increasingly specific HPLC fractions allowed us to isolate compounds with gregarizing properties. Hatchling gregarization was triggered only by certain fractions and was dose dependent. In a final series of experiments, we characterized the most specific gregarizing fraction by nuclear magnetic resonance (NMR) spectroscopy. Here we present tentative structural features of the primary locust maternal gregarizing agent, which appears to be an alkylated l-dopa analogue. In addition, we propose a mechanism for phase-dependent regulation of this compound's activity.