Endocrine mediated phenotypic plasticity: Condition-dependent effects of juvenile hormone on dominance and fertility of wasp queens

There has been increasing interest in the mechanisms that mediate behavioral and physiological plasticity across individuals with similar genotypes. Some of the most dramatic plasticity is found within and between social insect castes. For example, Polistes wasp queens can nest alone, dominate a group of cooperative queens, or act as worker-like subordinates who rarely reproduce. Previous work suggests that condition-dependent endocrine responses may play a role in plasticity between castes in the hymenoptera. Here, we test whether condition-dependent endocrine responses influence plasticity within castes in the wasp Polistes dominulus. We experimentally manipulate juvenile hormone (JH) titers in nest-founding queens and assess whether JH mediates variation in behavior and physiology. JH generally increased dominance and fertility of queens, but JH's effects were not uniform across individuals. JH had a stronger effect on the dominance and fertility of large individuals and individuals with facial patterns advertising high quality than on the dominance and fertility of small individuals and those advertising low quality. These results demonstrate that JH has condition-dependent effects. As such, they clarify how JH can mediate different behaviors in well nourished queens and poorly nourished workers. Many Polistes queens nest cooperatively with other queens, so condition-dependent hormonal responses provide a mechanism for queens to adaptively allocate energy based on their probability of successfully becoming the dominant queen. Research on the endocrine basis of plasticity often focuses on variation in endocrine titers alone. However, differential endocrine responses are likely to be a widespread mechanism mediating behavioral and physiological plasticity.     

Endocrine mediated phenotypic plasticity: Condition-dependent effects of juvenile hormone on dominance and fertility of wasp queens

There has been increasing interest in the mechanisms that mediate behavioral and physiological plasticity across individuals with similar genotypes. Some of the most dramatic plasticity is found within and between social insect castes. For example, Polistes wasp queens can nest alone, dominate a group of cooperative queens, or act as worker-like subordinates who rarely reproduce. Previous work suggests that condition-dependent endocrine responses may play a role in plasticity between castes in the hymenoptera. Here, we test whether condition-dependent endocrine responses influence plasticity within castes in the wasp Polistes dominulus. We experimentally manipulate juvenile hormone (JH) titers in nest-founding queens and assess whether JH mediates variation in behavior and physiology. JH generally increased dominance and fertility of queens, but JH's effects were not uniform across individuals. JH had a stronger effect on the dominance and fertility of large individuals and individuals with facial patterns advertising high quality than on the dominance and fertility of small individuals and those advertising low quality. These results demonstrate that JH has condition-dependent effects. As such, they clarify how JH can mediate different behaviors in well nourished queens and poorly nourished workers. Many Polistes queens nest cooperatively with other queens, so condition-dependent hormonal responses provide a mechanism for queens to adaptively allocate energy based on their probability of successfully becoming the dominant queen. Research on the endocrine basis of plasticity often focuses on variation in endocrine titers alone. However, differential endocrine responses are likely to be a widespread mechanism mediating behavioral and physiological plasticity.     

Lipid stores, ovary development, and brain gene expression in Polistes metricus females

In order to gain insights into the mechanistic basis of caste and behavioral differences in Polistes paper wasps, we examined abdominal lipid stores and ovary development in Polistes metricus females in four groups: foundresses, queens, workers, and gynes. Queens had the largest ovaries, followed by foundresses, workers, and gynes. Gynes had 6x higher lipid stores than the other groups, and lipid stores were lower in foragers (foundresses, workers) than non-foragers (queens, gynes). Lipid levels and ovary development were negatively correlated across the four groups, but removing gynes from the analysis revealed a significant positive correlation for foundresses, workers, and queens, suggesting different energy allocation strategies for gynes vs. other groups. Expression levels of 9 genes (including three in the insulin pathway), examined in a previous study, correlated with either lipid stores or ovary development. These correlative results suggest important relationships between nutrition, reproduction, and division of labor in primitively social insects. We also show that it is possible to assign P. metricus females to one of the four female groups on the basis of wing wear (an indicator of foraging experience), lipid stores, and ovary development, which can facilitate caste-specific collections for future studies. Received 23 May 2008; revised 6 November 2008; accepted 20 November 2008.     

Reproductive plasticity in Polistes paper wasp workers and the evolutionary origins of sociality

Regulatory pathways in solitary species provide the raw materials for the evolution of sociality. Therefore, comparing the mechanisms that mediate reproductive plasticity in social species and their solitary ancestors can provide insight into the evolutionary origin of sociality. In many solitary insects, the effect of juvenile hormone (JH) on fertility is mediated through the fat body; individuals in good physical condition show a stronger fertility response to JH than individuals in poor physical condition. Here, we test whether a similar, condition-dependent JH response mediates fertility in workers of the primitively eusocial Polistes dominulus wasps. We test how body weight, JH, and adult nutrition influence worker ovarian development. Both JH-treatment and adult nutrition dramatically increased ovarian development. Body weight also influenced ovarian development, as large workers developed more eggs than smaller workers. Body weight and fat are strongly linked in P. dominulus workers, so these results suggest that the fat-dependent JH responsiveness common in solitary insects is conserved in social wasps. The simple, ancestral relationship between reproductive investment and physical condition may facilitate cooperation by allowing workers to adaptively allocate energy into reproduction based on their probability of successfully becoming a queen.     

Incipient morphological castes in <Emphasis Type="Italic">Polistes gallicus</Emphasis> (Vespidae,Hymenoptera)

The occurrence of a pre-imaginal caste determination represents a sort of “point of no return” to eusociality. In some social insect taxa, including Polistes species, the occurrence of distinct queen and worker castes is still debated. Before this report, no clear morphological differences between reproductive and non-reproductive individuals were known. Here we show that several pre-imaginal morphological differences of the Van der Vecht organ occur between foundresses (queens) and workers. Geometric morphometrics revealed that queens are characterized by shape deformations of this organ, which is responsible for a typical allometric growth of the secretory area. This organ is predicted to be larger in foundresses compared to workers because its secretion is involved in defense against ants, in nestmate recognition and in preventing workers from challenging for direct reproduction. The results presented here indicate the existence of an incipient morphological caste determination Polistes gallicus and suggest that this species may have passed the “point of no return” for eusociality.     

Reproductive and worker castes in the primitively eusocial wasp Belonogaster petiolata (DeGeer) (Hymenoptera: Vespidae): evidence for pre-imaginal differentiation

Until recently, morphological differences between castes in independent-founding polistine wasps were considered absent. This paper investigates the extent of morphological and physiological differences between reproductive (foundress and gyne) and worker castes of Belonogaster petiolata, and tests the hypothesis that caste differentiation in this species occurs pre-imaginally.Foundresses were significantly larger than workers, to the extent that foundress/worker ratios were comparable with those between queens and workers in some swarm-founding Polistinae. Early emerging workers were small, but body size increased over the colony cycle such that late-season workers were similar in size to gynes. In proportion to body size, workers possessed broader heads while foundresses and gynes had broader thoraces and gasters. All queens, 98% of subordinate foundresses, and 95% of over-wintering gynes were inseminated. Workers were never inseminated and lacked mature ovaries in colonies with active queens. However, in the absence of the queen (and other foundresses), 11% of workers developed mature ovaries. Ovarian size and fat content of foundresses and gynes was significantly greater than that of workers. The differences in external morphology and reproductive physiology between castes support the hypothesis that differentiation occurs pre-imaginally. However, imaginal factors, in particular social dominance of the queen, maintain the reproductive subordinance of workers.     

Social dominance molds cuticular and egg chemical blends in a paper wasp

Hamilton's theory [1] for the evolution of social behaviour predicts that helpers may renounce direct reproduction to help their more fertile kin. Intra-colony recognition among queens and helpers (subordinate queens or workers) is consequently a central issue in insect sociobiology. In social insects, cuticular hydrocarbons (CHCs) are involved in recognition, and egg-laying and non-egg-laying individuals often differ in CHC composition. These differences are assumed to be directly determined by fertility status [2,3]. In several ants and in Polistes wasps, when egg-layers disappear they are substituted by helpers, which develop their ovaries and become chemically similar to their former queens [2,3]. Sometimes helpers lay eggs in the presence of queens, which recognize and destroy the subordinates' eggs [4]. In ponerine ants, eggs often have the same chemical signature as the maternal cuticle [2]. If chemical signatures depend on fertility, egg-laying subordinates should match the queen's signature even when she is present, making egg recognition and differential oophagy impossible. In the study reported here, we experimentally separated fertility from dominance and analyzed the dynamics of hydrocarbon profiles of the cuticle of Polistes dominulus foundresses and the shell surface of their eggs. We have demonstrated that, contrary to the widely accepted view, dominance, rather than fertility, determines chemical signatures in Polistes wasps. This explains why queens can recognize their own eggs and police reproduction by subordinates if they become fertile and lay eggs.     

Advertised quality,caste and food availability influence the survival cost of juvenile hormone in paper wasps

Life-history trade-offs are often hormonally mediated. Here, we provide a comparative perspective on the endocrine basis of life-history trade-offs by examining the invertebrate hormone juvenile hormone (JH). JH is often associated with benefits, including increased dominance and reproductive success. We tested whether JH reduced survival of Polistes dominulus wasps and whether this survival cost was influenced by factors such as advertised quality, food availability, caste and body size. Overall, JH reduced individual survival. Among fed and unfed queens, JH reduced survival in a dose-dependent manner. Among workers, JH had a stronger effect on survival of fed workers than unfed workers. Unfed workers died quickly and body size was the best predictor of survival. Surprisingly, queens and workers treated with JH survived longer when they had signals advertising high quality than when they had signals advertising low quality. The relationship between advertised quality and ability to withstand high levels of JH suggests that there are differential physiological costs associated with ornament elaboration that could play a role in maintaining signal accuracy over evolutionary time. Overall, the convergence of endocrine-mediated costs across diverse systems suggests that endocrine-mediated trade-offs may be an adaptive way to optimize resource allocation rather than a non-adaptive constraint specific to a particular hormone.     

Juvenile hormone titer and advertised quality are associated with timing of early spring activity in <Emphasis Type="Italic">Polistes dominulus</Emphasis> foundresses

Timing of diapause termination has an important influence on individual reproductive success, but relatively little research has explored how individuals differ in their response to diapause termination cues. We tested individual variation in the timing of post-diapause activity in Polistes dominulus paper wasps. Wasps were overwintered in a temperature-controlled chamber. In the spring, ambient temperature was gradually increased and the time each foundress became active was recorded. Timing of post-diapause activity was most strongly associated with the facial patterns that function as a conventional signal of quality. Foundresses with facial patterns indicating high quality became active at lower temperatures than individuals with facial patterns indicating low quality. Early diapause termination is associated with dominance, so the relationship between diapause termination and facial patterns provides a mechanism linking facial patterns with dominance. Body weight and mating status did not influence timing of post-diapause activity. Juvenile hormone (JH) titer at the time of diapause termination was also measured in a subset of foundresses. There was no JH titer threshold for diapause termination. Instead, our results suggest that individuals may have different threshold responses to JH, as individuals that became active at a lower temperature had lower JH titers than individuals that became active at a higher temperature. Overall, there is substantial individual variation in response to diapause termination cues and the variation is likely to have important impacts on the fitness of nest-founding females.     

The morphology of Van der Vecth's organ as a tool to measure caste dimorphism in Polistes paper wasps: a comparative approach

Given the centrality of chemical communication in social insects, there are many selective pressures acting on morpho‐functional traits that mediate chemical pheromones. On the last gastral sternite of Polistes females, there is an important exocrine surface secreting chemical pheromone, named Van der Vecht's organ. It is involved in chemical defence of the nest, in rank and nestmate recognition, preventing workers from direct reproduction. Allometric differential growth of phenotypic traits between castes of social insects is generally considered as an indication of incipient physical castes. European Polistes present different nesting strategies and reproductive choices. Here, we carry out a comparison of Van der Vecht's organ size between castes of four European Polistes to provide a general measure of dimorphism. We show that Van der Vecht's organ of Polistes dominula and Polistes nimphus foundresses shows an allometric development being enlarged with respect to workers. Otherwise, no allometries have been highlighted in the other two studied species (i.e. Polistes associus and Polistes biglumis). Therefore, our data show that neither rigid monogyny nor specific nesting habits foster the evolution of true morphological castes in primitively eusocial taxa. Thus, at least two other species of Ezuropean Polistes show real evidence of incipient morphological castes.     

The effect of juvenile hormone on temporal polyethism in the paper wasp Polistes dominulus

Juvenile hormone (JH) has an important role in the behavior of eusocial Hymenoptera. Previous work has shown that JH influences aggression and dominance behavior in primitive eusocial insects that lack discrete queen and worker castes (e.g. Bombus bees and Polistes wasps). In contrast, JH is one of the factors that mediates temporal polyethism among workers in advanced eusocial insects that have reproductive castes (e.g. Apis bees and Polybiawasps). Therefore, initial observations suggest that JH may have different roles in primitive and advanced eusocial taxa. Here, we use detailed behavioral observations of marked individuals to test whether JH influences temporal polyethism in the primitive eusocial wasp Polistes dominulus. First, we show that workers in P. dominulus have an age-related division of labor, as workers switch from nest work to foraging as they mature. Then, we show that application of JH accelerates the onset of foraging behavior.Workers treated with JH start foraging at a younger age than control workers. Therefore, JH mediates temporal polyethism in the primitively eusocial insect Polistes dominulus. Received 23 April 2008; revised 6 August 2008; accepted 11 August 2008     

Quantitative Differences in Nourishment Affect Caste-Related Physiology and Development in the Paper Wasp Polistes metricus

The distinction between worker and reproductive castes of social insects is receiving increased attention from a developmental rather than adaptive perspective. In the wasp genus Polistes, colonies are founded by one or more females, and the female offspring that emerge in that colony are either non-reproducing workers or future reproductives of the following generation (gynes). A growing number of studies now indicate that workers emerge with activated reproductive physiology, whereas the future reproductive gynes do not. Low nourishment levels for larvae during the worker-rearing phase of the colony cycle and higher nourishment levels for larvae when gynes are reared are now strongly suspected of playing a major role in this difference.Here, we present the results of a laboratory rearing experiment in which Polistes metricus single foundresses were held in environmental conditions with a higher level of control than in any previously published study, and the amount of protein nourishment made available to feed larvae was the only input variable. Three experimental feeding treatments were tested: restricted, unrestricted, and hand-supplemented. Analysis of multiple response variables shows that wasps reared on restricted protein nourishment, which would be the case for wasps reared in field conditions that subsequently become workers, tend toward trait values that characterize active reproductive physiology. Wasps reared on unrestricted and hand-supplemented protein, which replicates higher feeding levels for larvae in field conditions that subsequently become gynes, tend toward trait values that characterize inactive reproductive physiology. Although the experiment was not designed to test for worker behavior per se, our results further implicate activated reproductive physiology as a developmental response to low larval nourishment as a fundamental aspect of worker behavior in Polistes.     

Pleiotropic effects of juvenile hormone in ant queens and the escape from the reproduction–immunocompetence trade-off

The ubiquitous trade-off between survival and costly reproduction is one of the most fundamental constraints governing life-history evolution. In numerous animals, gonadotropic hormones antagonistically suppressing immunocompetence cause this trade-off. The queens of many social insects defy the reproduction–survival trade-off, achieving both an extraordinarily long life and high reproductive output, but how they achieve this is unknown. Here we show experimentally, by integrating quantification of gene expression, physiology and behaviour, that the long-lived queens of the ant Lasius niger have escaped the reproduction–immunocompetence trade-off by decoupling the effects of a key endocrine regulator of fertility and immunocompetence in solitary insects, juvenile hormone (JH). This modification of the regulatory architecture enables queens to sustain a high reproductive output without elevated JH titres and suppressed immunocompetence, providing an escape from the reproduction–immunocompetence trade-off that may contribute to the extraordinary lifespan of many social insect queens.     

Differential gene expression and phenotypic plasticity in behavioural castes of the primitively eusocial wasp, Polistes canadensis

Understanding how a single genome can produce a variety of different phenotypes is of fundamental importance in evolutionary and developmental biology. One of the most striking examples of phenotypic plasticity is the female caste system found in eusocial insects, where variation in reproductive (queens) and non-reproductive (workers) phenotypes results in a broad spectrum of caste types, ranging from behavioural through to morphological castes. Recent advances in genomic techniques allow novel comparisons on the nature of caste phenotypes to be made at the level of the genes in organisms for which there is little genome information, facilitating new approaches in studying social evolution and behaviour. Using the paper wasp Polistes canadensis as a model system, we investigated for the first time how behavioural castes in primitively eusocial insect societies are associated with differential expression of shared genes. We found that queens and newly emerged females express gene expression patterns that are distinct from each other whilst workers generally expressed intermediate patterns, as predicted by Polistes biology. We compared caste-associated genes in P. canadensis with those expressed in adult queens and workers of more advanced eusocial societies, which represent four independent origins of eusociality. Nine genes were conserved across the four taxa, although their patterns of expression and putative functions varied. Thus, we identify several genes that are putatively of evolutionary importance in the molecular biology that underlies a number of caste systems of independent evolutionary origin.     

The role of juvenile hormone in dominance behavior,reproduction and cuticular pheromone signaling in the caste-flexible epiponine wasp, <Emphasis Type="Italic">Synoeca surinama</Emphasis>

Background

The popular view on insect sociality is that of a harmonious division of labor among two morphologically distinct and functionally non-overlapping castes. But this is a highly derived state and not a prerequisite for a functional society. Rather, caste-flexibility is a central feature in many eusocial wasps, where adult females have the potential to become queens or workers, depending on the social environment. In non-swarming paper wasps (e.g., Polistes), prospective queens fight one another to assert their dominance, with losers becoming workers if they remain on the nest. This aggression is fueled by juvenile hormone (JH) and ecdysteroids, major factors involved in caste differentiation in most eusocial insects. We tested whether these hormones have conserved aggression-promoting functions in Synoeca surinama, a caste-flexible swarm-founding wasp (Epiponini) where reproductive competition is high and aggressive displays are common.

Results

We observed the behavioral interactions of S. surinama females in field nests before and after we had removed the egg-laying queen(s). We measured the ovarian reproductive status, hemolymph JH and ecdysteroid titers, ovarian ecdysteroid content, and analyzed the cuticular hydrocarbon (CHC) composition of females engaged in competitive interactions in both queenright and queenless contexts. These data, in combination with hormone manipulation experiments, revealed that neither JH nor ecdysteroids are necessary for the expression of dominance behaviors in S. surinama. Instead, we show that JH likely functions as a gonadotropin and directly modifies the cuticular hydrocarbon blend of young workers to match that of a reproductive. Hemolymph ecdysteroids, in contrast, are not different between queens and workers despite great differences in ovarian ecdysteroid content.

Conclusions

The endocrine profile of S. surinama shows surprising differences from those of other caste-flexible wasps, although a rise in JH titers in replacement queens is a common theme. Extensive remodeling of hormone functions is also evident in the highly eusocial bees, which has been attributed to the evolution of morphologically defined castes. Our results show that hormones which regulate caste-plasticity can lose these roles even while caste-plasticity is preserved.

     

Inhibition of Host Queen Reproductive Capacity by the Obligate Social Parasite Polistes atrimandibularis (Hymenoptera,Vespidae)

Polistes atrimandibularis is the obligate and permanent parasite of the social paper wasp Polistes biglumis bimaculatus. This parasite lives together with the host foundress for a considerable period on the invaded nest. Ovarian development was measured in females of the host species (foundresses and, when present, workers) from 18 parasitized and 14 non-parasitized colonies. The reproductive capacity of foundresses from parasitized nests decreased faster than that of foundresses from non-parasitized nests. These results indicate that the presence of the parasite lowers reproductive capacity of the host queen. Simultaneously, the fertility of the host worker is inhibited.     

Hormonal modulation of reproduction and fertility signaling in polistine wasps

In social insects, it has been suggested that reproduction and the production of particular fertility-linked cuticular hydrocarbons (CHC) may be under shared juvenile hormone (JH) control, and this could have been key in predisposing such cues to later evolve into full-fledged queen pheromone signals. However, to date, only few studies have experimentally tested this “hormonal pleiotropy” hypothesis. Here, we formally test this hypothesis using data from four species of Polistine wasps, Polistes dominula, Polistes satan, Mischocyttarus metathoracicus, and Mischocyttarus cassununga, and experimental treatments with JH using the JH analogue methoprene and the anti-JH precocene. In line with reproduction being under JH control, our results show that across these four species, precocene significantly decreased ovary development when compared with both the acetone solvent-only control and the methoprene treatment. Consistent with the hormonal pleiotropy hypothesis, these effects on reproduction were further matched by subtle shifts in the CHC profiles, with univariate analyses showing that in P. dominula and P. satan the abundance of particular linear alkanes and mono-methylated alkanes were affected by ovary development and our hormonal treatments. The results indicate that in primitively eusocial wasps, and particularly in Polistes, reproduction and the production of some CHC cues are under joint JH control. We suggest that pleiotropic links between reproduction and the production of such hydrocarbon cues have been key enablers for the origin of true fertility and queen signals in more derived, advanced eusocial insects.     

Effects of social conditions on Juvenile Hormone mediated reproductive development in Bombus terrestris workers

Abstract. During the annual life cycle of the bumble bee Bombus terrestris (L.) colony, there is a stage characterized by worker reproduction in the presence of the queen. It has been proposed that this is a result of a decrease in queen inhibition. This hypothesis was examined by studying the effects of queens taken from colonies at different stages of development on several aspects of worker physiology and behaviour: rates of Juvenile Hormone (JH) release in vitro , ovary development, and behaviour associated with reproduction. After optimizing and validating the radiochemical assay for JH release for bumble bee workers, we found that queenless workers had significantly more developed ovaries and higher rates of release of JH than did queenright workers, confirming and extending previous findings that suggest that bumblebee ovarian development is under JH control. Mated queens, separated from their colony and brood, can have the same inhibitory effect on the reproductive development of callow workers. In contrast, workers confined with virgin queens or in queenless groups demonstrated a significantly higher rate of release of JH, overt aggression and threatening behaviours. However, there were no differences in rates of release of JH between workers confined in groups in the laboratory with queens taken from colonies either before or after the onset of worker reproduction. Furthermore, overt aggression and threatening behaviours were similar and low in both types of groups. These results gave no support to the hypothesis that a decrease in queen inhibition is associated with the onset of worker reproduction. We also show that young workers reared in colonies either before or after worker reproduction occurs, or in queenless colonies, all demonstrated similar, low rates of release of JH. These results suggest that older workers may inhibit the corpora allata of younger workers in queenless colonies.     

Orphaning does not affect the colony productivity of the primitive eusocial wasp Polistes snelleni

In colonies of primitively eusocial wasps, some dominant workers become successive queens and inherit queenship after the death of the foundress queens. Although workers in many species do not mate, workers of Polistes snelleni are capable of mating and female production. In this study, we removed foundress queens from colonies of P. snelleni to evaluate the effects of queen loss on the dominant–subordinate relationships among the remaining workers and the productivity of colonies in the species. The foundress queens were the sole egg layers in almost all of the queenright colonies. The frequency of dominance behaviour among the wasps in the queenright colonies was significantly less than in the orphan colonies. The frequency of dominance behaviour in the successive queens after queen removal was significantly more than in the foundress queens. Multiple workers had developed ovaries, including the successive queens in 66.7 % (10/15) of the orphan colonies after queen removal. The orphan colonies produced significantly more cells and eggs than the queenright colonies. Our results suggest that the reproductive potential of the successive queens in the orphan colonies is not lower than that of the foundress queens, and that the productivity of the orphan colonies is maintained rather than causing potential conflict over direct reproduction among workers.