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.     

Castes and asynchronous colony cycle in Polybia bistriata (Fabricius) (Hymenoptera: Vespidae)

Neotropical swarm-founding wasps, the Epiponini, are an outstanding group of social insects whose societies are polygynic and complex nest builders. Caste dimorphism in these wasps ranges from incipient to highly distinct. Morphometric analyses of nine body parts, ovarian status, relative age and development of the 5th gastral sternite gland (Richards' gland) of Polybia bistriata Fabricius were undertaken in order to estimate caste differentiation in nine colonies. Morphological and physiological data were used in multivariate analyses in order to evaluate the level of discrimination between inseminated and non-inseminated females. Clear physiological differences were found: queens had highly developed ovaries and they were inseminated, and workers had totally undeveloped ovaries or they had few developed oocytes (only in two colonies), but in both cases insemination was not detected. ANOVA and discriminant function analysis detected slight, but significant differences between castes. In relation to colony cycle, colonies were considered to be in the following stages: one in pre-emergence, four in worker production and four in sexuals production. Richards' gland analyses indicated that in small colonies (<100 females) queens had a less developed gland than in medium (100-200 females), and large colonies (>200 females). Taking the whole data, it was possible to conclude that caste differences were slight, but more evident in some phases of the colony cycle, a phenomenon previously described for other epiponines.     

Queen–worker caste ratio depends on colony size in the pharaoh ant (<Emphasis Type="Italic">Monomorium pharaonis</Emphasis>)

The success of an ant colony depends on the simultaneous presence of reproducing queens and non-reproducing workers in a ratio that will maximize colony growth and reproduction. Despite its presumably crucial role, queen–worker caste ratios (the ratio of adult queens to workers) and the factors affecting this variable remain scarcely studied. Maintaining polygynous pharaoh ant (Monomorium pharaonis) colonies in the laboratory has provided us with the opportunity to experimentally manipulate colony size, one of the key factors that can be expected to affect colony level queen–worker caste ratios and body size of eclosing workers, gynes and males. We found that smaller colonies produced more new queens relative to workers, and that these queens and workers both tended to be larger. However, colony size had no effect on the size of males or on the sex ratio of the individuals reared. Furthermore, for the first time in a social insect, we confirmed the general life history prediction by Smith and Fretwell (Am Nat 108:499–506, 1974) that offspring number varies more than offspring size. Our findings document a high level of plasticity in energy allocation toward female castes and suggest that polygynous species with budding colonies may adaptively adjust caste ratios to ensure rapid growth.     

Caste totipotency and conflict in a large-colony social insect

In most social insects with large, complex colonies workers and queens are morphologically quite distinct. This means that caste determination must occur prior to adulthood. However workers and queens in the swarm-founding epiponine wasps are often morphologically indistinguishable, or nearly so, suggesting that caste determination in these wasps could be quite different. To determine the extent of caste lability in the epiponine, Parachartergus colobopterus, we removed all the queens from one colony and all but one from another colony. Worker aggression diminished after queen removal. A week later the colony with no queens had a new, young cohort of mated queens. These must have been either adults or pupae at the time of queen removal, and so could not have been fed any differently from workers. Relatedness patterns confirmed that these new queens would normally have been workers and not queens. A model of inclusive fitness interests shows that workers ought to suppress new queen production, except at low queen numbers, a prediction supported by our empirical results. The patterns of social conflict over queenship resulting from swarm founding in a many-queen society may help to explain the unusually weak caste differentiation in the epiponines.     

The role of queens in colonies of the swarm-founding wasp Parachartergus colobopterus

Social insect queens reproduce while workers generally do not. Queens may also have other behavioural roles in the colony. In small, independent-founding colonies of social wasps, the dominant queen physically enforces her interests over those of the workers and serves as a pacemaker of the colony, stimulating workers to forage and engage in other tasks. By contrast, in large-colony, swarm-founding wasps, the collective interests of the workers are fulfilled in sex allocation and production of males, whether or not they coincide with the interests of the queens. The behavioural role of the queens in such species has not been extensively studied. We investigated the role of the queens both in regulating worker activity and in reducing the numbers of reproductively active queens in the swarm-founding epiponine wasp Parachartergus colobopterus. We found no evidence that queens regulate worker activity, as they were rarely involved in any interactions. Worker activity may be self-organized, without centralized active control by anyone. Furthermore, we found no evidence that the reduction in queen number characteristic of this tribe of wasps occurs in response to aggression among queens. The reduction in queen number may be a result of worker treatment of queens, although worker discrimination against some queens was not obvious in our data. i Copyright 2000 The Association for the Study of Animal Behaviour.     

Maternal effect on female caste determination in a social insect

Caste differentiation and division of labor are the hallmarks of social insect colonies [1, 2]. The current dogma for female caste differentiation is that female eggs are totipotent, with morphological and physiological differences between queens and workers stemming from a developmental switch during the larval stage controlled by nutritional and other environmental factors (e.g., [3-8]). In this study, we tested whether maternal effects influence caste differentiation in Pogonomyrmex harvester ants. By conducting crossfostering experiments we identified two key factors in the process of caste determination. New queens were produced only from eggs laid by queens exposed to cold. Moreover, there was a strong age effect, with development into queens occurring only in eggs laid by queens that were at least two years old. Biochemical analyses further revealed that the level of ecdysteroids was significantly lower in eggs developing into queens than workers. By contrast, we found no significant effect of colony size or worker exposure to cold, suggesting that the trigger for caste differentiation may be independent of the quantity and quality of resources provided to larvae. Altogether these data demonstrate that the developmental fate of female brood is strongly influenced by maternal effects in ants of the genus Pogonomyrmex.     

Morphological caste and sex differences in the Taiwanese stingless bee Trigona ventralis hoozana (Hymenoptera: Apidae)

To examine morphological differences among queens, workers and males, 14 external body characters were measured in two colonies of the Taiwanese stingless bee Trigona ventralis hoozana. Queens were largest in all of the body parts measured except eye width and mesoscutum length, and values for most variables in queens did not overlap with those of workers and males. In contrast, the worker : male size ratios for 11 variables were close to 1.0, showing that overall body size and shape of workers resembled that of males rather than of queens. Males had the largest eyes and their mesoscutum length was comparable to that of queens. ancova between 14 morphometric variables and mesoscutum width chosen as standard body size showed that allometric growth in most variables was not linear. Plotting of some variables on mesoscutum width showed that queens had a proportionally wider first metasomal tergum and longer antennal scape, but a proportionally narrower head and eye than workers and males. These tests suggest that the morphological caste differences in this species belong to a category of complete dimorphism.     

MODELING THE MAINTENANCE OF A DEPENDENT LINEAGE SYSTEM: THE INFLUENCE OF POSITIVE FREQUENCY-DEPENDENT SELECTION ON SEX RATIO

In insect societies, worker versus queen development (reproductive caste) is typically governed by environmental factors, but some Pogonomyrmex seed-harvester ants exhibit strict genetic caste determination, resulting in an obligate mutualism between two reproductively isolated lineages. Queens mate randomly with multiple males from each lineage and intralineage crosses produce new queens, whereas interlineage crosses produce workers. Early colony survival is negatively frequency dependent; when lineage frequencies are unequal, queens from the rarer lineage benefit because they acquire more interlineage sperm, and produce more workers. Here we examine theoretically and empirically the effect of relative lineage frequency on sex ratio. We predict that the ratio of inter- to intralineage sperm acquired by queens of each lineage will affect the sex ratio produced at colony maturity. Consistent with model predictions, we found that gyne production in mature colonies was positively frequency dependent, increasing significantly with increasing lineage frequency across 15 populations. Unequal lineage frequencies are common and likely maintained by a complex interplay between an ecological advantage specific to one lineage, and opposing frequency-dependent selection pressures experienced throughout the colonies life-cycle; rare lineage colonies benefit during early colony growth, and common lineage colonies benefit at reproductive maturity.     

Aggression and worker control of caste fate in a multiple-queen wasp, Parachartergus colobopterus

Although famously cooperative, social insect colonies harbour considerable potential for genetic conflict among colonymates. This conflict may be expressed behaviourally as aggression by workers. We investigated aggression in 34 colonies of the wasp Parachartergus colobopterus, by evaluating the characteristics of both instigators and victims of aggressive interactions. We estimated genetic relatedness and queen number using DNA microsatellites and found that workers and emerging females should be most in conflict over the caste of the latter when there are many queens on the nest. We found that aggressive interactions are more likely to involve older workers attacking either males or younger workers, and that victim and aggressor females have more ovarian development than randomly sampled colonymates. Moreover, mated females with low levels of ovarian development relative to active queens were also more likely to be aggressors and victims than were randomly sampled females. Aggression among females supports the hypothesis that older workers use aggression towards younger females as a means of policing the development of emerging females into queens. Workers also may use aggression to suppress the reproduction of some mated females. Our findings thus support the hypothesis that genetic conflicts of interest motivate worker aggression in swarm-founding wasp colonies.     

Cryptically dimorphic caste differences in a Neotropical, swarm-founding paper wasp genus, Parachartergus (Hymenoptera: Vespidae)

Morphological differences in body shape between females of different reproductive conditions (in terms of insemination and ovarian development) were examined in two species of the Neotropical polistine genus Parachartergus: P. smithii and P. fraternus. The present study shows, for the first time, that non‐size‐based morphological divergence between queens and workers occurs in Parachartergus, an epiponine genus once believed to have little or no morphological caste differences. In the P. smithii colony examined, queens were significantly larger than workers in five of the eight body parts measured (head width, eye width, genal width, mesosomal length, wing length, first metasomal tergum width, and width and length of the second tergum), but the mean values of wing length and first and second tergum widths were not significantly different between them. The queen : worker size ratios tended to be greater anteriorly and smaller posteriorly, although the size ratio was greatest in second tergum length. Analysis of covariance (ancova ) with mesosomal length as covariate showed that queens had proportionally wider heads and narrower first terga than did workers. In the P. fraternus colony, size differences between queens and workers were not significant, and there was little or no difference in shape, but queens had significantly proportionally wider first terga than did workers.     

Sex-ratio conflict between queens and workers in eusocial Hymenoptera: mechanisms, costs, and the evolution of split colony sex ratios

Because workers in the eusocial Hymenoptera are more closely related to sisters than to brothers, theory predicts that natural selection should act on them to bias (change) sex allocation to favor reproductive females over males. However, selection should also act on queens to prevent worker bias. We use a simulation approach to analyze the coevolution of this conflict in colonies with single, once-mated queens. We assume that queens bias the primary (egg) sex ratio and workers bias the secondary (adult) sex ratio, both at some cost to colony productivity. Workers can bias either by eliminating males or by directly increasing female caste determination. Although variation among colonies in kin structure is absent, simulations often result in bimodal (split) colony sex ratios. This occurs because of the evolution of two alternative queen or two alternative worker biasing strategies, one that biases strongly and another that does not bias at all. Alternative strategies evolve because the mechanisms of biasing result in accelerating benefits per unit cost with increasing bias, resulting in greater fitness for strategies that bias more and bias less than the population equilibrium. Strategies biasing more gain from increased biasing efficiency whereas strategies biasing less gain from decreased biasing cost. Our study predicts that whether queens or workers evolve alternative strategies depends upon the mechanisms that workers use to bias the sex ratio, the relative cost of queen and worker biasing, and the rates at which queen and worker strategies evolve. Our study also predicts that population and colony level sex allocation, as well as colony productivity, will differ diagnostically according to whether queens or workers evolve alternative biasing strategies and according to what mechanism workers use to bias sex allocation.     

Sociogenetic organisation of two desert ants

Desert ants of the genus Cataglyphis evolved a remarkable diversity in their reproductive strategies. In Cataglyphis species where social organisation was described so far, colonies are headed by one or multiple queens, queens being singly or multiply mated, and workers and/or queens possess the ability to reproduce asexually via thelytokous parthenogenesis. Here, we investigate the social organisation of C. bombycina (group bombycinus) and C. theryi (group albicans) using highly polymorphic microsatellite markers. Our results show that both species are characterized by monogynous colonies and multiply mated queens, supporting the idea that polyandry is an ancestral trait of the genus. No evidence for parthenogenetic reproduction by queens was found. One distinctive feature of the species C. bombycina among the genus is the presence of a morphologically distinct soldier caste, with highly developed scythe blades jaws. In the only colony where a significant number of soldiers have been sampled, the distribution of patrilines is fundamentally different between the soldier and the worker caste. This result suggests a genetic contribution to worker caste determination in this species, and certainly awaits further investigation.     

Workers and queens of the European hornetVespa crabro L. have colony-specific cuticular hydrocarbon profiles (Hymenoptera: Vespidae)

Summary Cuticular hydrocarbons were extracted from sixty individual workers from six colonies ofVespa crabro L. and analyzed by combined gas-chromatography/mass spectrometry. Discriminant analysis of the cuticular hydrocarbon profiles of workers and queens showed that the wasps could be grouped by colony and by caste. Stepwise discriminant analysis selected the components which were weighted most heavily in these analyses. Different combinations of cuticular hydrocarbons were important in grouping workers by colony, queens and workers by colony, and workers and queens by caste.     

Genetic components to caste allocation in a multiple-queen ant species

Reproductive division of labor and the coexistence of distinct castes are hallmarks of insect societies. In social insect species with multiple queens per colony, the fitness of nestmate queens directly depends on the process of caste allocation (i.e., the relative investment in queen, sterile worker and male production). The aim of this study is to investigate the genetic components to the process of caste allocation in a multiple-queen ant species. We conducted controlled crosses in the Argentine ant Linepithema humile and established single-queen colonies to identify maternal and paternal family effects on the relative production of new queens, workers, and males. There were significant effects of parental genetic backgrounds on various aspects of caste allocation: the paternal lineage affected the proportion of queens and workers produced whereas the proportions of queens and males, and females and males were influenced by the interaction between parental lineages. In addition to revealing nonadditive genetic effects on female caste determination in a multiple-queen ant species, this study reveals strong genetic compatibility effects between parental genomes on caste allocation components.     

Nutritional versus genetic correlates of caste differentiation in a desert ant

1. In many ant species, caste differentiation stems from trophic differences at the larval stage. Adult workers that feed larvae have great control over the allocation of colony resources to growth (production of workers) versus reproduction (production of queens). However, larval caste fate may also be constrained very early on through direct genetic effects or non‐genetic maternal effects. 2. Here, we combined isotopic and genetic analyses to study the developmental origin of queens and workers in a desert‐dwelling ant, Cataglyphis tartessica (Amor & Ortega, 2014). Queens do not found new colonies alone but rather disperse with workers. As the latter are always wingless, selection pressures on specific queen traits such as flight ability have become relaxed. Though the phylogenetically related species, C. emmae (Forel, 1909) only produces winged queens much larger than workers, C. tartessica produces two types of small queens relative to workers: brachypterous (short‐winged) queens and permanently apterous ergatoid (wingless and worker‐like) queens. 3. Upon emergence, workers and ergatoids have similar δ15N isotopic values, which were lower than those of brachypters, suggesting the latter are fed more protein as larvae. Microsatellite analyses indicated that: (i) colonies are mostly monogynous and monandrous; (ii) both ergatoids and brachypters are equally related to workers; and (iii) in the few polyandrous colonies, patrilines were evenly represented across workers, brachypters and ergatoids. 4. Overall, there was no evidence of genetic caste determination. We suggest that, in contrast to brachypters, ergatoids are selfish individuals that escape the nutritional castration carried out by workers and develop into queens in spite of the colony's collective interests.     

Worker demography in a large-colony,swarm-founding wasp

Neotropical swarm-founding wasps build nests enclosed in a covering envelope, which makes it difficult to count individual births and deaths. Thus, knowledge of worker demography is very limited for swarm-founding species compared with that for independent-founding species. In this study, we explored the worker demography of the swarm-founding wasp Polybia paulista, the colony size of which usually exceeds several thousand adults. We considered each wasp colony as an open-population and estimated the survival probability, recruitment rate, and population size of workers using the developments of the Cormack–Jolly–Seber model. We found that capture probability varied considerably among the workers, probably due to age polyethism and/or task specialization. The daily survival rate of workers was high (around 0.97) throughout the season and was not related to the phase of colony development. On the other hand, the recruitment rate ranged from 0 to 0.37, suggesting that worker production was substantially less important than worker survival in determining worker population fluctuations. When we compared survival rates among worker groups of one colony, the mean daily survival rate was lower for founding workers than for progeny workers and tended to be higher in progeny workers that emerged in winter. These differences in survivorship patterns among worker cohorts would be related to worker foraging activity and/or level of parasitism.     

Shift from independent to dependent colony foundation and evolution of 'multi-purpose' ergatoid queens in Mystrium ants (subfamily Amblyoponinae)

Division of labour improves fitness in animal societies. In ants, queens reproduce, whereas workers perform all other tasks. However, during independent colony founding, queens live as solitary insects and must be totipotent, especially in species where they need to forage. In many ants, solitary founding has been replaced by dependent founding, where queens are continuously helped by nestmate workers. Little is known about the details of this evolutionary transition. Mystrium rogeri from Madagascar and Mystrium camillae from Southeast Asia (subfamily Amblyoponinae) have winged queens, but three congeneric species from Madagascar reproduce with permanently wingless queens instead. We show that this 'ergatoid' caste has distinct body proportions in all three species, expressing a mixture of both queen and worker traits. Ergatoid queens have functional ovaries and spermatheca, and tiny wing rudiments. They can be as numerous as workers within a colony, but only a few mate and reproduce, whereas most behave as sterile helpers. The shape of their mandibles makes them unsuited for hunting and, together with a lack of metabolic reserves (i.e. in the form of wing muscles), this means that ergatoid queens cannot be solitary foundresses. In comparison with winged queens, ergatoid queens are less costly per capita and they experience lower mortality. They remain in their natal colonies where they can either reproduce or function as helpers, making them a 'multi-purpose' caste. Within the Amblyoponinae, ergatoid queens replace winged queens in Onychomyrmex as well. However, in this genus, ergatoid queens are 'sole-purpose', few are produced each year and they reproduce but do not work. Hence, different types of ergatoid queens evolved to replace winged queens in ants.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 198–207.     

Behavior during artifically induced swarm emigration in an old world polistine wasp,Ropalidia romandi (Hymenoptera: Vespidae)

Absconding swarms of the Australian swarm-founding wasp,Ropalidia romandi, were artificially induced. Workers may initially search for new nesting sites and once a new nest site is decided, they recruit their nest mates including multiple queens there. During the swarm emigration, workers often land on prominent objects, such as tree leaves, and drag the metasoma; those that do not drag usually antennate the object surfaces. These findings suggest thatR. romandi workers use a scent trail during swarm emigration. Artificial transfer of colonies resulted in clustering of wasps from different colonies, indicating thatR. romandi may not act exclusively against non-nestmates. In the formation of tentative aggregation of wasps prior to commencing emigration, and also in recognizing a new nesting site by following swarm members that have been guided by a scent trail, a visual cue (aerial swarm formed around an aggregation of wasps) seems to play an important role.     

Does the diapause experience of bumblebee queens Bombus terrestris affect colony characteristics?

1. Bumblebee colonies show much variation in the number of workers, drones, and queens produced. Because this variation prevails even when colonies are kept under identical conditions, it does not seem to be caused by extrinsic factors but rather by differences between founding queens. 2. The most likely factor that could cause differences between queens is diapause. Although colonies are raised under standardised conditions, the queens often experience diapause of different length. If there are costs associated with diapause that influence post‐diapause reproduction, the diapause history of the queens could affect colony characteristics. 3. Here, several colony characteristics are compared: number of first and second brood workers; total number of workers, drones, and queens; energy spent on sexuals; sex ratio; rate of worker production; time to emergence of first reproductive; and colony lifetime. Colonies were used where the queens experienced a diapause treatment of 0 (nondiapause queens), 2, and 4 months. 4. Although no proof was found for the existence of costs associated with diapause, the colony characteristics of nondiapause queens were significantly different from those of diapause queens. Colonies of nondiapause queens produced the lowest number of workers but the highest number of young queens. 5. It is argued that these nondiapause colonies are more time‐constrained than diapause colonies because nondiapause colonies produce two generations within the same season and should therefore be more efficient in producing sexual offspring. 6. Moreover, nondiapause colonies should rear a more female‐biased sex ratio because they can be certain of the presence of males produced by other (diapause) colonies.     

Brain organization mirrors caste differences,colony founding and nest architecture in paper wasps (Hymenoptera: Vespidae)

The cognitive challenges that social animals face depend on species differences in social organization and may affect mosaic brain evolution. We asked whether the relative size of functionally distinct brain regions corresponds to species differences in social behaviour among paper wasps (Hymenoptera: Vespidae). We measured the volumes of targeted brain regions in eight species of paper wasps. We found species variation in functionally distinct brain regions, which was especially strong in queens. Queens from species with open-comb nests had larger central processing regions dedicated to vision (mushroom body (MB) calyx collars) than those with enclosed nests. Queens from advanced eusocial species (swarm founders), who rely on pheromones in several contexts, had larger antennal lobes than primitively eusocial independent founders. Queens from species with morphologically distinct castes had augmented central processing regions dedicated to antennal input (MB lips) relative to caste monomorphic species. Intraspecific caste differences also varied with mode of colony founding. Independent-founding queens had larger MB collars than their workers. Conversely, workers in swarm-founding species with decentralized colony regulation had larger MB calyx collars and optic lobes than their queens. Our results suggest that brain organization is affected by evolutionary transitions in social interactions and is related to the environmental stimuli group members face.