Queen Worker Research Articles

Ecological traits of social insects: Colony, queen and worker size relationships reveal a nexus trait with broad ecological relevance

Abstract Trait‐based ecology has surged in recent decades but a framework to assess eusocial taxa, such as ants and termites, has been left underdeveloped because ecological trait‐based approaches have yet to examine the ecological relationships and potential mechanisms between different trait components (i.e. queen, colony and worker). We collected worker‐, queen‐ and colony‐scale traits of the ants of Florida, an ant faunae comprised of ~25% nonnative species, based on morphology, natural and life histories and combined these traits with geospatial data. We used these data to assess and develop the beginnings of a hypothesis‐testing framework to aid in better understanding trait ecology for eusocial organisms and to understand the spatial structure and drivers of eusocial insect traits by examining individual‐ and colony‐scale trait relationships. Results show that queen–worker dimorphism, the morphological difference between workers and queens, shows exceptional potential as a nexus trait for which other colony‐scale traits, especially colony size, have significant relationships with. Colony size is therefore the key trait that should be measured, whenever possible, to better understand these other, emergent relationships. We also find differences between native and nonnative regional patterns of colony size and queen–worker dimorphism across environmental gradients. We show the potential for a more holistic trait approach to ants and potentially other eusocial taxa given the state of current trait knowledge, providing an advanced path forward in functional and trait ecology of eusocial organisms. Read the free Plain Language Summary for this article on the Journal blog.

Efficient Allocation of Labor Maximizes Brood Development and Explains Why Intermediate-Sized Groups Perform Best During Colony-Founding in the Ant, Pogonomyrmex californicus

Cooperation in nature is usually between relatives, but unrelated individuals can also cooperate, requiring significant benefits to outweigh the costs of helping non-kin. Unrelated queens of the ant, Pogonomyrmex californicus, work together to found a new colony, a phenomenon known as pleometrosis. While previous studies have shown that pleometrosis improves queen survival and worker production, little is known of the behavioral interactions within nests that explain these advantages. We aimed to determine how the optimal group size for a small, simple social group is related to group productivity and the organization of work. Collecting queens from a known pleometrotic population, we established nests with either one, three, six, or nine foundresses and observed the resulting nascent colonies for 50 days. We found that queens in social founding groups survived longer and had higher productivity. While all social groups were equally successful in producing workers, intermediate-sized groups were most successful in terms of per capita production. Inactivity increased with group size. In addition, the proportion of essential colony growth tasks performed (such as foraging and brood care) was lowest in both solitary-founded groups and in groups of nine queens. As a result, intermediate sized groups outperformed both solitary queens and groups of nine in the efficiency with which they converted eggs into workers. These results emphasize the benefits of cooperation and the ways in which group size can influence fitness and the allocation of labor in social groups.

Genetic diversity among queen bee, worker bees and larvae in terms of retrotransposon movements

Honey bee (Apis mellifera L.) is a model organism, contributing significant effect on global ecology by pollination and examining due to its social behaviour. In this study, barley-specific Sukkula and Nikita retrotransposons were analysed using IRAP (Inter-Retrotransposon Amplification Polymorphism) marker technique, and the relationships between retrotransposon movements and development were also investigated in three different colonies of the Caucasian honey bee (A. mellifera caucasica). Furthermore, transposon sequences belonging to A. mellifera, Bombus terrestris, Triticum turgidum L. and Hordeum vulgare L. were also examined to figure out evolutionary relationships. For this purpose, a queen bee, five worker bees, and five larvae from each colony were studied. Both retrotransposons were found in all samples in three colonies with different polymorphism ratios (0–100% for Nikita and 0–67% for Sukkula). We also determined polymorphisms in queen–worker (0–83% for Nikita, 0–63% for Sukkula), queen–larvae (0–83% for Nikita, 0–43% for Sukkula) and worker–larvae comparisons (0–100% for Nikita, 0–63% for Sukkula) in colonies. Moreover, close relationships among transposons found in plant and insect genomes as a result of in silico evaluations to verify experimental results. This work could be one of the first studies to analyse plant-specific retrotransposons’ movements in honey bee genome. Results are expected to understand evolutionary relationships in terms of horizontal transfer of transposons among kingdoms.

Colony composition, phasic reproduction, and queen–worker dimorphism of an oriental non-army ant doryline Cerapachys sulcinodis species complex in northern Thailand

Colony composition, phasic reproduction, and queen–worker dimorphism of an oriental non-army ant doryline Cerapachys sulcinodis species complex in northern Thailand

Comprehensive analysis of male-free reproduction in Monomorium triviale (Formicidae: Myrmicinae).

We report comprehensive evidence for obligatory thelytokous parthenogenesis in an ant Monomorium triviale. This species is characterized by distinct queen-worker dimorphism with strict reproductive division of labor: queens produce both workers and new queens without mating, whereas workers are completely sterile. We collected 333 nests of this species from 14 localities and three laboratory-reared populations in Japan. All wild queens dissected had no sperm in their spermathecae. Laboratory observation confirmed that virgin queens produced workers without mating. Furthermore, microsatellite genotyping showed identical heterozygous genotypes between mothers and their respective daughters, suggesting an extremely low probability of sexual reproduction. Microbial analysis detected no bacterial genera that are known to induce thelytokous parthenogenesis in Hymenoptera. Finally, the lack of variation in partial sequences of mitochondrial DNA among individuals sampled from across Japan suggests recent rapid spread or selective sweep. M. triviale would be a promising model system of superorganism-like adaptation through comparative analysis with well-studied sexual congeners, including the pharaoh ant M. pharaonis.

Assessing Agrochemical Risk to Mated Honey Bee Queens.

Current risk assessment strategies for honey bees rely heavily upon laboratory tests performed on adult or immature worker bees, but these methods may not accurately capture the effects of agrochemical exposure on honey bee queens. As the sole producer of fertilized eggs inside a honeybee colony, the queen is arguably the most important single member of a functioning colony unit. Therefore, understanding how agrochemicals affect queen health and productivity should be considered a critical aspect of pesticide risk assessment. Here, an adapted method is presented to expose honey bee queens and worker queen attendants to agrochemical stressors administered through a worker diet, followed by tracking egg production in the laboratory and assessing first instar eclosion using a specialized cage, referred to as a Queen Monitoring Cage. To illustrate the method's intended use, results of an experiment in which worker queen attendants were fed diet containing sublethal doses of imidacloprid and effects on queens were monitored are described.

Caste differentiation of spermatheca and organs related to sperm use and oviposition in the honeybee, Apis mellifera

Striking queen–worker dimorphism associated with each task is a key feature of highly eusocial insects, including Apis mellifera. The present study histologically examined the developmental process of the internal reproductive organs including spermatheca, valve-fold in the vagina, semi-circular muscle surrounding the common oviduct, and abdominal ganglia in honeybee queens and workers. During the pupal stage, queens showed an increased spermathecal reservoir, development of the tracheal network surrounding the spermathecal reservoir, and elongation of the spermathecal gland. Compared with queens, these developmental processes were never observed during the pupal stage in workers. Moreover, development of the valve-fold and semi-circular muscle was aborted, and they became rudimentary at the middle pupal stage in workers. Morphological caste differences in the abdominal ganglia were observed from the prepupal stages, showing that the most posterior ganglion was fused with the anterior ganglia in queens but not in workers.

Mating status and the evolution of eusociality: Oogenesis is independent of mating status in the solitary bee Osmia bicornis

The fundamental trait underlying eusociality is the reproductive division of labour. In honeybees (Apis mellifera), queens lay eggs while workers forage, defend and care for brood. The division of labour is maintained by pheromones including queen mandibular pheromone (QMP) produced by the queen. QMP constrains reproduction in adult honeybee workers, but in the absence of their queen workers can activate their ovaries and, although they cannot mate, they lay haploid male eggs. The reproductive ground plan hypothesis suggests that reproductive constraint may have evolved by co-opting mechanisms of reproductive control in solitary ancestors. In many insects mating is required to activate or accelerate oogenesis. Here, we use the solitary bee Osmia bicornis (Megachilidae) to test whether reproductive constraint evolved from ancestral control of reproduction by mating status. We present a structural study of the O. bicornis ovary, and compare key stages of oogenesis with honeybee workers. Importantly, we show that mating does not affect any aspect of the reproductive physiology of O. bicornis. We therefore conclude that mechanisms governing reproductive constraint in honeybees were unlikely to have been co-opted from mechanisms pertaining to mating status.

Queen–worker conflict can drive the evolution of social polymorphism and split sex ratios in facultatively eusocial life cycles*

Hamilton's idea that haplodiploidy favors the evolution of altruism-the haplodiploidy hypothesis-relies on the relatedness asymmetry between the sexes caused by the sex-specific ploidies. Theoretical work on the consequences of relatedness asymmetries has significantly improved our understanding of sex allocation and intracolony conflicts, but the importance of haplodiploidy for the evolution of altruism came to be seen as minor. However, recently it was shown that haplodiploidy can strongly favor the evolution of eusociality, provided additional "preadaptations" are also present, such as the production of multiple broods per season and maternal ability to bias offspring sex ratios. These results were obtained assuming no influence of workers on the sex ratio, even though worker control of the sex ratio is known to occur. Here, we model the evolution of sex-specific fratricide as a mechanism of worker control over the sex ratio. We show that fratricide can facilitate the initial evolution of helping. However, fratricide can also hamper the evolution of unconditional help. Instead, social polymorphism evolves a mixture of helping and dispersing offspring. Finally, we show that the co-evolution of sex-allocation strategies of workers (fratricide) and queens leads to a split production of the sexes, with some colonies specializing in males and others in females. Thus, the model predicts that fratricide spawns a diversity of co-existing life cycles that strongly vary in degree of sociality and sex ratios.

Queen–worker aggression in the facultatively eusocial bee Megalopta genalis

The establishment of reproductive division of labor in primitively eusocial insects is typically based on dominance interactions from the queen toward her worker daughters. We used a standardized behavioral assay, the circle tube, to observe aggressive and other behaviors between the queen and workers of field-collected social colonies of the sweat bee, Megalopta genalis. Queens generally expressed higher levels of aggressive and dominant behaviors than workers. Workers performed two behaviors, ‘C-posture’ and ‘reverse’, more frequently than queens. Our data suggest a defensive function for C-posture in workers, as it correlated with the expression of queen aggression. Within queens, the aggressive behaviors ‘nudge’, ‘bite’, ‘push’, and ‘chase’ correlated with each other in their frequencies of expression; the other two behaviors that were performed more often by queens (‘back up’ and ‘back into’) did not. The two behaviors performed more often by workers, ‘C-posture’ and ‘reverse’, were not correlated in workers. Queen and worker activity levels were correlated. Body size correlated with increased expression of queen-like behavior in both the queen and worker castes. Queens generally were bigger and had larger ovaries than workers. Queen–worker body size and ovary size differences correlated with behavioral differences. The effects of body size suggest an influence of developmental nutrition on adult behavior.

Israeli Acute Paralysis Virus: Honey Bee Queen⁻Worker Interaction and Potential Virus Transmission Pathways.

Queen loss or failure is an important cause of honey bee colony loss. A functional queen is essential to a colony, and the queen is predicted to be well protected by worker bees and other mechanisms of social immunity. Nevertheless, several honey bee pathogens (including viruses) can infect queens. Here, we report a series of experiments to test how virus infection influences queen–worker interactions and the consequences for virus transmission. We used Israeli acute paralysis virus (IAPV) as an experimental pathogen because it is relevant to bee health but is not omnipresent. Queens were observed spending 50% of their time with healthy workers, 32% with infected workers, and 18% without interaction. However, the overall bias toward healthy workers was not statistically significant, and there was considerable individual to individual variability. We found that physical contact between infected workers and queens leads to high queen infection in some cases, suggesting that IAPV infections also spread through close bodily contact. Across experiments, queens exhibited lower IAPV titers than surrounding workers. Thus, our results indicate that honey bee queens are better protected by individual and social immunity, but this protection is insufficient to prevent IAPV infections completely.

The influence of sociality, caste, and size on behavior in a facultatively eusocial bee

Social cooperation requires increased tolerance of other individuals. We used social and solitary individuals of the facultatively eusocial bee Megalopta genalis to compare interactions with non-nestmate individuals in a standardized behavioral assay, a circle tube. We set up interactions between bees from different nests matched for caste (solitary, social: queen or worker). We found more tolerance in social than solitary pairs, but found no difference in aggression. We also found that workers continued expressing caste-typical behavior even when matched against other workers from different nests. However, there was no difference in expression of queen-typical behaviors between the three groups. Our data on social caste show that outside of the queen–worker social context, both queens and workers express similar levels of queen-like behavior. However, workers still express higher levels of worker-like behavior than do queens. We found no effect of variation in ovary size on behavior. We found that body size correlated positively with queen-like behaviors, and negatively with worker-like behaviors. Our body size data suggest that the worker phenotype may result from naturally occurring size-correlated variation in behavior, combined with maternal manipulation of both body size through nutrition and behavior and ovary development through social aggression.

Hungry for the queen: Honeybee nutritional environment affects worker pheromone response in a life stage‐dependent manner

Abstract Animal nutritional state can profoundly affect behaviour, including an individual's tendency to cooperate with others. We investigated how nutritional restriction at different life stages affects cooperative behaviour in a highly social species, Apis mellifera honeybees. We found that nutritional restriction affects a worker's queen pheromone response, a behavioural indicator of investment in group vs. individual reproduction. Nutritional restriction at the larval stage led to reduced ovary size and increased queen pheromone response, whereas nutritional restriction at the adult stage led to reduced lipid stores and reduced queen pheromone response. We argue that these differences depend upon the extent of reproductive plasticity at these life stages and that individual worker honeybees may adjust their behavioural and physiological traits in response to nutritional stress to invest nutritional resources in either their own or their colony's reproduction. These results support the role of nutritional stress in the maintenance of cooperative behaviour, and we suggest that historical nutritional scarcity may be an important contributor to the evolution of extreme forms of cooperation. A plain language summary is available for this article.

Repeated switches from cooperative to selfish worker oviposition during stingless bee evolution.

Reproductive division of labour is a defining feature of insect societies. Stingless bees (Meliponini) are an interesting exception among the highly eusocial insects in that workers of many species contribute significantly to the production of males. Since workers remain sterile in other species of this large tropical tribe, it has been hypothesized that, in the latter species, ancestral queens have won the conflict over who produces the males. The fact that sterile workers of some species lay trophic eggs to feed the queen and display ritualized behaviours towards her during oviposition has been interpreted as an evolutionary relic of this ancient conflict. Here, I used ancestral state estimation to test whether worker reproduction is indeed the ancestral condition and worker sterility a derived state in stingless bees. Contrary to this hypothesis, data suggest that trophic egg laying was the ancestral condition, whereas selfish worker reproduction in queenright colonies evolved subsequently during stingless bee diversification. The appearance of worker reproduction in queenright conditions was tightly linked to the laying of trophic eggs, which suggests that having activated ovaries in queen presence facilitates the evolution of worker reproduction. Worker reproduction is also linked to brood cell architecture, but surprisingly not to colony size or queen-worker dimorphism. The reason for this association between brood cell architecture and worker oviposition is currently unknown. These results suggest that trophic eggs are not a relic of an ancient conflict, but a sign of overlapping interests between the queen and workers about who produces the males.

Phenotypically distinct female castes in honey bees are defined by alternative chromatin states during larval development.

The capacity of the honey bee to produce three phenotypically distinct organisms (two female castes; queens and sterile workers, and haploid male drones) from one genotype represents one of the most remarkable examples of developmental plasticity in any phylum. The queen–worker morphological and reproductive divide is environmentally controlled during post-embryonic development by differential feeding. Previous studies implicated metabolic flux acting via epigenetic regulation, in particular DNA methylation and microRNAs, in establishing distinct patterns of gene expression underlying caste-specific developmental trajectories. We produce the first genome-wide maps of chromatin structure in the honey bee at a key larval stage in which developmental canalization into queen or worker is virtually irreversible. We find extensive genome-wide differences in H3K4me3, H3K27ac, and H3K36me3, many of which correlate with caste-specific transcription. Furthermore, we identify H3K27ac as a key chromatin modification, with caste-specific regions of intronic H3K27ac directing the worker caste. These regions may harbor the first examples of caste-specific enhancer elements in the honey bee. Our results demonstrate a key role for chromatin modifications in the establishment and maintenance of caste-specific transcriptional programs in the honey bee. We show that at 96 h of larval growth, the queen-specific chromatin pattern is already established, whereas the worker determination is not, thus providing experimental support for the perceived timing of this critical point in developmental heterochrony in two types of honey bee females. In a broader context, our study provides novel data on environmentally regulated organismal plasticity and the molecular foundation of the evolutionary origins of eusociality.

Proteomic analysis in the Dufour's gland of Africanized Apis mellifera workers (Hymenoptera: Apidae).

The colony of eusocial bee Apis mellifera has a reproductive queen and sterile workers performing tasks such as brood care and foraging. Chemical communication plays a crucial role in the maintenance of sociability in bees with many compounds released by the exocrine glands. The Dufour’s gland is a non-paired gland associated with the sting apparatus with important functions in the communication between members of the colony, releasing volatile chemicals that influence workers roles and tasks. However, the protein content in this gland is not well studied. This study identified differentially expressed proteins in the Dufour’s glands of nurse and forager workers of A. mellifera through 2D-gel electrophoresis and mass spectrometry. A total of 131 spots showed different expression between nurse and forager bees, and 28 proteins were identified. The identified proteins were categorized into different functions groups including protein, carbohydrate, energy and lipid metabolisms, cytoskeleton-associated proteins, detoxification, homeostasis, cell communication, constitutive and allergen. This study provides new insights of the protein content in the Dufour’s gland contributing to a more complete understanding of the biological functions of this gland in honeybees.

Levels of selection shaping caste interactions during queen replacement in the honey bee, Apis mellifera

We investigated the different levels of selection that may influence the queen replacement process in honey bees by monitoring queen–queen and worker–queen interactions in queenless observation colonies containing populations of paint-marked workers that were either related or not related to introduced virgin queens experimentally reared to be of low- and high-quality, as estimated by differences in size and potential reproductive capacities. The high-quality queens were more likely to survive and become the new laying queen of their colonies. Queen survival was positively associated with both queen fighting ability and worker–queen interactions. Surviving queens were more aggressive and had greater fighting success. However, surviving queens were not bigger than killed queens nor were they the first queens to emerge, suggesting that queen size and early emergence alone are not the determining factors of queen fighting ability. The worker–queen interaction that was most strongly associated with the outcome of queen replacement was the vibration signal, which is a communication signal that workers perform on virgin queens. Surviving queens were vibrated at rates 3–4 times those experienced by killed queens and a queen’s vibration rate was positively correlated with her fighting ability and the number of rivals killed. Workers showed no consistent preferences for related vs. unrelated queens and the proportion of interactions received from related workers was not associated with any aspect of queen fighting ability and success monitored. Our results suggests that caste interactions during queen replacement have been shaped by both selection acting at the level of the individual queen (which favors higher quality queens with greater fighting ability) and selection acting at the level of the colony (which favors workers directing vibration signals towards queens with greater fighting potential), ultimately resulting in a higher quality queen becoming the new laying queen of the colony. Selection acting at the level of the individual worker through kin selection, which would favor preferential treatment of related queens, did not consistently influence caste interactions or the outcome of the replacement process. Thus, the outcome of queen replacement in honey bees may be determined primarily by a combination of a queen’s inherent fighting ability coupled with the rate at which she receives some interactions (particularly vibration signals) from workers.

Queen personality type predicts nest-guarding behaviour, colony size and the subsequent collective aggressiveness of the colony

The founding individuals of a new group or lineage are uniquely poised to have long-lasting effects on their descendants. Here we explore how the behavioural traits of new foundresses influence their task allocation strategies and the downstream collective behaviour of their colonies. We collected individual paper wasp (Polistes metricus) queens from the wild and measured their boldness, aggressiveness, exploration, activity and time budgets in the laboratory. We then provided queens with food and nest materials and waited for their workers to eclose. We subsequently tested for links between queen behavioural traits (e.g. boldness, aggressiveness) and the collective aggressiveness of their colonies, as estimated by the number of workers that responded to and stung a model predator. We found that bolder queens (i.e. queens that were less likely to leave their nests after repeated exposure to an agonistic stimulus) spent more time nest guarding, reared more workers and produced groups that were less responsive to a model predator. Thus, there appears to be parent–offspring, or queen–worker, resemblance in the degree to which individuals remain on their nests in different social contexts (solitary queen versus queen + her worker group). To our knowledge this represents the first study to explicitly test for links between individual variation in foundress behaviour, worker production and intercolony collective behaviour; the results highlight the possibility that cross-contextual and transgenerational behavioural links could be a reasonably common and potentially important phenomenon in complex societies.

The sternal brush of the Van der Vecht organ scales isometrically with body size: implications for the study of incipient morphological castes in primitively eusocial wasps

Recent studies have revealed a slight degree of queen–worker morphological divergence in wasps with a primitively eusocial organization, even though, by definition, they lack physical castes. Previously, we showed that in the Neotropical paper wasp Polistes versicolor, the cuticular excretory area of the Van der Vecht organ is disproportionally bigger in queens than in workers (non-isometric difference). Here, we look for caste difference in another component of the same organ, the sternal brush – a tuft of hairs arising from the excretory area. We found that the number of hairs in the sternal brush is higher in queens than in workers. Contrary to our expectation, such a difference is the result of an isometric growth, as caste difference vanishes after correcting for the wasp’s overall body size. For the reason that only non-isometric relations are considered evidence of queen–worker divergence, we state that there is no morphological caste difference regarding the sternal brush of the Van der Vecht organ. Thus, the number of hairs in the sternal brush and the excretory area from which these hairs emerge have different scaling relations with the wasp’s body size (isometric and non-isometric, respectively). We discuss implications for the study of incipient morphological castes in primitively eusocial wasps.

Reproductive and aggressive behaviours of queen–worker intercastes in the ant Mystrium rogeri and caste evolution

Evolutionary novelty can arise through the recombination of ancestral characters. In ants, colonies occasionally produce anomalous individuals that are mosaics of queen and worker characters. These intermediate individuals may be the first step in the evolution of new castes such as wingless queens and soldiers. While the morphological features of intercastes support this hypothesis, their rarity has meant their behaviour is unknown. In this study we carried out a behavioural study of 32 intercastes of Mystrium rogeri to assess whether they are a burden for their colonies, and whether their unusual morphologies are associated with consistent behavioural profiles and new functions. In addition, we measured dominance interactions within colonies, reproductive behaviours during dyadic encounters with males and aggressive interactions during dyadic encounters with heterospecific workers. We found that intercastes were active in colonies and that their behavioural profiles did not differ from those of virgin queens and workers. However, some intercastes were dominant, had developed ovaries and attracted males. One intercaste successfully mated and laid diploid eggs, demonstrating her reproductive potential. Moreover, we found that intercastes were as capable of fighting as workers because of their large size and could thus be useful for colony defence. Our results show that the production of intercastes in M. rogeri inflicts few costs but can yield benefits. Accordingly, certain ecological circumstances could select for increased frequency of intercaste production, ultimately resulting in the evolution of new reproductive and defensive castes.