The Mechanistic, Genetic, and Evolutionary Basis of Worker Sterility in the Social Hymenoptera

Phenomenon of reproductive plasticity in ants

Ant colonies are organized in castes with distinct behaviors that together allow the colony to strive. Reproduction relies on one or a few queens that stay in the nest producing eggs, while females of the worker caste do not reproduce and instead engage in colony maintenance and brood caretaking. Yet, in spite of this clear separation of functions, workers can become reproductive under defined circumstances. Here, we review the context in which workers become reproductive, exhibiting asexual or sexual reproduction depending on the species. Remarkably, the activation of reproduction in these workers can be quite stable, with changes that include behavior and a dramatic extension of lifespan. We compare these changes between species that do or do not have a queen caste. We discuss how the mechanisms underlying reproductive plasticity include changes in hormonal functions and in epigenetic configurations. Further studies are warranted to elucidate not only how reproductive functions have been gradually restricted to the queen caste during evolution but also how reproductive plasticity remains possible in workers of some species.

Conflict and conflict resolution in the major transitions

Conflict and conflict resolution have been argued to be fundamental to the major transitions in evolution. These were key events in life's history in which previously independently living individuals cooperatively formed a higher-level individual, such as a multicellular organism or eusocial colony. Conflict has its central role because, to proceed stably, the evolution of individuality in each major transition required within-individual conflict to be held in check. This review revisits the role of conflict and conflict resolution in the major transitions, addressing recent work arguing for a minor role. Inclusive fitness logic suggests that differences between the kin structures of clones and sexual families support the absence of conflict at the origin of multicellularity but, by contrast, suggest that key conflicts existed at the origin of eusociality. A principal example is conflict over replacing the founding queen (queen replacement). Following the origin of each transition, conflict remained important, because within-individual conflict potentially disrupts the attainment of maximal individuality (organismality) in the system. The conclusion is that conflict remains central to understanding the major transitions, essentially because conflict arises from differences in inclusive fitness optima while conflict resolution can help the system attain a high degree of coincidence of inclusive fitness interests.

Unexpected worker mating and colony-founding in a superorganism

The emergence of caste-differentiated colonies, which have been defined as 'superorganisms', in ants, bees, and wasps represents a major transition in evolution. Lifetime mating commitment by queens, pre-imaginal caste determination and lifetime unmatedness of workers are key features of these animal societies. Workers in superorganismal species like honey bees and many ants have consequently lost, or retain only vestigial spermathecal structures. However, bumble bee workers retain complete spermathecae despite 25-40 million years since their origin of superorganismality, which remains an evolutionary mystery. Here, we show (i) that bumble bee workers retain queen-like reproductive traits, being able to mate and produce colonies, underlain by queen-like gene expression, (ii) the social conditions required for worker mating, and (iii) that these abilities may be selected for by early queen-loss in these annual species. These results challenge the idea of lifetime worker unmatedness in superorganisms, and provide an exciting new tool for the conservation of endangered bumble bee species.

A key gene for the climatic adaptation of Apis cerana populations in China according to selective sweep analysis

BACKGROUND

Apis cerana is widely distributed in China and, prior to the introduction of western honeybees, was the only bee species kept in China. During the long-term natural evolutionary process, many unique phenotypic variations have occurred among A. cerana populations in different geographical regions under varied climates. Understanding the molecular genetic basis and the effects of climate change on the adaptive evolution of A. cerana can promote A. cerana conservation in face of climate change and allow for the effective utilization of its genetic resources.

RESULT

To investigate the genetic basis of phenotypic variations and the impact of climate change on adaptive evolution, A. cerana workers from 100 colonies located at similar geographical latitudes or longitudes were analyzed. Our results revealed an important relationship between climate types and the genetic variation of A. cerana in China, and a greater influence of latitude compared with longitude was observed. Upon selection and morphometry analyses combination for populations under different climate types, we identified a key gene RAPTOR, which was deeply involved in developmental processes and influenced the body size.

CONCLUSION

The selection of RAPTOR at the genomic level during adaptive evolution could allow A. cerana to actively regulate its metabolism, thereby fine-tuning body sizes in response to harsh conditions caused by climate change, such as food shortages and extreme temperatures, which may partially elucidate the size differences of A. cerana populations. This study provides crucial support for the molecular genetic basis of the expansion and evolution of naturally distributed honeybee populations.

Queens control caste allocation in the ant Cardiocondyla obscurior

Social insect queens and workers can engage in conflict over reproductive allocation when they have different fitness optima. Here, we show that queens have control over queen-worker caste allocation in the ant Cardiocondyla obscurior, a species in which workers lack reproductive organs. We describe crystalline deposits that distinguish castes from the egg stage onwards, providing the first report of a discrete trait that can be used to identify ant caste throughout pre-imaginal development. The comparison of queen and worker-destined eggs and larvae revealed size and weight differences in late development, but no discernible differences in traits that may be used in social interactions, including hair morphology and cuticular odours. In line with a lack of caste-specific traits, adult workers treated developing queens and workers indiscriminately. Together with previous studies demonstrating queen control over sex allocation, these results show that queens control reproductive allocation in C. obscurior and suggest that the fitness interests of colony members are aligned to optimize resource allocation in this ant.

Sex- and caste-specific transcriptomes of larval honey bee (Apis mellifera L.) gonads: DMRT A2 and Hsp83 are differentially expressed and regulated by juvenile hormone

The gonads of honey bee, Apis mellifera, queens and drones are each composed of hundreds of serial units, the ovarioles and testioles, while the ovaries of the adult subfertile workers consist of only few ovarioles. We performed a comparative RNA-seq analysis on early fifth-instar (L5F1) larval gonads, which is a critical stage in gonad development of honey bee larvae. A total of 1834 genes were identified as differentially expressed (Padj < 0.01) among the three sex and caste phenotypes. The Gene Ontology analysis showed significant enrichment for metabolism, protein or ion binding, and oxidoreductase activity, and a KEGG analysis revealed metabolic pathways as enriched. In a principal component analysis for the total transcriptomes and hierarchical clustering of the DEGs, we found higher similarity between the queen and worker ovary transcriptomes compared to the drone testis, despite the onset of programmed cell death in the worker ovaries. Four DEGs were selected for RT-qPCR analyses, including their response to juvenile hormone (JH), which is a critical factor in the caste-specific development of the ovaries. Among these, DMRT A2 and Hsp83 were found upregulated by JH and, thus, emerged as potential molecular markers for sex- and caste-specific gonad development in honey bees.

Reproductive plasticity and oogenesis in the queen honey bee (Apis mellifera)

In the honey bee (Apis mellifera), queen and worker castes originate from identical genetic templates but develop into different phenotypes. Queens lay up to 2000 eggs daily whereas workers are sterile in the queen's presence. Periodically queens stop laying: during swarming, when resources are scarce in winter, and when they are confined to a cage by beekeepers. We used confocal microscopy and gene expression assays to investigate the control of oogenesis in the ovaries of honey bee queens that were caged inside and outside the colony. We find evidence that queens use a different combination of 'checkpoints' to regulate oogenesis compared to honey bee workers and other insect species. However, both queen and worker castes likely use the same programmed cell death pathways to terminate oocyte development at their caste-specific checkpoints. Our results also suggest that a key factor driving the termination of oogenesis in queens is nutritional stress. Thus, queens may regulate oogenesis via the same regulatory pathways that were utilised by ancestral solitary species but likely have adjusted physiological checkpoints to suit their highly-derived life history.

Humoral and Cellular Defense Mechanisms in Rebel Workers of Apis mellifera

The physiological state of an insect depends on efficiently functioning immune mechanisms such as cellular and humoral defenses. However, compounds participating in these mechanisms also regulate reproductive caste formation and are responsible for reproductive division of labor as well as for labor division in sterile workers. Divergent reaction of the same genotype yielding reproductive queens and worker castes led to shaping of the physiological and behavioral plasticity of sterile or reproductive workers. Rebels that can lay eggs while maintaining tasks inside and outside the colony exhibit both queen and worker traits. So, we expected that the phagocytic index, JH3 titer, and Vg concentration would be higher in rebels than in normal workers and would increase with their age. We also assumed that the numbers of oenocytes and their sizes would be greater in rebels than in normal workers. The rebels and the normal workers were collected at the age of 1, 7, 14, and 21 days, respectively. Hemolymph and fat bodies were collected for biochemical and morphological analyses. The high levels of JH, Vg, and the phagocytic index, as well as increased numbers and sizes of oenocytes in the fat body cells demonstrate the physiological and phenotypic adaptation of rebels to the eusocial life of honeybees.

Founder effects on sex determination systems in invasive social insects

Invasive populations are often established from a small number of individuals, and thus have low genetic diversity relative to native-range populations. Social ants, bees and wasps (social Hymenoptera) should be vulnerable to such founder effects on genetic diversity because sex in these species is determined genetically via Complementary Sex Determination (CSD). Under CSD, individuals homozygous at one or more critical sex loci are inviable or develop as infertile diploid males. Low diversity at sex loci leads to increased homozygosity and diploid male production, increasing the chance of colony death. In this review, we identify behavioral, social and reproductive traits that preserve allele richness at sex loci, allow colonies to cope with diploid male production, and eventually restore sex allele diversity in invasive populations of social Hymenoptera that experience founding bottlenecks.

Vitellogenin expression in the ovaries of adult honeybee workers provides insights into the evolution of reproductive and social traits

Social insects are notable for having two female castes that exhibit extreme differences in their reproductive capacity. The molecular basis of these differences is largely unknown. Vitellogenin (Vg) is a powerful antioxidant and insulin-signalling regulator used in oocyte development. Here we investigate how Royal Jelly (the major food of honeybee queens) and queen mandibular pheromone (a major regulator of worker fertility), affect the longevity and reproductive status of honey bee workers, the expression of Vg, its receptor VgR and associated regulatory proteins. We find that Vg is expressed in the ovaries of workers and that workers fed a queen diet of Royal Jelly have increased Vg expression in the ovaries. Surprisingly, we find that expression of Vg is not associated with ovary activation in workers, suggesting that this gene has potentially acquired non-reproductive functions. Therefore, Vg expression in the ovaries of honeybee workers provides further support for the Ovarian Ground Plan Hypothesis, which argues that genes implicated in the regulation of reproduction have been co-opted to regulate behavioural differences between queens and workers.

Ageing and sociality: why, when and how does sociality change ageing patterns?

Individual lifespans vary tremendously between and also within species, but the proximate and ultimate causes of different ageing speeds are still not well understood. Sociality appears to be associated with the evolution of greater longevity and probably also with a larger plasticity of the shape and pace of ageing. For example, reproductives of several termites and ants reach lifespans that surpass those of their non-reproductive nestmates by one or two decades. In this issue, 15 papers explore the interrelations between sociality and individual longevity in both, group-living vertebrates and social insects. Here, we briefly give an overview of the contents of the various contributions, including theoretical and comparative studies, and we explore the similarities and dissimilarities in proximate mechanisms underlying ageing among taxa, with particular emphasis on nutrient-sensing pathways and, in insects, juvenile hormone. These studies point to an underestimated role of more downstream processes. We highlight the need for reliable transcriptomic markers of ageing and a comprehensive ageing theory of social animals, which includes the reproductive potential of workers, and considers the fact that social insect queens reach maturity only after a prolonged period of producing non-reproductive workers. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

A small family business: synergistic and additive effects of the queen and the brood on worker reproduction in a primitively eusocial bee

The mechanisms that maintain reproductive division of labor in social insects are still incompletely understood. Most studies focus on the relationship between adults, overlooking another important stakeholder - the juveniles. Recent studies show that not only the queen but also the brood regulate worker reproduction. However, how the two coordinate to maintain reproductive monopoly remained unexplored. Here, we disentangled the roles of the brood and the queen in primitively eusocial bees (Bombus impatiens) by examining their separated and combined effects on worker behavioral, physiological and brain gene expression. We found that young larvae produce a releaser effect on workers, decreasing oviposition and aggression, while the queen produces both releaser and primer effects, modifying worker behavior and reproductive physiology. The expression of reproduction- and aggression-related genes was altered in the presence of both queen and brood but was stronger or the same in the presence of the queen. We identified two types of interactions between the queen and the brood in regulating worker reproduction: (1) synergistic interactions regulating worker physiology, where the combined effect of the queen and the brood on worker physiology was greater than their separate effects; (2) additive interactions, where the combined effect of the queen and the brood on worker behavior was similar to the sum of their separate effects. Our results suggest that the queen and the brood interact synergistically and additively to regulate worker behavior and reproduction, and this interaction exists at multiple regulatory levels.

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.

Effects of larval Age at Grafting and Juvenile Hormone on Morphometry and Reproductive Quality Parameters of in Vitro Reared Honey Bees (Hymenoptera: Apidae)

The honey bee queen plays a central role in the Apis mellifera L. (Hymenoptera: Apidae) colony, and her high reproductive capacity is fundamental for building up the workforce of a colony. Caste development in honey bee females involves elaborate physiological pathways unleashed at the beginning of the first larval instars, with juvenile hormone (JH) playing a crucial role. Here we took advantage of established in vitro rearing techniques to conduct a 2 × 2 experimental design and test initial rearing age (young vs old) and JH treatment (JH III vs solvent control) to enlighten the role of nutrient quality and JH in shaping honey bee female fertility, morphological features related to queenliness, and key physiological parameters (hemolymph vitellogenin/Vg, sugar levels, and Vg transcript levels). Our results show that while the age at initial larval rearing had major impacts on external morphology development, where younger larvae exhibited a higher probability to develop into queen-like adults morphotypes, the JH application during the larval stage improved physiological pathways related to ovary development and metabolism during the ontogenic development. We detected that the supplementation of queen larvae with JH promoted important benefits regarding queen fertility as the increase of ovariole number and vg levels at hemolymph, both crucial factors at eggs production. The data presented here provide guidance in efforts to improve honey bee queen quality, especially in light of frequent episodes of queen failures in the beekeeping industry.

Conflict and major transitions - why we need true queens

In contrast to human societies, where kings and queens can be sources of conflict, we argue that the morphologically distinct queens of insect colonies are central to the minimization of conflict within their societies. Thus, the evolution of irreversible queen and worker castes represents a major transition in social evolution. Queens are selected to become better reproducers, and workers are selected to become better workers. The reproductive success of queens and workers are, therefore, inextricably linked. Workers achieve reproductive success by assisting the queen, whereas the queen needs her workers to provide her with the wherewithal to raise her brood. The tighter the mutual dependence, the lower conflict, and the larger insect societies can become. As the queen becomes a better breeder, workers are selected to become better at raising their siblings. Yet, nothing in nature is ever free of conflict and with the evolution of a true worker caste a new set of conflicts arises. Multiple mating by queens in particular opens the door to a new set of conflicts. Ironically, multiple mating can only evolve once within-colony conflict is reduced by evolving a true worker caste.

Fat body lipolysis connects poor nutrition to hypopharyngeal gland degradation in Apis mellifera

The hypopharyngeal glands (HGs) of honey bee nurse workers secrete the major protein fraction of jelly, a protein and lipid rich substance fed to developing larvae, other worker bees, and queens. A hallmark of poorly nourished nurses is their small HGs, which actively degrade due to hormone-induced autophagy. To better connect nutritional stress with HG degradation, we looked to honey bees and other insect systems, where nutrient stress is often accompanied by fat body degradation. The fat body contains stored lipids that are likely a substrate for ecdysteroid synthesis, so we tested whether starvation caused increased fat body lipolysis. Ecdysteroid signaling and response pathways and IIS/TOR are tied to nutrient-dependent autophagy in honey bees and other insects, and so we also tested whether and where genes in these pathways were differentially regulated in the head and fat body. Last, we injected nurse-aged bees with the honey bee ecdysteroid makisterone A to determine whether this hormone influenced HG size and autophagy. We find that starved nurse aged bees exhibited increased fat body lipolysis and increased expression of ecdysteroid production and response genes in the head. Genes in the IIS/TOR pathway were not impacted by starvation in either the head or fat body. Additionally, bees injected with makisterone A had smaller HGs and increased expression of autophagy genes. These data support the hypothesis that nutritional stress induces fat body lipolysis, which may liberate the sterols important for ecdysteroid production, and that increased ecdysteroid levels induce autophagic HG degradation.

The possible role of ant larvae in the defence against social parasites

Temporary social parasite ant queens initiate new colonies by entering colonies of host species, where they begin laying eggs. As the resident queen can be killed during this process, host colonies may lose their entire future reproductive output. Selection thus favours the evolution of defence mechanisms, before and after parasite intrusion. Most studies on social parasites focus on host worker discrimination of parasite queens and their offspring. However, ant larvae can also influence brood composition by consuming eggs. This raises the question whether host larvae can aid in preventing colony takeover by consuming eggs laid by parasite queens. To test whether larvae could play a role in anti-parasite defence, we compared the rates at which larvae of a common host species, Formica fusca, consumed eggs laid by social parasite, non-parasite, nest-mate, or conspecific non-nest-mate queens. Larvae consumed social parasite eggs more than eggs laid by a heterospecific non-parasite queen, irrespective of the chemical distance between the egg cuticular profiles. Also, larvae consumed eggs laid by conspecific non-nest-mate queens more than those laid by nest-mate queens. Our study suggests that larvae may act as players in colony defence against social parasitism, and that social parasitism is a key factor shaping discrimination behaviour in ants.

Monogamy promotes altruistic sterility in insect societies

Monogamy is associated with sibling-directed altruism in multiple animal taxa, including insects, birds and mammals. Inclusive-fitness theory readily explains this pattern by identifying high relatedness as a promoter of altruism. In keeping with this prediction, monogamy should promote the evolution of voluntary sterility in insect societies if sterile workers make for better helpers. However, a recent mathematical population-genetics analysis failed to identify a consistent effect of monogamy on voluntary worker sterility. Here, we revisit that analysis. First, we relax genetic assumptions, considering not only alleles of extreme effect-encoding either no sterility or complete sterility-but also alleles with intermediate effects on worker sterility. Second, we broaden the stability analysis-which focused on the invasibility of populations where either all workers are fully sterile or all workers are fully reproductive-to identify where intermediate pure or mixed evolutionarily stable states may occur. Third, we consider a broader range of demographically explicit ecological scenarios relevant to altruistic worker non-reproduction and to the evolution of eusociality more generally. We find that, in the absence of genetic constraints, monogamy always promotes altruistic worker sterility and may inhibit spiteful worker sterility. Our extended analysis demonstrates that an exact population-genetics approach strongly supports the prediction of inclusive-fitness theory that monogamy promotes sib-directed altruism in social insects.

The dynamic association between ovariole loss and sterility in adult honeybee workers

In the social insects, ovary state (the presence or absence of mature oocytes) and ovary size (the number of ovarioles) are often used as proxies for the reproductive capacity of an individual worker. Ovary size is assumed to be fixed post-eclosion whereas ovary state is demonstrably plastic post-eclosion. Here, we show that in fact ovary size declines as honeybee workers age. This finding is robust across two honeybee species: Apis mellifera and A. cerana The ovariole loss is likely to be due to the regression of particular ovarioles via programmed cell death. We also provide further support for the observation that honeybee workers with activated ovaries (mature oocytes present) most commonly have five ovarioles rather than a greater or smaller number. This result suggests that workers with more than five ovarioles are unable to physiologically support more than five activated ovarioles and that workers with fewer than five ovarioles are below a threshold necessary for ovary activation. As a worker's ovariole number declines with age, studies on worker ovariole number need to take this plasticity into account.

The evolution of queen control over worker reproduction in the social Hymenoptera

A trademark of eusocial insect species is reproductive division of labor, in which workers forego their own reproduction while the queen produces almost all offspring. The presence of the queen is key for maintaining social harmony, but the specific role of the queen in the evolution of eusociality remains unclear. A long‐discussed scenario is that a queen either behaviorally or chemically sterilizes her workers. However, the demographic and ecological conditions that enable such manipulation are still debated. We study a simple model of evolutionary dynamics based on haplodiploid genetics. Our model is set in the commonly observed case where workers have lost the ability to lay female (diploid) eggs by mating, but retain the ability to lay male (haploid) eggs. We consider a mutation that acts in a queen, causing her to control the reproductive behavior of her workers. Our mathematical analysis yields precise conditions for the evolutionary emergence and stability of queen‐induced worker sterility. These conditions do not depend on the queen's mating frequency. We find that queen control is always established if it increases colony reproductive efficiency, but can evolve even if it decreases colony efficiency. We further derive the conditions under which queen control is evolutionarily stable against invasion by mutant workers who have recovered the ability to lay male eggs.

The effects of juvenile hormone on Lasius niger reproduction

Reproduction has been shown to be costly for survival in a wide diversity of taxa. The resulting trade-off, termed the reproduction-survival trade-off, is thought to be one of the most fundamental forces of life-history evolution. In insects the pleiotropic effect of juvenile hormone (JH), antagonistically regulating reproduction and pathogen resistance, is suggested to underlie this phenomenon. In contrast to the majority of insects, reproductive individuals in many eusocial insects defy this trade-off and live both long and prosper. By remodelling the gonadotropic effects of JH in reproductive regulation, the queens of the long-lived black garden ant Lasius niger (living up to 27 years), have circumvented the reproduction-survival trade off enabling them to maximize both reproduction and pathogen resistance simultaneously. In this study we measure fertility, vitellogenin gene expression and protein levels after experimental manipulation of hormone levels. We use these measurements to investigate the mechanistic basis of endocrinological role remodelling in reproduction and determine how JH suppresses reproduction in this species, rather then stimulating it, like in the majority of insects. We find that JH likely inhibits three key aspects of reproduction both during vitellogenesis and oogenesis, including two previously unknown mechanisms. In addition, we document that juvenile hormone, as in the majority of insects, has retained some stimulatory function in regulating vitellogenin expression. We discuss the evolutionary consequences of this complex regulatory architecture of reproduction in L. niger, which might enable the evolution of similar reproductive phenotypes by alternate regulatory pathways, and the surprising flexibility regulatory role of juvenile hormone in this process.