Cooperative Nesting in the Multivoltine Large Carpenter Bee Xylocopa sulcatipes Maa (Apoidea: Anthophoridae): Do Helpers Gain or Lose to Solitary Females?

Xylocopa caerulea and Xylocopa auripennis harbor a homologous gut microbiome related to that of eusocial bees

Background

Eusocial bees, such as bumblebees and honey bees, harbor host-specific gut microbiota through their social behaviors. Conversely, the gut microbiota of solitary bees is erratic owing to their lack of eusocial activities. Carpenter bees (genus Xylocopa) are long-lived bees that do not exhibit advanced eusociality like honey bees. However, they often compete for nests to reproduce. Xylocopa caerulea and Xylocopa auripennis are important pollinators of wild plants on Hainan Island. Whether they have host-specific bacteria in their guts similar to eusocial bees remains unknown.

Methods

We targeted the bacterial 16S rRNA V3-V4 region to investigate the diversity of bacterial symbionts in the fore-midgut and hindgut of two carpenter bees, X. caerulea and X. auripennis.

Results

A maximum of 4,429 unique amplicon sequence variants (ASVs) were detected from all samples, belonging to 10 different phyla. X. caerulea and X. auripennis shared similar bacterial community profiles, with Lactobacillaceae, Bifidobacteriaceae, and Orbaceae being dominant in their entire guts. X. caerulea and X. auripennis harbor a highly conserved core set of bacteria, including the genera Candidatus Schmidhempelia and Bombiscardovia. These two bacterial taxa from carpenter bees are closely related to those isolated from bumblebees. The LEfSe analysis showed that Lactobacillaceae, Bifidobacteriaceae, and the genus Bombilactobacillus were significantly enriched in the hindguts of both carpenter bees. Functional prediction suggested that the most enriched pathways were involved in carbohydrate and lipid metabolism.

Conclusions

Our results revealed the structure of the gut microbiota in two carpenter bees and confirmed the presence of some core bacterial taxa that were previously only found in the guts of social bees.

Extreme reproductive skew at the dawn of sociality is consistent with inclusive fitness theory but problematic for routes to eusociality

To understand the earliest stages of social evolution, we need to identify species that are undergoing the initial steps into sociality. Amphylaeus morosus is the only unambiguously known social species in the bee family Colletidae and represents an independent origin of sociality within the Apoidea. This allows us to investigate the selective factors promoting the transition from solitary to social nesting. Using genome-wide SNP genotyping, we infer robust pedigree relationships to identify maternity of brood and intracolony relatedness for colonies at the end of the reproductive season. We show that A. morosus forms both matrifilial and full-sibling colonies, both involving complete or almost complete monopolization over reproduction. In social colonies, the reproductive primary was also the primary forager with the secondary female remaining in the nest, presumably as a guard. Social nesting provided significant protection against parasitism and increased brood survivorship in general. We show that secondary females gain large indirect fitness benefits from defensive outcomes, enough to satisfy the conditions of inclusive fitness theory, despite an over-production of males in social colonies. These results suggest an avenue to sociality that involves high relatedness and, very surprisingly, extreme reproductive skew in its earliest stages and raises important questions about the evolutionary steps in pathways to eusociality.

Hamilton's rule in economic decision-making

SignificanceKin selection-helping genetically related individuals even at a cost to oneself-can be evolutionarily advantageous. This is the main theoretical explanation for altruism in the natural world. Hamilton's rule provides a simple algebraic relationship that captures this profound idea. While behavior consistent with Hamilton's rule has been observed in many species, a direct and sharp test of this rule has not yet been performed. In this paper, we employ techniques borrowed from experimental economics to test the predictions of Hamilton's rule. We find strong support for the rule. This result sheds light on the dominant role played by evolutionary biology in explaining human behavior.

Social consequences of energetically costly nest construction in a facultatively social bee

Social groups form when the costs of breeding independently exceed fitness costs imposed by group living. The costs of independent breeding can often be energetic, especially for animals performing expensive behaviours, such as nest construction. To test the hypothesis that nesting costs can drive sociality by disincentivizing independent nest founding, we measured the energetics of nest construction and inheritance in a facultatively social carpenter bee (Xylocopa sonorina Smith), which bores tunnel nests in wood. We measured metabolic rates of bees excavating wood and used computerized tomography images of nesting logs to measure excavation volumes. From these data, we demonstrate costly energetic investments in nest excavation of a minimum 4.3 kJ per offspring provisioned, an expense equivalent to nearly 7 h of flight. This high, potentially prohibitive cost of nest founding may explain why females compete for existing nests rather than constructing new ones, often leading to the formation of social groups. Further, we found that nest inheritors varied considerably in their investment in nest renovation, with costs ranging more than 12-fold (from 7.08 to 89.1 kJ energy), probably reflecting differences in inherited nest quality. On average, renovation costs were lower than estimated new nest construction costs, with some nests providing major savings. These results suggest that females may join social groups to avoid steep energetic costs, but that the benefits of this strategy are not experienced equally.

The evolution of eusociality: no risk-return tradeoff but the ecology matters

The origin of eusociality in the Hymenoptera is a question of major interest. Theory has tended to focus on genetic relatedness, but ecology can be just as important a determinant of whether eusociality evolves. Using the model of Fu et al. (2015), we show how ecological assumptions critically affect the conclusions drawn. Fu et al. inferred that eusociality rarely evolves because it faces a fundamental 'risk-return tradeoff'. The intuitive logic was that worker production represents an opportunity cost because it delays realising a reproductive payoff. However, making empirically justified assumptions that (1) workers take over egg-laying following queen death and (2) productivity increases gradually with each additional worker, we find that the risk-return tradeoff disappears. We then survey Hymenoptera with more specialised morphological castes, and show how the interaction between two common features of eusociality - saturating birth rates and group size-dependent helping decisions - can determine whether eusociality outperforms other strategies.

Polyandrous bee provides extended offspring care biparentally as an alternative to monandry based eusociality

Parental care behavior evolves to increase the survival of offspring. When offspring care becomes complicated for ecological reasons, cooperation of multiple individuals can be beneficial. There are two types of cooperative care: biparental care and worker (helper)-based care (e.g., eusociality). Although biparental care is common in several groups of vertebrates, it is generally rare in arthropods. Conversely, eusociality is widespread in insects, especially the aculeate Hymenoptera. Here, we present a case of biparental care in bees, in Ceratina nigrolabiata (Apidae, Xylocopinae). Similar to eusocial behavior, biparental care leads to greater brood protection in this species. Male guarding increases provisioning of nests because females are liberated from the tradeoff between provisioning and nest protection. The main benefit of parental care for males should be increased paternity. Interestingly though, we found that paternity of offspring by guard males is extraordinarily low (10% of offspring). Generally, we found that nests were not guarded by the same male for the whole provisioning season, meaning that males arrive to nests as stepfathers. However, we show that long-term guarding performed by a single male does increase paternity. We suggest that the multiple-mating strategy of these bees increased the amount of time for interactions between the sexes, and this longer period of potential interaction supported the origin of biparental care. Eusociality based on monandry was thought to be the main type of extended brood protection in bees. We show that biparental care based on polyandry provides an interesting evolutionary alternative.

Foraging strategies and physiological adaptations in large carpenter bees

Large carpenter bees are charismatic and ubiquitous flower visitors in the tropics and sub-tropics. Unlike honeybees and bumblebees that have been popular subjects of extensive studies on their neuroethology, behaviour and ecology, carpenter bees have received little attention. This review integrates what is known about their foraging behaviour as well as sensory, physiological and cognitive adaptations and is motivated by their versatility as flower visitors and pollinators. This is evident from their extremely generalist foraging and adeptness at handling diverse flower types as legitimate pollinators and as illegitimate nectar robbers. They purportedly use traplining to forage between isolated patches and are long-distance flyers over several kilometres suggesting well-developed spatial learning, route memory and navigational capabilities. They have a broad range of temperature tolerance and thermoregulatory capabilities which are likely employed in their forays into crepuscular and nocturnal time periods. Such temporal extensions into dim-light periods invoke a suite of visual adaptations in their apposition optics. Thus, we propose that carpenter bees are an excellent though understudied group for exploring the complex nature of plant-pollinator mutualisms from ecological and mechanistic perspectives.

Genetic accommodation and the role of ancestral plasticity in the evolution of insect eusociality

For over a century, biologists have proposed a role for phenotypic plasticity in evolution, providing an avenue for adaptation in addition to 'mutation-first' models of evolutionary change. According to the various versions of this idea, the ability of organisms to respond adaptively to their environment through phenotypic plasticity may lead to novel phenotypes that can be screened by natural selection. If these initially environmentally induced phenotypes increase fitness, then genetic accommodation can lead to allele frequency change, influencing the expression of those phenotypes. Despite the long history of 'plasticity-first' models, the importance of genetic accommodation in shaping evolutionary change has remained controversial - it is neither fully embraced nor completely discarded by most evolutionary biologists. We suggest that the lack of acceptance of genetic accommodation in some cases is related to a lack of information on its molecular mechanisms. However, recent reports of epigenetic transgenerational inheritance now provide a plausible mechanism through which genetic accommodation may act, and we review this research here. We also discuss current evidence supporting a role for genetic accommodation in the evolution of eusociality in social insects, which have long been models for studying the influence of the environment on phenotypic variation, and may be particularly good models for testing hypotheses related to genetic accommodation. Finally, we introduce 'eusocial engineering', a method by which novel social phenotypes are first induced by environmental modification and then studied mechanistically to understand how environmentally induced plasticity may lead to heritable changes in social behavior. We believe the time is right to incorporate genetic accommodation into models of the evolution of complex traits, armed with new molecular tools and a better understanding of non-genetic heritable elements.

Can Hamilton's rule be violated?

How generally Hamilton’s rule holds is a much debated question. The answer to that question depends on how costs and benefits are defined. When using the regression method to define costs and benefits, there is no scope for violations of Hamilton’s rule. We introduce a general model for assortative group compositions to show that, when using the counterfactual method for computing costs and benefits, there is room for violations. The model also shows that there are limitations to observing violations in equilibrium, as the discrepancies between Hamilton’s rule and the direction of selection may imply that selection will take the population out of the region of disagreement, precluding observations of violations in equilibrium. Given what it takes to create a violation, empirical tests of Hamilton’s rule, both in and out of equilibrium, require the use of statistical models that allow for identifying non-linearities in the fitness function.

The benefits of grouping as a main driver of social evolution in a halictine bee

Over the past decade, the cause of sociality has been much debated. Inclusive fitness [br in Hamilton's rule (br - c > 0)] has been criticized but is still useful in the organization of a framework by elucidating mechanisms through which br (benefit × relatedness) becomes larger than c (cost). The bee Lasioglossum baleicum is suitable for investigation of this issue because of the sympatric occurrence of both social and solitary nesting in its populations. We show that a large part (approximately 92%) of the inclusive fitness of a eusocial worker can be attributed to the benefits of grouping. A 1.5-fold relatedness asymmetry benefit in singly mated haplo-diploids explains a small part (approximately 8.5%) of the observed inclusive fitness. Sociality enables this species to conduct foraging and nest defense simultaneously, which is not the case in solitary nests. Our results indicate that this benefit of grouping is the main source of the increased inclusive fitness of eusocial workers.

Nestmate discrimination based on familiarity but not relatedness in eastern carpenter bees

How animals recognize conspecific individuals has important outcomes in many contexts, but interactions among group members are particularly important. Two recognition criteria are often implicated in these interactions: kin recognition is based on relatedness cues and nestmate recognition is based on familiarity. For social insects, both types of recognition are possible, as many nestmates are close kin and familiarity can develop among individuals that encounter each other repeatedly. To discern whether social insects use kin or nestmate recognition, it is necessary to simultaneously assess how relatedness and familiarity influence behaviour. The facultatively social eastern carpenter bee, Xylocopa virginica, offers an excellent opportunity to study how either nestmate or kin recognition (or both) may influence interactions among nestmates, as many females disperse from their natal nests in spring, and often attempt to join new colonies that may contain unrelated individuals. This leads to frequent behavioural interactions among females that may be related or unrelated, and familiar or unfamiliar. We used observation nests and microsatellite loci to assess the influence of familiarity and relatedness on behavioural interactions during the early phase of colony development, when females establish reproductive queues prior to brood production. Females were more likely to feed and were less aggressive to familiar rather than related nestmates, regardless of their relatedness. This suggests that eastern carpenter bees primarily use learned cues to discriminate among nestmates. Interactions with nestmates were also context-dependent, as females returning to the nest without food were the recipients of more aggression than those returning with food. If spring dispersal leads to reduced relatedness in X. virginica colonies, then nestmate recognition based on familiarity would be an important factor in maintaining group cohesion.

The general form of Hamilton's rule makes no predictions and cannot be tested empirically

Hamilton's rule asserts that a trait is favored by natural selection if the benefit to others, [Formula: see text], multiplied by relatedness, [Formula: see text], exceeds the cost to self, [Formula: see text] Specifically, Hamilton's rule states that the change in average trait value in a population is proportional to [Formula: see text] This rule is commonly believed to be a natural law making important predictions in biology, and its influence has spread from evolutionary biology to other fields including the social sciences. Whereas many feel that Hamilton's rule provides valuable intuition, there is disagreement even among experts as to how the quantities [Formula: see text], [Formula: see text], and [Formula: see text] should be defined for a given system. Here, we investigate a widely endorsed formulation of Hamilton's rule, which is said to be as general as natural selection itself. We show that, in this formulation, Hamilton's rule does not make predictions and cannot be tested empirically. It turns out that the parameters [Formula: see text] and [Formula: see text] depend on the change in average trait value and therefore cannot predict that change. In this formulation, which has been called "exact and general" by its proponents, Hamilton's rule can "predict" only the data that have already been given.

Hamilton's rule

This paper reviews and addresses a variety of issues relating to inclusive fitness. The main question is: are there limits to the generality of inclusive fitness, and if so, what are the perimeters of the domain within which inclusive fitness works? This question is addressed using two well-known tools from evolutionary theory: the replicator dynamics, and adaptive dynamics. Both are combined with population structure. How generally Hamilton's rule applies depends on how costs and benefits are defined. We therefore consider costs and benefits following from Karlin and Matessi's (1983) "counterfactual method", and costs and benefits as defined by the "regression method" (Gardner et al., 2011). With the latter definition of costs and benefits, Hamilton's rule always indicates the direction of selection correctly, and with the former it does not. How these two definitions can meaningfully be interpreted is also discussed. We also consider cases where the qualitative claim that relatedness fosters cooperation holds, even if Hamilton's rule as a quantitative prediction does not. We furthermore find out what the relation is between Hamilton's rule and Fisher's Fundamental Theorem of Natural Selection. We also consider cancellation effects - which is the most important deepening of our understanding of when altruism is selected for. Finally we also explore the remarkable (im)possibilities for empirical testing with either definition of costs and benefits in Hamilton's rule.

Kinship, parental manipulation and evolutionary origins of eusociality

One of the hallmarks of eusociality is that workers forego their own reproduction to assist their mother in raising siblings. This seemingly altruistic behaviour may benefit workers if gains in indirect fitness from rearing siblings outweigh the loss of direct fitness. If worker presence is advantageous to mothers, however, eusociality may evolve without net benefits to workers. Indirect fitness benefits are often cited as evidence for the importance of inclusive fitness in eusociality, but have rarely been measured in natural populations. We compared inclusive fitness of alternative social strategies in the tropical sweat bee, Megalopta genalis, for which eusociality is optional. Our results show that workers have significantly lower inclusive fitness than females that found their own nests. In mathematical simulations based on M. genalis field data, eusociality cannot evolve with reduced intra-nest relatedness. The simulated distribution of alternative social strategies matched observed distributions of M. genalis social strategies when helping behaviour was simulated as the result of maternal manipulation, but not as worker altruism. Thus, eusociality in M. genalis is best explained through kin selection, but the underlying mechanism is likely maternal manipulation.

Ergonomic skew and reproductive queuing based on social and seasonal variation in foraging activity of eastern carpenter bees (Xylocopa virginica)

Reproductive division of labour in social carpenter bees differs from that in classically eusocial insects because reproductive output and ergonomic inputs are positively correlated—dominant females monopolize both foraging and reproduction. We quantified ergonomic skew in the facultatively social bee Xylocopa virginica (L., 1771) (eastern carpenter bee) based on detailed observations of foraging activity by individually marked females in 2009. Unusually for a univoltine bee, this species exhibits a spring foraging phase during which females feed pollen to other adults, probably as part of behavioural interactions to establish dominance hierarchies. During brood-provisioning, foraging in social nests was dominated by one female at a time, with replacement by a succession of foragers as dominants disappeared and were succeeded by a subordinate. The principal foragers (individuals that did the largest share of foraging in each colony) did 85%–100% of all pollen trips, so contributions to pollen-provisioning by female nest mates were highly uneven. Individual foraging rate was unaffected by group size and total colony foraging effort was a function of the number of foragers per group. Transient females that moved to new nests were as successful in achieving dominant forager status as females resident in their natal nests. This evidence indicates that colony social organisation is based on reproductive queues, whereby the first-ranked bee is the dominant forager and subordinates queue for opportunities to replace her.

The risk-return trade-off between solitary and eusocial reproduction

Social insect colonies can be seen as a distinct form of biological organisation because they function as superorganisms. Understanding how natural selection acts on the emergence and maintenance of these colonies remains a major question in evolutionary biology and ecology. Here, we explore this by using multi-type branching processes to calculate the basic reproductive ratios and the extinction probabilities for solitary vs. eusocial reproductive strategies. We find that eusociality, albeit being hugely successful once established, is generally less stable than solitary reproduction unless large demographic advantages of eusociality arise for small colony sizes. We also demonstrate how such demographic constraints can be overcome by the presence of ecological niches that strongly favour eusociality. Our results characterise the risk-return trade-offs between solitary and eusocial reproduction, and help to explain why eusociality is taxonomically rare: eusociality is a high-risk, high-reward strategy, whereas solitary reproduction is more conservative.

Hamilton's rule and the causes of social evolution

Hamilton's rule is a central theorem of inclusive fitness (kin selection) theory and predicts that social behaviour evolves under specific combinations of relatedness, benefit and cost. This review provides evidence for Hamilton's rule by presenting novel syntheses of results from two kinds of study in diverse taxa, including cooperatively breeding birds and mammals and eusocial insects. These are, first, studies that empirically parametrize Hamilton's rule in natural populations and, second, comparative phylogenetic analyses of the genetic, life-history and ecological correlates of sociality. Studies parametrizing Hamilton's rule are not rare and demonstrate quantitatively that (i) altruism (net loss of direct fitness) occurs even when sociality is facultative, (ii) in most cases, altruism is under positive selection via indirect fitness benefits that exceed direct fitness costs and (iii) social behaviour commonly generates indirect benefits by enhancing the productivity or survivorship of kin. Comparative phylogenetic analyses show that cooperative breeding and eusociality are promoted by (i) high relatedness and monogamy and, potentially, by (ii) life-history factors facilitating family structure and high benefits of helping and (iii) ecological factors generating low costs of social behaviour. Overall, the focal studies strongly confirm the predictions of Hamilton's rule regarding conditions for social evolution and their causes.

Secondarily solitary: the evolutionary loss of social behavior

Studies of social behavior frequently assume that evolution proceeds from a solitary state to a social one, and social to social lineages give rise to line are also social, excluding parasitic taxa. Recent phylogenetic studies of some bees contradict this assumption, and more examples are known or hypothesized in other animals. Social behaviour can be lost to give rise to species that are secondarily solitary. Studies of the conditions to the suppression or loss of social behavior can help to illuminate those factors that lead to its origins and maintenance.

The large carpenter bees of central Saudi Arabia, with notes on the biology of Xylocopa sulcatipes Maa (Hymenoptera, Apidae, Xylocopinae)

The large carpenter bees (Xylocopinae, Xylocopa Latreille) occurring in central Saudi Arabia are reviewed. Two species are recognized in the fauna, Xylocopa (Koptortosoma) aestuans (Linnaeus) and Xylocopa (Ctenoxylocopa) sulcatipes Maa. Diagnoses for and keys to the species of these prominent components of the central Saudi Arabian bee fauna are provided to aid their identification by pollination researchers active in the region. Females and males of both species are figured and biological notes provided for Xylocopa sulcatipes. Notes on the nesting biology and ecology of Xylocopa sulcatipes are appended. As in studies for this species from elsewhere, nests were found in dried stems of Calotropis procera (Aiton) (Asclepiadaceae) and Phoenix dactylifera L. (Arecaceae).

A mid-cretaceous origin of sociality in xylocopine bees with only two origins of true worker castes indicates severe barriers to eusociality

The origin of sterile worker castes, resulting in eusociality, represents one of the major evolutionary transitions in the history of life. Understanding how eusociality has evolved is therefore an important issue for understanding life on earth. Here we show that in the large bee subfamily Xylocopinae, a simple form of sociality was present in the ancestral lineage and there have been at least four reversions to purely solitary nesting. The ancestral form of sociality did not involve morphological worker castes and maximum colony sizes were very small. True worker castes, entailing a life-time commitment to non-reproductive roles, have evolved only twice, and only one of these resulted in discrete queen-worker morphologies. Our results indicate extremely high barriers to the evolution of eusociality. Its origins are likely to have required very unusual life-history and ecological circumstances, rather than the amount of time that selection can operate on more simple forms of sociality.

The validity and value of inclusive fitness theory

Social evolution is a central topic in evolutionary biology, with the evolution of eusociality (societies with altruistic, non-reproductive helpers) representing a long-standing evolutionary conundrum. Recent critiques have questioned the validity of the leading theory for explaining social evolution and eusociality, namely inclusive fitness (kin selection) theory. I review recent and past literature to argue that these critiques do not succeed. Inclusive fitness theory has added fundamental insights to natural selection theory. These are the realization that selection on a gene for social behaviour depends on its effects on co-bearers, the explanation of social behaviours as unalike as altruism and selfishness using the same underlying parameters, and the explanation of within-group conflict in terms of non-coinciding inclusive fitness optima. A proposed alternative theory for eusocial evolution assumes mistakenly that workers' interests are subordinate to the queen's, contains no new elements and fails to make novel predictions. The haplodiploidy hypothesis has yet to be rigorously tested and positive relatedness within diploid eusocial societies supports inclusive fitness theory. The theory has made unique, falsifiable predictions that have been confirmed, and its evidence base is extensive and robust. Hence, inclusive fitness theory deserves to keep its position as the leading theory for social evolution.

On the evolution of multicelluarity and eusociality

In this article versions of the abstract NKC model are used to examine the conditions under which two significant evolutionary phenomena - multicellularity and eusociality - are likely to occur and why. First, comparisons in evolutionary performance are made between simulations of unicellular organisms and very simple multicellular-like organisms, under varying conditions. The results show that such multicellularity without differentiation appears selectively neutral, but that differentiation to soma (nonreproductives) proves beneficial as the amount of epistasis in the fitness landscape increases. This is explained by considering mutations in the generation of daughter cells and their subsequent effect on the propagule's fitness. This is interpreted as a simple example of the Baldwin effect. Second, the correspondences between multicellularity and eusociality are highlighted, particularly that both contain individuals who do not reproduce. The same process is then used to explain the emergence of eusocial colonies.