Unequal subfamily proportions among honey bee queen and worker brood

Collective decision-making during reproduction in social insects: a conceptual model for queen supersedure in honey bees (Apis mellifera)

Insect societies have served as excellent examples for co-ordinated decision-making. The production of sexuals is the most important group decision that social insects face since it affects both direct and indirect fitness. The behavioral processes by which queens are selected have been of particular interest since they are the primary egg layers that enable colony function. As a model system, previous research on honey bee reproduction has focused on swarming behavior and nest site selection. One significant gap in our knowledge of the collective decision-making process over reproduction is how daughter queens simply replace old or failing queens (=supersedure) rather than being reared for the purposes of colony fission (=swarming) or queen loss (=emergency queen rearing). Here, I present a conceptual model that provides a framework for understanding the collective decisions by colonies to supersede their mother queens, as well as provide some key recommendations on future empirical work.

Strength surpasses relatedness-queen larva selection in honeybees

Nepotism was initially theoretically predicted and sometimes found to trigger the selection of specific larvae to be reared as queens in the honeybee Apis mellifera. Although the importance of selecting the next queen for a colony indicates that it should not occur at random, nepotism is increasingly considered unlikely in eusocial insect societies. Different prenatal maternal supplies of embryos have been found to impact fitness in many other species and therefore could be a possible trigger underlying the likelihood of being raised as a queen. We offered related or unrelated larvae from six colonies originating from eggs of different weights for emergency queen rearing in queenless units with worker bees from these six colonies. We showed that nurses did not significantly prefer related larvae during queen rearing, which confirms the theory that different relatedness-driven kin preferences within a colony cannot be converted into a colony-level decision. However, we found that larvae originating from heavier eggs were significantly preferred for queen breeding. Studies on other species have shown that superior maternal supply is important for later reproductive success. However, we did observe tendencies in the expected direction (e.g., queens that hatched from heavier eggs had both more ovarioles and a shorter preoviposition period). Nevertheless, our data do not allow for a significant conclusion that the selection of larvae from heavy eggs truly offers fitness advantages.

Pesticide Exposure During Development Does Not Affect the Larval Pheromones, Feeding Rates, or Morphology of Adult Honey Bee (Apis mellifera) Queens

Recent work demonstrated that honey bee (Apis mellifera L.) queens reared in pesticide-laden beeswax exhibit significant changes in the composition of the chemicals produced by their mandibular glands including those that comprise queen mandibular pheromone, which is a critical signal used in mating as well as queen tending behavior. For the present study, we hypothesized that pesticide exposure during development would alter other queen-produced chemicals, including brood pheromone in immature queens, thus resulting in differential feeding of queen larvae by nurse workers, ultimately impacting adult queen morphology. We tested these hypotheses by rearing queens in beeswax containing field-relevant concentrations of (1) a combination of tau-fluvalinate and coumaphos, (2) amitraz, or (3) a combination of chlorothalonil and chlorpyrifos. These pesticides are ubiquitous in most commercial beekeeping operations in North America. We observed nurse feeding rates of queen larvae grafted into pesticide-laden beeswax, analyzed the chemical composition of larval queen pheromones and measured morphological markers in adult queens. Neither the nurse feeding rates, nor the chemical profiles of immature queen pheromones, differed significantly between queens reared in pesticide-laden wax compared to queens reared in pesticide-free wax. Moreover, pesticide exposure during development did not cause virgin or mated adult queens to exhibit differences in morphological markers (i.e., body weight, head width, or thorax width). These results were unexpected given our previous research and indicate that future work is needed to fully understand how pesticide exposure during development affects honey bee queen physiology, as well as how various adult queen quality metrics relate to each other.

Parent-of-origin effects, allele-specific expression, genomic imprinting and paternal manipulation in social insects

Haplo-diploidy and the relatedness asymmetries it generates mean that social insects are prime candidates for the evolution of genomic imprinting. In single-mating social insect species, some genes may be selected to evolve genomic mechanisms that enhance reproduction by workers when they are inherited from a female. This situation reverses in multiple mating species, where genes inherited from fathers can be under selection to enhance the reproductive success of daughters. Reciprocal crosses between subspecies of honeybees have shown strong parent-of-origin effects on worker reproductive phenotypes, and this could be evidence of such genomic imprinting affecting genes related to worker reproduction. It is also possible that social insect fathers directly affect gene expression in their daughters, for example, by placing small interfering RNA molecules in semen. Gene expression studies have repeatedly found evidence of parent-specific gene expression in social insects, but it is unclear at this time whether this arises from genomic imprinting, paternal manipulation, an artefact of cyto-nuclear interactions, or all of these. This article is part of the theme issue 'How does epigenetics influence the course of evolution?'

The brood parasite's guide to inclusive fitness theory

Hamilton's theory of inclusive fitness provides a framework for understanding the evolution of social behaviour between kin, including parental and alloparental care. Brood parasitism is a reproductive tactic in which parasites exploit the care of other individuals of the same species (conspecific parasitism) or different species (interspecific parasitism) to rear their brood. Here, drawing from examples in birds and social insects, we identify two insights into brood parasitism that stem from inclusive fitness theory. First, the kin structure within nests, or between neighbouring nests, can create a niche space favouring the evolution of conspecific parasitism. For example, low average relatedness within social insect nests can increase selection for reproductive cheats. Likewise, high average relatedness between adjacent nests of some birds can increase a female's tolerance of parasitism by her neighbour. Second, intrabrood conflict will be high in parasitized broods, from the perspective of both parasite and host young, relative to unparasitized broods. We also discuss offspring recognition by hosts as an example of discrimination in a kin-selected social behaviour. We conclude that the inclusive fitness framework is instructive for understanding aspects of brood parasite and host evolution. In turn, brood parasites present some unique opportunities to test the predictions of inclusive fitness theory. This article is part of the theme issue 'The coevolutionary biology of brood parasitism: from mechanism to pattern'.

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.

Cryptic "royal" subfamilies in honey bee (Apis mellifera) colonies

During emergency queen rearing, worker honey bees (Apis mellifera) select several otherwise worker-destined larvae to instead rear as candidates to replace their dead or failing queen. This choice is crucial as the queen is the sole reproductive in the colony and her quality is essential to its success. Because honey bee queens mate with and store sperm from multiple drones, emergency queen selection presents workers with an opportunity to increase fitness by selecting full- (0.75 relatedness), rather than half- (0.25 relatedness), sisters as new queen candidates. Through patriline analysis of colonies along with large numbers of emergency queens reared by each we affirm the purported “royal” patriline theory that, instead of competing nepotistically, workers exhibit bias towards selecting individuals from particular “royal” subfamilies during emergency queen rearing events, Further, we show that these “royal” patrilines are cryptic in honey bee colonies; occurring in such low frequency in the overall colony population that they are frequently undetected in traditional tests of queen mating number and colony composition. The identification of these cryptic “royal” subfamilies reveals that honey bee queens, already considered “hyperpolyandrous,” are mating with even more males than has been previously recognized. These results alter our understanding of reproductive behavior in honey bees, raising questions about the evolutionary implications of this phenomenon.

Genetics and Evolution of Social Behavior in Insects

The study of insect social behavior has offered tremendous insight into the molecular mechanisms mediating behavioral and phenotypic plasticity. Genomic applications to the study of eusocial insect species, in particular, have led to several hypotheses for the processes underlying the molecular evolution of behavior. Advances in understanding the genetic control of social organization have also been made, suggesting an important role for supergenes in the evolution of divergent behavioral phenotypes. Intensive study of social phenotypes across species has revealed that behavior and caste are controlled by an interaction between genetic and environmentally mediated effects and, further, that gene expression and regulation mediate plastic responses to environmental signals. However, several key methodological flaws that are hindering progress in the study of insect social behavior remain. After reviewing the current state of knowledge, we outline ongoing challenges in experimental design that remain to be overcome in order to advance the field.

THE EVOLUTION OF WORKER STERILITY IN HONEY BEES: AN INVESTIGATION INTO A BEHAVIORAL MUTANT CAUSING FAILURE OF WORKER POLICING

Normally, worker honey bees (Apis mellifera) only lay eggs when their colony is queenless. When a queen is present, worker egg-laying is controlled by mutual "policing" behavior in which any rare worker-laid eggs are eaten by other workers. However, an extremely rare behavioral phenotype arises in which workers develop functional ovaries and lay large numbers of eggs despite the presence of the queen. In this study, microsatellite analysis was used to determine the maternity of drones produced in such a colony under various conditions. One subfamily was found to account for about 90% of drone progeny, with the remainder being laid by other subfamilies or the queen. No evidence of queen policing was found. After a one-month period of extreme worker oviposition in spring, the colony studied reverted to normal behavior and showed no signs of worker oviposition. However, upon removal of the queen, workers commenced oviposition very quickly. Significantly, the subfamily that laid eggs when the queen was present did not contribute to the drone production when the colony was queenless. However, another subfamily contributed a disproportionately large number of drones. The frequency of worker oviposition appears to be determined by opposing selective forces. Individual bees benefit from personal reproduction, whereas other bees and the colony are disadvantaged by it. Thus a behavioral polymorphism can be maintained in the population in which some workers can escape worker policing, with balancing selection at the colony level to detect and eliminate these mutations.

Molecular Evolution of Insect Sociality: An Eco-Evo-Devo Perspective

The evolution of eusociality is a perennial issue in evolutionary biology, and genomic advances have fueled steadily growing interest in the genetic changes underlying social evolution. Along with a recent flurry of research on comparative and evolutionary genomics in different eusocial insect groups (bees, ants, wasps, and termites), several mechanistic explanations have emerged to describe the molecular evolution of eusociality from solitary behavior. These include solitary physiological ground plans, genetic toolkits of deeply conserved genes, evolutionary changes in protein-coding genes, cis regulation, and the structure of gene networks, epigenetics, and novel genes. Despite this proliferation of ideas, there has been little synthesis, even though these ideas are not mutually exclusive and may in fact be complementary. We review available data on molecular evolution of insect sociality and highlight key biotic and abiotic factors influencing social insect genomes. We then suggest both phylogenetic and ecological evolutionary developmental biology (eco-evo-devo) perspectives for a more synthetic view of molecular evolution in insect societies.

Mating frequencies of honey bee queens (Apis mellifera L.) in a population of feral colonies in the Northeastern United States

Across their introduced range in North America, populations of feral honey bee (Apis mellifera L.) colonies have supposedly declined in recent decades as a result of exotic parasites, most notably the ectoparasitic mite Varroa destructor. Nonetheless, recent studies have documented several wild populations of colonies that have persisted. The extreme polyandry of honey bee queens-and the increased intracolony genetic diversity it confers-has been attributed, in part, to improved disease resistance and may be a factor in the survival of these populations of feral colonies. We estimated the mating frequencies of queens in feral colonies in the Arnot Forest in New York State to determine if the level of polyandry of these queens is especially high and so might contribute to their survival success. We genotyped the worker offspring from 10 feral colonies in the Arnot Forest of upstate New York, as well as those from 20 managed colonies closest to this forest. We found no significant differences in mean mating frequency between the feral and managed queens, suggesting that queens in the remote, low-density population of colonies in the Arnot Forest are neither mate-limited nor adapted to mate at an especially high frequency. These findings support the hypothesis that the hyperpolyandry of honey bees has been shaped on an evolutionary timescale rather than on an ecological one.

A parent-of-origin effect on honeybee worker ovary size

Apis mellifera capensis is unique among honeybees in that unmated workers can produce pseudo-clonal female offspring via thelytokous parthenogenesis. Workers use this ability to compete among themselves and with their queen to be the mother of new queens. Males could therefore enhance their reproductive success by imprinting genes that enhance fertility in their daughter workers. This possibility sets the scene for intragenomic conflict between queens and drones over worker reproductive traits. Here, we show a strong parent-of-origin effect for ovary size (number of ovarioles) in reciprocal crosses between two honeybee subspecies, A. m. capensis and Apis mellifera scutellata. In this cross, workers with an A. m. capensis father had 30% more ovarioles than genotypically matched workers with an A. m. scutellata father. Other traits we measured (worker weight at emergence and the presence/absence of a spermatheca) are influenced more by rearing conditions than by parent-of-origin effects. Our study is the first to show a strong epigenetic (or, less likely, cytoplasmic maternal) effect for a reproductive trait in the honeybee and suggests that a search for parent-of-origin effects in other social insects may be fruitful.

Public goods dilemma in asexual ant societies

Cooperation in biological, social, and economic groups is underpinned by public goods that are generated by group members at some personal cost. Theory predicts that public goods will be exploited by cheaters who benefit from the goods by not paying for them, thereby leading to the collapse of cooperation. This situation, described as the "public goods dilemma" in game theory, makes the ubiquity of cooperation a major evolutionary puzzle. Despite this generalization, the demonstration of genetic background and fitness effects of the public goods dilemma has been limited to interactions between viruses and between cells, and thus its relevance at higher levels of organismal complexity is still largely unexplored. Here we provide experimental evidence for the public goods dilemma in a social insect, the ant Pristomyrmex punctatus. In this species, all workers are involved in both asexual reproduction and cooperative tasks. Genetic cheaters infiltrate field colonies, reproducing more than the workers but shunning cooperative tasks. In laboratory experiments, cheaters outcompeted coexisting workers in both survival and reproduction, although a group composed only of cheaters failed to produce offspring. The operations of the public goods dilemma in P. punctatus showed a remarkable convergence with those in microbial societies, not only in fitness consequences but also in behavioral mechanisms. Our study reinforces the evolutionary impact of cheaters on diverse cooperative systems in the laboratory and in the field.

Intragenomic conflict over queen determination favours genomic imprinting in eusocial Hymenoptera

Colonies of eusocial Hymenoptera, such as ants, bees and wasps, have long been recognized as candidates for the study of genomic imprinting on the grounds of evolutionary conflicts that arise from close interactions among colony members and relatedness asymmetry owing to haplodiploidy. Although a general kinship theory of genomic imprinting predicts its occurrence under various circumstances of the colony life cycle, new theoretical approaches are required to account for the specifics of real colonies based on recent advances in molecular-level understanding of ants and honeybees. Using a multivariate quantitative genetic model, we examined the potential impact of genomic imprinting on genes that determine the carrier female's propensity to develop into the queen caste. When queen overproduction owing to the increased propensity comes at a colony-level cost, the conflict between maternally and paternally inherited genes in polyandrous (queen multiple mating) colonies favours genomic imprinting. Moreover, we show that the genomic imprinting can occur even under monandry (queen single mating), once incorporating the costs differentially experienced by new males and new queens. Our model predicts the existence of imprinted 'genetic royal cheats' with patriline-specific expression in polyandrous colonies, and seems consistent with the paternal effect on queen determination in monandrous Argentine ants.

Nutritional asymmetries are related to division of labor in a queenless ant

Eusocial species exhibit pronounced division of labor, most notably between reproductive and non-reproductive castes, but also within non-reproductive castes via morphological specialization and temporal polyethism. For species with distinct worker and queen castes, age-related differences in behavior among workers (e.g. within-nest tasks versus foraging) appear to result from physiological changes such as decreased lipid content. However, we know little about how labor is divided among individuals in species that lack a distinct queen caste. In this study, we investigated how fat storage varied among individuals in a species of ant (Dinoponera australis) that lacks a distinct queen caste and in which all individuals are morphologically similar and capable of reproduction (totipotent at birth). We distinguish between two hypotheses, 1) all individuals are physiologically similar, consistent with the possibility that any non-reproductive may eventually become reproductive, and 2) non-reproductive individuals vary in stored fat, similar to highly eusocial species, where depletion is associated with foraging and non-reproductives have lower lipid stores than reproducing individuals. Our data support the latter hypothesis. Location in the nest, the probability of foraging, and foraging effort, were all associated with decreased fat storage.

Genetic caste polymorphism and the evolution of polyandry in Atta leaf-cutting ants

Multiple mating by females with different males (polyandry) is difficult to explain in many taxa because it carries significant costs to females, yet benefits are often hard to identify. Polyandry is a derived trait in social insects, the evolutionary origins of which remain unclear. One of several leading hypotheses for its evolution is that it improves division of labour by increasing intra-colonial genetic diversity. Division of labour is a key player in the ecological success of social insects, and in many successful species of ants is based on morphologically distinct castes of workers, each with their own task specialisations. Atta leaf-cutting ants exhibit one of the most extreme and complicated forms of morphologically specialised worker castes and have been reported to be polyandrous but with relatively low mating frequencies (~2.5 on average). Here, we show for the first time that there is a significant genetic influence on worker size in Atta colombica leaf-cutting ants. We also provide the first estimate of the mating frequency of Atta cephalotes (four matings) and, by analysing much higher within-colony sample sizes, find that Atta are more polyandrous than previously thought (approximately six to seven matings). The results show that high polyandry and a genetic influence on worker caste are present in both genera of leaf-cutting ants and add weight to the hypothesis that division of labour is a potential driver of the evolution of polyandry in this clade of ants.

Patriline shifting leads to apparent genetic caste determination in harvester ants

The harvester ant, Pogonomyrmex occidentalis, is characterized by high levels of intracolonial genetic diversity resulting from multiple mating by the queen. Within reproductively mature colonies, the relative frequency of different male genotypes (patrilines) is not stable. The difference between samples increases with time, nearing an asymptote after a year. Patriline distributions in gynes and workers display similar patterns of change. A consequence of changing patriline distributions is that workers and gynes appear to have different fathers. However, apparent genetic differences between castes are caused by changing paternity among all females. Temporal variation in the relative frequency of patrilines may be a consequence of processes that reflect sexual conflict, such as sperm clumping. Recent work documenting genotype differences between physical castes (workers and gynes; major and minor workers) in several species of ants has been interpreted as evidence of genetic caste determination. Reanalysis of these studies found little support for this hypothesis. Apparent caste determination may result from temporal variation in sperm use, rather than from fertilization bias among male ejaculates.

Cheating honeybee workers produce royal offspring

The Cape bee (Apis mellifera capensis) is unique among honeybees in that workers can lay eggs that instead of developing into males develop into females via thelytokous parthenogenesis. We show that this ability allows workers to compete directly with the queen over the production of new queens. Genetic analyses using microsatellites revealed that 23 out of 39 new queens produced by seven colonies were offspring of workers and not the resident queen. Of these, eight were laid by resident workers, but the majority were offspring of parasitic workers from other colonies. The parasites were derived from several clonal lineages that entered the colonies and successfully targeted queen cells for parasitism. Hence, these parasitic workers had the potential to become genetically reincarnated as queens. Of the daughter queens laid by the resident queen, three were produced asexually, suggesting that queens can 'choose' to produce daughter queens clonally and thus have the potential for genetic immortality.

Caste determination in a polymorphic social insect: nutritional, social, and genetic factors

We examined how dietary, social, and genetic factors affect individual size and caste in the Florida harvester ant Pogonomyrmex badius, which has three discrete female castes. The diet that a larva consumed, as indicated by delta(13)C, delta(15)N, and C:N, varied with caste. Both N content and estimated trophic position of dietary input was higher for major than for minor workers and was highest for gynes (reproductive females). The size and resources of a colony affected the size of only minor workers, not that of gynes and major workers. Approximately 19% of patrilines showed a bias in which female caste they produced. There were significant genetic effects on female size, and the average sizes of a major worker and a gyne produced by a patriline were correlated, but neither was correlated with minor worker size. Thus, genetic factors influence both caste and size within caste. We conclude that environmental, social, and genetic variation interact to create morphological and physiological variation among females in P. badius. However, the relative importance of each type of factor affecting caste determination is caste specific.

Genetic and genomic analyses of the division of labour in insect societies

Division of labour--individuals specializing in different activities--features prominently in the spectacular success of the social insects. Until recently, genetic and genomic analyses of division of labour were limited to just a few species. However, research on an ever-increasing number of species has provided new insight, from which we highlight two results. First, heritable influences on division of labour are more pervasive than previously imagined. Second, different forms of division of labour, in lineages in which eusociality has arisen independently, have evolved through changes in the regulation of highly conserved molecular pathways associated with several basic life-history traits, including nutrition, metabolism and reproduction.

Genetic royal cheats in leaf-cutting ant societies

Social groups are vulnerable to cheating because the reproductive interests of group members are rarely identical. All cooperative systems are therefore predicted to involve a mix of cooperative and cheating genotypes, with the frequency of the latter being constrained by the suppressive abilities of the former. The most significant potential conflict in social insect colonies is over which individuals become reproductive queens rather than sterile workers. This reproductive division of labor is a defining characteristic of eusocial societies, but individual larvae will maximize their fitness by becoming queens whereas their nestmates will generally maximize fitness by forcing larvae to become workers. However, evolutionary constraints are thought to prevent cheating by removing genetic variation in caste propensity. Here, we show that one-fifth of leaf-cutting ant patrilines cheat their nestmates by biasing their larval development toward becoming queens rather than workers. Two distinct mechanisms appear to be involved, one most probably involving a general tendency to become a larger adult and the other relating specifically to the queen-worker developmental switch. Just as evolutionary theory predicts, these "royal" genotypes are rare both in the population and within individual colonies. The rarity of royal cheats is best explained as an evolutionary strategy to avoid suppression by cooperative genotypes, the efficiency of which is frequency-dependent. The results demonstrate that cheating can be widespread in even the most cooperative of societies and illustrate that identical principles govern social evolution in highly diverse systems.

Nepotism absent in insect societies - or is it?

Inclusive fitness theory, put forward by English biologist William Hamilton in 1964, is considered by many as the most important addition to the theory of natural selection since Darwin. One prediction of the theory is that animals should often show a tendency to nepotistically favour close relatives. Goodisman et al. (2007) test this theory for the first time using molecular methods in a vespine wasp, the eastern yellowjacket, Vespula maculifrons. Somewhat surprisingly, nepotism was found to be absent. This begs the question why nepotism is predicted by theory, yet in a growing list of species is shown to be absent. Is inclusive fitness theory in trouble? As we show, it is not: costs and constraints explain the general absence of queen rearing nepotism, and nepotism in insect societies in fact is well supported in the context of male rearing and manipulation of colony sex ratios.

Lack of conflict during queen production in the social wasp Vespula maculifrons

Social insects display extreme cooperative and helping behaviours. However, social insect colonies are also arenas of intense competitive interactions. One particularly important matter over which colony members may compete centres on the development of sexual offspring. Specifically, colony members may engage in selfish behaviours leading to reproductive competition, whereby individuals either strive to develop as sexuals or assist kin so that close relatives emerge as new reproductives. We investigated whether reproductive competition occurred in the polyandrous social wasp Vespula maculifrons. We genotyped V. maculifrons workers and new queens at eight polymorphic microsatellite loci to determine if larvae of particular genotypes were reared as gynes more frequently than expected by chance. However, we found no significant evidence of reproductive competition in this species. The proportional contributions of males to workers and new queens did not vary within colonies. Moreover, male reproductive skew did not differ between workers and new queens. Finally, novel statistical techniques uncovered no evidence of patriline reversal, the phenomenon whereby males that contribute little to worker production contribute substantially to new queen production. Consequently, we conclude that individual level selection operating to increase the frequency of selfish behaviours that would lead to reproductive competition has been nullified by colony-level selection acting to maintain colony efficiency and cooperation.

Rare royal families in honeybees, Apis mellifera

The queen is the dominant female in the honeybee colony, Apis mellifera, and controls reproduction. Queen larvae are selected by the workers and are fed a special diet (royal jelly), which determines caste. Because queens mate with many males a large number of subfamilies coexist in the colony. As a consequence, there is a considerable potential for conflict among the subfamilies over queen rearing. Here we show that honeybee queens are not reared at random but are preferentially reared from rare "royal" subfamilies, which have extremely low frequencies in the colony's worker force but a high frequency in the queens reared.

Extreme genetic differences between queens and workers in hybridizing Pogonomyrmex harvester ants

The process of reproductive caste determination in eusocial insect colonies is generally understood to be mediated by environmental, rather than genetic factors. We present data demonstrating unexpected genetic differences between reproductive castes in a variant of the rough harvester ant, Pogonomyrmex rugosus var. fuscatus. Across multiple loci, queens were consistently more homozygous than expected, while workers were more heterozygous. Adult colony queens were divided into two highly divergent genetic groups, indicating the presence of two cryptic species, rather than a single population. The observed genetic differences between castes reflect differential representation of heterospecific and conspecific patrilines in these offspring groups. All workers were hybrids; by contrast, winged queens were nearly all pure-species. The complete lack of pure-species workers indicates a loss of worker potential in pure-species female offspring. Hybrids appear to be bipotential, but do not normally develop into reproductives because they are displaced by pure-species females in the reproductive pool. Genetic differences between reproductive castes are expected to be rare in non-hybridizing populations, but within hybrid zones they may be evolutionarily stable and thus much more likely to occur.

Possible causes of reproductive dominance during emergency queen rearing by honeybees

When queenless honeybees, Apis mellifera, rear new queens, the relative frequencies of subfamilies found in the queen and worker brood are often very different, suggesting that certain subfamilies are reproductively dominant. At least two mechanisms could account for the observed differences in queen and worker broods. First, kin selection theory predicts that if honeybee workers are able to distinguish levels of relatedness, they should act nepotistically by favouring super-sisters over less-related half-sisters during emergency queen rearing. Alternatively, selection might result in royalty alleles that make their possessors more favoured for rearing as queens. Documented genetically based tendencies to rear queen or worker brood might interact with either of these mechanisms. To determine which of these effects might best explain reproductive dominance, we removed brood from the queenright section of one colony and offered it to the queenless section of the same colony and to three unrelated queenless colonies. We used two microsatellite loci to determine the paternity of queen and worker brood reared by these colonies. Variance in the proportions of subfamilies in queen and worker brood was greatest when the rearing bees were related to the brood. The results suggest that nepotistic interactions are more important than royalty alleles or other factors in causing reproductive dominance, but that there are complex interactions between the genotype of the nursing workers, and the genotypes of the larvae favoured for rearing as queens. Copyright 1999 The Association for the Study of Animal Behaviour.