Social Hymenopterans Research Articles

Queen–worker conflict over male production and the sex ratio in a facultatively polyandrous bumblebee,Bombus hypnorum: the consequences of nest usurpation

AbstractEvolutionary conflicts among social hymenopteran nestmates are theoretically likely to arise over the production of males and the sex ratio. Analysis of these conflicts has become an important focus of research into the role of kin selection in shaping social traits of hymenopteran colonies. We employ microsatellite analysis of nestmates of one social hymenopteran, the primitively eusocial and monogynous bumblebeeBombus hypnorum, to evaluate these conflicts. In our 14 study colonies,B. hypnorumqueens mated between one and six times (arithmetic mean 2.5). One male generally predominated, fathering most of the offspring, thus the effective number of matings was substantially lower (1–3.13; harmonic mean 1.26). In addition, microsatellite analysis allowed the detection of alien workers, those who could not have been the offspring of the queen, in approximately half the colonies. Alien workers within the same colony were probably sisters. Polyandry and alien workers resulted in high variation among colonies in their sociogenetic organization. Genetic data were consistent with the view that all males (n = 233 examined) were produced by a colony’s queen. Male parentage was therefore independent of the sociogenetic organization of the colony, suggesting that the queen, and not the workers, was in control of the laying of male‐destined eggs. The population‐wide sex ratio (fresh weight investment ratio) was weakly female biased. No evidence for colony‐level adaptive sex ratio biasing could be detected.

No evidence for Wolbachia-induced parthenogenesis in the social Hymenoptera

Abstract In some parasitoid wasps, infection by the micro-organism Wolbachia leads to asexual reproduction. Within the Hymenoptera, the limits of distribution of parthenogenesis inducing Wolbachia have not yet been established. To address this issue, we screened all known thelytokous social hymenopteran species using a PCR assay. None was infected, and therefore we conclude that worker thelytoky evolves independently of Wolbachia in ants and bees. This supports the previously proposed hypothesis that a sex determining system based on heterozygosity may form a proximate limitation to Wolbachia-induced parthenogenesis.

Navigation in Context: Grand Theories and Basic Mechanisms

Among the movements of animals across the surface of our planet, the wide-ranging migratory journeys of birds and the smaller-scale foraging excursions of social (hymenopteran) insects provide some of the most intriguing examples of biological systems of navigation. Many sensory cues have been found to be involved in accomplishing these tasks, but how this sensory information is integrated into the animal's overall system of navigation has remained elusive. Several over-arching concepts such as sunor star-based systems of astronavigation, E-vector-based spherical geometry, map-and-compass and bi-coordinate position-fixing schemes have been developed to account for the animals' performances. Although these metaphors have some heuristic value, they are potentially distracting and might obscure some of the most important computational strategies used by the brain. Moreover, these top-down approaches are especially inappropriate in trying to understand the evolutionary design of an animal's navigational system. Instead, we must go back to basics, use modern recording technology to unravel the detailed spatial and temporal structures of migratory routes and foraging trajectories, study the animal's sensory and computational abilities by combining behavioural and neurophysiological approaches, then work bottom-up, as evolution did, by trying to integrate the individual navigational methods. Rather than being part of a general-purpose navigational toolkit, the various guiding mechanisms have most certainly arisen from an opportunistic grafting of particular special-purpose modules on to pre-existing sensory-motor control systems.

Kin selection, kin avoidance and correlated strategies

Kin selection of correlated strategies is examined for both weak and strong altruism under simple haploid inheritance. While kin assortment enhances the range of evolutionary stability for (strongly altruistic) correlated strategies (defined herein), kin avoidance is possible under a weakly altruistic correlated strategy. When social competition induces role assignments of variable fitness, group mates may ‘prefer’ association with non-relatives. Even when group life is mandatory, an individual may accept the risk of abandonment (and reproductive death) rather then associate with kin: a competitive superior may behave altruistically by permitting competitively inferior kin to emigrate. Thus, kin selection and social competition are not necessarily mutually supportive processes within groups. I conclude by interpreting dominance as a strongly altruistic correlated strategy in two social hymenopteran contexts.

Diploid males and colony-level selection inFormicaants

It is suggested that the evolution of polyandry by social hymenopteran queens is caused by colony-level selection, either because polyandry affects the distribution of non-functional diploid males in colonies (the load hypothesis) or because it increases the genetic diversity of the worker force (the diversity hypothesis). Diploid males that arise from fertilized eggs thai are homozygous at the sex-determining locus (or loci) are inviable or infertile. Models of the load hypothesis analysed in this study suggest that slow growth and high mortality of colonies with diploid males favour single mating by queens. The longer the period ol colony growth (the period with selective differences) and the heavier the mortality, the stronger is the selection for monandry. The load hypothesis also predicts an association between monogyny and monandry. In contrast, the diversity hypothesis predicts an association between monogyny and polyandry, as multiple mating offers a way by which a monogynous colony could increase...

Coloniality and Inducible Polymorphism

Although zooid polymorphism is a common phenomenon among colonial marine invertebrates, the proximate mechanisms governing transformations and coordination among the different units of a colony are largely unknown. Zooids in these often genetically homogeneous colonies may be coordinated by intrinsic, centralized, signaling mechanisms at the colony level (e.g., by giant axons in siphonophores) or by more decentralized mechanisms at the zooid level, such as environmental cues activating same-aged zooids. This second class of mechanism is responsible for coordinated defense in colonies of the marine bryozoan Membranipora membranacea. Membranipora membranacea colonies are initially populated by identical looking zooids, some of which can rapidly differentiate into an organized pattern of defensive zooids on the periphery of the colony in response to chemical cues from predators. To test the hypothesis that the age of the zooids rather than their position within the colony determines which zooids develop spines, I manipulated the normal center-to-edge age gradient. With these manipulated colonies and localized chemical cues, I established that spination is evoked independently by local action of inducers on each zooid of the appropriate age group, irrespective of its position within the colony. Thus, spines were produced in the centers of colonies as well as on the edge when the central zooids were the same age as the edge zooids. Similar age-dependent, "self-organizing" mechanisms regulate the distribution of specialized tasks within colonies of social hymenopterans. For example, honey bees form colonies of related units, and, in these, the principal basis for specialization of labor among the units is ontogeny; similarly aged individuals respond similarly to the same environmental cues. Inducible defensive polymorphisms such as those shown by bryozoans, hydrozoans, and anthozoans may be one pathway by which fixed polymorphisms in zooid form and function arise. If the transition from facultative to fixed is common, then processes such as genetic assimilation may be more important than has been thought. It seems possible that the relative ubiquity of both facultative and constitutive polymorphisms in colonial invertebrates may be a result of the compartmentalized, iterated nature of modular organisms, which allows the production of variant zooids through slight changes in developmental timing.

The evolution of queen‐rearing nepotism in social Hymenoptera: Effects of discrimination costs in swarming species

AbstractPolyandry by social hymenopteran queens leads to a potential worker reproductive strategy of rearing full‐sister queens in preference to half‐sister queens. If there is no cost to discrimination, discriminatory queen rearing will be the ESS. However, if discrimination has a cost this conclusion is weakened. This study examines the effect of a “worker efficiency cost” (i.e. discriminatory, nepotistic workers are less effective than non‐nepotistic workers in working to increase total colony reproductive output) on the invasion of rare “nepotist” and “non‐nepotist” alleles in a population in which the other allele is near fixation. Efficiency costs are considered of special relevance to species with swarm founded colonies (e.g., army ants, honey bees).Two analyses are presented: 1) A general model exploring the effects of efficiency costs, the queen‐rearing‐biasing ability of nepotists, and queen mating frequency on invasion of nepotists and non‐nepotists; 2) a discriminatory removal model, in which the actual amount of biasing is dependent on recognition abilities and the discriminatory intensity of nepotists, queen mating frequency, and whether or not removed immature queens are replaced.The results of the removal model indicate that when queen mating frequency is close to one (i.e., because of double mating with unequal sperm use or mixed double and single mating) non‐nepotist is likely to be the ESS. At a higher mating frequency nepotist will become the ESS. At even higher mating frequencies, non‐nepotist may reinvade. This latter conclusion is robust across all models. In particular, non‐nepotists may reinvade at the high mating frequencies found in the honey bee (10–20) if their work efficiency is only a few percent higher than nepotists (the exact amount depends on recognition errors and the intensity of discrimination), resulting in a genetic polymorphism of both nepotists and non‐nepotists. Results of a recent study of patriline discrimination in honey bees (Page et al., 1989) are consistent with such a polymorphism.

Reproductive competition involving oophagy in the socially nesting beeXylocopa sulcatipes

Several types of nesting associations, termed quasisocial (females more or less identical, no division of labor), semisocial (females of the same generation, with division of labor) and matrifilial (overlap of generations, with division of labor) are described in many primitively social hymenopteran species. In the latter two types, at least one potentially reproductive female forgoes her own reproduction in favor of that of a dominant female. Quasisociality is generally of a temporary nature, because dominance relations develop. It turns then into semisociality [11. The large carpenter bee Xylocopa sulcatipes in southern Israel has been described as forming both solitary and social nests [2, 3]. This paper shows that in certain associative nests founded by two females an extreme example of reproductive competition, oophagy, occurs. A X. sulcatipes egg measures approximately 11 mm in length and 2.2 mm in diameter; for an adult female of 22 mm body length, egg cannibalism therefore represents a significant loss of investment. X. su/catipes typically nests in twigs o f dead branches where the female excavates a straight tunnel. Females also use pre-existing cavities in stems of Arundo, Ferula and other plants existing naturally in the habitat. The female builds a linear array of cells [4]. Each is provisioned with pollen and nectar, and an egg is deposited upon the loaf-like food mass. Finally the cell is sealed with masticated wood. As part of a study on Xylocopa at Hazeva Field Centre, Arava Valley,

Evolution of sex ratios in social hymenoptera: kin selection, local mate competition, polyandry and kin recognition

A model is constructed to study the effects of local mate competition and multiple mating on the optimum allocation of resources between the male and female reproductive brood in social hymenopteran colonies from the ‘points of view’ of the queen (parental manipulation theory) as well as the workers (kin selection theory). Competition between pairs of alleles specifying different sex investment ratios is investigated in a game theoretic frame work. All other things being equal, local mate competition shifts the sex allocation ratio in favour of females both under queen and worker control. While multiple mating has no effect on the queen’s optimum investment ratio, it leads to a relatively male biased investment ratio under worker control. Under queen control a true Evolutionarily Stable Strategy(ess) does not exist but the ‘best’ strategy is merely immune from extinction. A trueess exists under worker control in colonies with singly mated queens but there is an asymmetry between the dominant and recessive alleles so that for some values of sex ratio a recessive allele goes to fixation but a dominant allele with the same properties fails to do so. Under multiple mating, again, a trueess does not exist but a frequency dependent region emerges. The best strategy here is one that is guaranteed fixation against any competing allele with a lower relative frequency. Our results emphasize the need to determine levels of local mate competition and multiple mating before drawing any conclusions regarding the outcome of queen-worker conflict in social hymenoptera. Multiple mating followed by sperm mixing, both of which are known to occur in social hymenoptera, lower average genetic relatedness between workers and their reproductive sisters. This not only shifts the optimum sex ratio from the workers’ ‘point of view’ in favour of males but also poses problems for the kin selection theory. We show that kin recognition resulting in the ability to invest in full but not in half sisters reverts the sex ratio back to that in the case of single mating and thus completely overcomes the hurdles for the operation of kin selection.

Furcula, a major component of the hymenopterous venom apparatus

This report represents observations from a 15 yr investigation of the furcula in hymenopterous insects. Significance of the furcula as one of the major sclerites in sting manipulation is emphasized. Support for the origin of the furcula from the base of gonapophyses from sternite 9 is based chiefly on the muscular associations present in the Symphyta, ectoparasitic and endoparasitic Terebrantia and aculeates. Loss of the furcula has occurred along with a modification of the defensive system in a number of aculeates. However, a structure representing the old furcula persists in social hymenopterans that Have phyletically lost their ability to sting. Even with significant reduction of venom apparatus sclerites, the furcula is represented in remnant form and retains its association with at least part of the musculature present in the most primitive forms of Hymenoptera.