Were Workers of Eusocial Hymenoptera Initially Altruistic or Oppressed?

Studies of a primitively eusocial halictid bee, Lasioglossum zephyrum, strongly suggest that a major factor in originating a worker caste is selection at the individual level for queens that control associated adult females. Even in this scarcely social form, the queen inhibits other adult females from becoming queens, perhaps by her high level of activity and frequent nudging in the nest. Queens are behaviorally less varied than workers and show specialization, particularly in frequency of nudging (which is concentrated on the worker with largest ovaries) and of backing. Backing draws workers, especially those with slender ovaries, down to lower parts of the burrows where the stimuli for cell construction and provisioning probably operate. Eating of worker-laid eggs by queens was also noted. In spite of the suggestion that queens have evolved to control their workers rather than that workers have evolved to help their queens, both may well have occurred, for these processes are not mutually exclusive; moreover, social attributes mutually beneficial to both castes no doubt have arisen.     

Interactions in Colonies of Primitively Social Bees: Artificial Colonies of Lasioglossum zephyrum

Lasioglossum zephyrum usually lives in small colonies but is facultatively solitary. Lone bees and colonies produced from female pupae of the same generation were established in artificial indoor nests. Both the length of the prereproductive period and the number of cells produced per bee per day decreased with increasing colony size. In most colonies, ovarially and behaviorally recognized castes arose, a queen and workers, but with all intergradations. The mean size of queens was larger than that of workers. Nearly all queens mated although few workers did so in rooms with a few males, but mating had no effect on subsequent behavior or ovarian development. In groups of diverse age there was a tendency for the oldest bees to be queens; queens also were larger on the average than workers. In groups of equal age, the largest bee was most often queen. As would be expected for a scarcely social species, mechanisms of social integration (resulting in division of labor and differentiation of castes) mostly appear to involve behavioral features of the solitary ancestors and accidental results of joint occupancy of nests. There is no evidence of direct food or pheromone transfer among adult bees.     

Demographic predisposition to the evolution of eusociality: a hierarchy of models.

I present a hierarchy of models that illustrate, within the framework of inclusive fitness theory, how demographic factors can predispose a species to the evolution of eusociality. Delayed reproductive maturation lowers the inclusive fitness of a solitary foundress relative to that of a worker. Variation in age at reproductive maturity makes the worker strategy more profitable to some individuals than to others and thus predicts the coexistence of single-foundress and multiple-foundress nesting associations. Delayed reproductive maturation and variation in age at reproductive maturity also select for mixed reproductive strategies so that some individuals whose reproductive maturation is expected to be delayed can first act as workers and later switch over to the role of a queen or foundress. Assured fitness returns shows how identical mortality rates can have different consequences for workers and solitary nest foundresses because a solitary foundress will have to necessarily survive for the entire duration of development of her brood, whereas a worker can hope to get proportional fitness returns for short periods of work. In concert with assured fitness returns, delayed reproductive maturation and variation in age at reproductive maturity become more powerful in selecting for worker behavior, and mixed reproductive strategies become available to a wider range of individuals. These phenomena provide a consistently more powerful selective advantage for the worker strategy than do genetic asymmetries created by haplodiploidy.     

A novel social polymorphism in a primitively eusocial bee

Halictine sweat bees (Hymenoptera, Halictidae) are model organisms for the evolution of altruism, reproductive castes, and eusocial colony organization. Halictine social behavior is not only extremely variable, but also ecologically and evolutionarily labile. Among social species, colony social organization ranges from communal societies of egalitarian females to eusocial and semisocial ones with reproductive queens and more or less sterile workers. A striking aspect of halictine social variation is the mutual exclusivity of communal and eusocial types of colony social organization within the same species, these two types of social behavior being characteristic of different genera and subgenera. We report a recently discovered exception to this rule in a population of Halictus sexcinctus (Fabricius) at Daimonia-Pyla in southern Greece, that contained both communal and eusocial colonies. Moreover, communal and eusocial females exhibit morphological differences that imply a preimaginal developmental switch, which could also underlie the two types of social behavior. That the communal and eusocial forms are not merely cryptic sister species with different social behavior is indicated by the comparison of mitochondrial DNA sequences of two sections of cytochrome oxidase I, which indicate that Greek specimens of both social types are more similar than they are to conspecifics from elsewhere in Europe. The phylogenetic position of Halictus sexcinctus suggests that this unusual communal/eusocial polymorphism may represent an unstable intermediate step in an evolutionary reversal from eusocial to solitary behavior.     

Evolution of sociality in a primitively eusocial lineage of bees

Eusociality is a major evolutionary innovation involving alterations in life history, morphology, and behavior. Advanced eusocial insects, such as ants, termites, and corbiculate bees, cannot provide insights into the earliest stages of eusocial evolution because eusociality in these taxa evolved long ago (in the Cretaceous) and close solitary relatives are no longer extant. In contrast, primitively eusocial insects, such as halictid bees, provide insights into the early stages of eusocial evolution because eusociality has arisen recently and repeatedly. By mapping social behavior onto well-corroborated phylogenies, I show that eusociality has arisen only three times within halictid bees (contrary to earlier estimates of six or more origins). Reversals from eusocial to solitary behavior have occurred as many as 12 times, indicating that social reversals are common in the earliest stages of eusocial evolution. Important attributes of social complexity (e.g., colony size, queen/worker dimorphism) show no obvious association with phylogeny, and some reversals to solitary nesting are related to host-plant switches (from polylecty to oligolecty). These results provide a glimpse of social evolution in its earliest stages and provide insights into the early evolution of advanced eusocial organisms.     

Comparative analysis of constraints and caste differences in brain investment among social paper wasps

We compared species mean data on the size of functionally distinct brain regions to test the relative rates at which investment in higher-order cognitive processing (mushroom body calyces) versus peripheral sensory processing (optic and antennal lobes) increased with increasing brain size. Subjects were eusocial paper wasps from queen and worker castes of 10 species from different genera. Relative investment in central processing tissue increased with brain size at a higher rate than peripheral structure investment, demonstrating that tissue devoted to higher-order cognitive processing is more constrained by brain size. This pattern held for raw data and for phylogenetically independent contrasts. These findings suggest that there is a minimum necessary investment in peripheral sensory processing brain tissue, with little to gain from additional investment. In contrast, increased brain size provides opportunities to invest in additional higher-order cognitive processing tissue. Reproductive castes differed within species in brain tissue investment, with higher central-to-peripheral brain tissue ratios in queens than in workers. Coupled with previous findings that paper wasp queen, but not worker, brain architecture corresponds to ecological and social variation, queen brain evolution appears to be most strongly shaped by cognitive demands, such as social interactions. These evolutionary patterns of neural investment echo findings in other animal lineages and have important implications, given that a greater investment in higher-order processing has been shown to increase the prevalence of complex and flexible behaviors across the animal kingdom.     

Morphological castes in a vertebrate

Morphological specialization for a specific role has, until now, been assumed to be restricted to social invertebrates. Herein we show that complete physical dimorphism has evolved between reproductives and helpers in the eusocial naked mole-rat. Dimorphism is a consequence of the lumbar vertebrae lengthening after the onset of reproduction in females. This is the only known example of morphological castes in a vertebrate and is distinct from continuous size variation between breeders and helpers in other species of cooperatively breeding vertebrates. The evolution of castes in a mammal and insects represents a striking example of convergent evolution for enhanced fecundity in societies characterized by high reproductive skew. Similarities in the selective environment between naked mole-rats and eusocial insect species highlight the selective conditions under which queen/worker castes are predicted to evolve in animal societies.     

Reproduction, social behavior, and aging trajectories in honeybee workers

While a negative correlation between reproduction and life span is commonly observed, specialized reproductive individuals outlive their non-reproductive nestmates in all eusocial species, including the honeybee, Apis mellifera (L). The consequences of reproduction for individual life expectancy can be studied directly by comparing reproductive and non-reproductive workers. We quantified the life span consequences of reproduction in honeybee workers by removal of the queen to trigger worker reproduction. Furthermore, we observed the social behavior of large cohorts of workers under experimental and control conditions to test for associations with individual life expectancy. Worker life expectancy was moderately increased by queen removal. Queenless colonies contained a few long-lived workers, and oviposition behavior was associated with a strong reduction in mortality risk, indicating that a reproductive role confers a significant survival advantage. This finding is further substantiated by an association between brood care behavior and worker longevity that depends on the social environment. In contrast, other in-hive activities, such as fanning, trophallaxis, and allogrooming did not consistently affect worker life expectancy. The influence of foraging varied among replicates. An earlier age of transitioning from in-hive tasks to outside foraging was always associated with shorter life spans, in accordance with previous studies. In sum, our studies quantify how individual mortality is affected by particular social roles and colony environments and demonstrate interactions between the two. The exceptional, positive association between reproduction and longevity in honeybees extends to within-caste plasticity, which may be exploited for mechanistic studies.     

Individual recognition and learning of queen odors by worker honeybees

A honeybee queen is usually attacked if she is placed among the workers of a colony other than her own. This rejection occurs even if environmental sources of odor, such as food, water, and genetic origin of the workers, are kept constant in laboratory conditions. The genetic similarity of queens determines how similar their recognition characteristics are; inbred sister queens were accepted in 35% of exchanges, outbred sister queens in 12%, and nonsister queens in 0%. Carbon dioxide narcosis results in worker honeybees accepting nonnestmate queens. A learning curve is presented, showing the time after narcosis required by workers to learn to recognize a new queen. In contrast, worker transfers result in only a small percentage of the workers being rejected. The reason for the difference between queens and workers may be because of worker and queen recognition cues having different sources.     

Caste and metamorphosis: hemolymph titers of juvenile hormone and ecdysteroids in last instar honeybee larvae

Juvenile hormone (JH) and ecdysteroid titers are critical factors for caste development and metamorphosis in the last larval instar of the honeybee, Apis mellifera. Two highly sensitive radioimmunoassays were used for the determination of these hormones in the hemolymph. For juvenile hormone, which is of prime importance for the control of caste development in honeybees, our data show a caste-specific peak in queen larvae of the early fifth instar. A second peak appears in prepupae of both castes which probably is responsible for the regulation of the pupal moult. A single peak of ecdysteroids was observed in prepupae of both castes. In queens, however, the titer increases distinctly earlier than in worker larvae. The ecdysteroid composition of this prepupal peak was determined by high-performance liquid chromatography separation followed by radioimmunoassay. Makisterone A proved to be the main ecdysteroid compound, but 20-hydroxyecdysone was also found in significant amounts.     

Vitellogenin, juvenile hormone, insulin signaling, and queen honey bee longevity

In most animals, longevity is achieved at the expense of fertility, but queen honey bees do not show this tradeoff. Queens are both long-lived and fertile, whereas workers, derived from the same genome, are both relatively short-lived and normally sterile. It has been suggested, on the basis of results from workers, that vitellogenin (Vg), best known as a yolk protein synthesized in the abdominal fat body, acts as an antioxidant to promote longevity in queen bees. We explored this hypothesis, as well as related roles of insulin-IGF-1 signaling and juvenile hormone. Vg was expressed in thorax and head fat body cells in an age-dependent manner, with old queens showing much higher expression than workers. In contrast, Vg expression in worker head was much lower. Queens also were more resistant to oxidative stress than workers. These results support the hypothesis that caste-specific differences in Vg expression are involved in queen longevity. Consistent with predictions from Drosophila, old queens had lower head expression of insulin-like peptide and its putative receptors than did old workers. Juvenile hormone affected the expression of Vg and insulin-IGF-1 signaling genes in opposite directions. These results suggest that conserved and species-specific mechanisms interact to regulate queen bee longevity without sacrificing fecundity.     

Reproductive queue without overt conflict in the primitively eusocial wasp Ropalidia marginata

Colonies of the primitively eusocial wasp Ropalidia marginata consist of a single egg layer (queen) and a number of non-egg-laying workers. Although the queen is a docile individual, not at the top of the behavioral dominance hierarchy of the colony, she maintains complete reproductive monopoly. If the queen is lost or removed, one and only one of the workers [potential queen (PQ)] becomes hyperaggressive and will become the next queen of the colony. The PQ is almost never challenged because she first becomes hyperaggressive and then gradually loses her aggression, develops her ovaries, and starts laying eggs. Although we are unable to identify the PQ when the queen is present, she appears to be a "cryptic heir designate." Here, we show that there is not just one heir designate but a long reproductive queue and that PQs take over the role of egg-laying, successively, without overt conflict, as the queen or previous PQs are removed. The dominance rank of an individual is not a significant predictor of its position in the succession hierarchy. The age of an individual is a significant predictor, but it is not a perfect predictor because PQs often bypass older individuals to become successors. We suggest that such a predesignated reproductive queue that is implemented without overt conflict is adaptive in the tropics, where conspecific usurpers from outside the colony, which can take advantage of the anarchy prevailing in a queenless colony and invade it, are likely to be present throughout the year.     

Genetic control of social organization in an ant

A central issue in evolutionary biology is the extent to which complex social organization is under genetic control. We have found that a single genomic element marked by the protein-encoding gene Gp-9 is responsible for the existence of two distinct forms of social organization in the fire ant Solenopsis invicta. This genetic factor influences the reproductive phenotypes and behavioral strategies of queens and determines whether workers tolerate a single fertile queen or multiple queens per colony. Furthermore, this factor affects worker tolerance of queens with alternate genotypes, thus explaining the dramatic differences in Gp-9 allele frequencies observed between the two social forms in the wild. These findings reveal how a single genetic factor can have major effects on complex social behavior and influence the nature of social organization.     

Evolution and mechanisms of long life and high fertility in queen honey bees

Honey bees (Apis mellifera) are eusocial insects that exhibit striking caste-specific differences in longevity. Queen honey bees live on average 1–2 years whereas workers live on average 15–38 days in the summer and 150–200 days in the winter. Previous studies of senescence in the honey bee have focused on establishing the importance of extrinsic mortality factors (predation, weather) and behavior (nursing and foraging) in worker bee longevity. However, few studies have tried to elucidate the mechanisms that allow queen honey bees to achieve their long lifespan without sacrificing fecundity. Here, we review both types of studies and emphasize the importance of understanding both proximate and ultimate causes of the unusual life history of honey bee queens.     

Age, caste, and behavior determine the replicative activity of intestinal stem cells in honeybees (Apis mellifera L.)

Honeybees (Apis mellifera L.) display a pronounced natural aging plasticity. The differences in aging rates between the alternative phenotypes and behavioral classes could reflect differences in protection against damage or in the ability to repair vulnerable tissues. As in other animals, including humans, the gut is continually exposed to environmental insults and harbors a large population of replicating stem cells that maintain the intestinal epithelium. Through studies of the major internal organs using incorporation and immunodetection of the mitotic marker bromo-deoxyuridine, the intestine was determined to be the main site of tissue renewal in adult honeybees. Proliferative activity of the intestinal stem cells was compared among queens, workers, and males of different ages. Simultaneous attempts to assess intestinal cell loss via apoptosis yielded inconclusive results. The relationship between intestinal cell proliferation and worker life-history was evaluated in greater depth by studying diutinus winter workers, reproductive workers, and by decoupling worker behavioral status from chronological age in a single-cohort colony. Intestinal cell proliferation was abundant in all groups and showed an age-related decline in workers, queens, and males. At young ages, workers exhibited relatively more intestinal cell proliferation than did queens and queens more than drones, but the caste and sex differences decreased with age. Cell proliferation did not decrease beyond 6 weeks of age in older queens and in diutinus workers. Ovary activation did not correlate with the amount of intestinal stem cell proliferation in workers, although the queenless hive condition was associated with lower overall counts. In the single-cohort colony, nurse bees exhibited more cell proliferation than foragers, regardless of age. The overall results do not support our hypothesis that longer-lived phenotypes exhibit increased somatic repair in the form of higher replicative activity of intestinal stem cells. Instead, the observed proliferation patterns reflect differential demands for digestive activity in the different groups, which result in different requirements for replacement of lost intestinal cells. The maintenance of proliferative capacity for over 1 year suggests that queen intestinal stem cells have a relatively high replicative potential, but further studies are needed to relate honeybee lifespan differences to cellular aging.     

Reproductive constraint is a developmental mechanism that maintains social harmony in advanced ant societies

A hallmark of eusociality in ants is the reproductive division of labor between queens and workers. Yet, nothing is known about the molecular mechanisms underlying reproduction in this group. We therefore compared the developmental genetic capacity of queens and workers to reproduce in several eusocially advanced species from the two largest subfamilies of ants, the Myrmicinae and Formicinae. In flies, the asymmetric localization of maternally encoded determinants (mRNAs and proteins) during oogenesis establishes oocyte polarity and subsequently ensures proper embryonic development. Vasa and nanos, two key maternal determinants, are properly localized in the posterior of queen oocytes, but their localization is impaired in those of the workers. This mislocalization leads to severe embryonic defects in worker progeny, and therefore, represents a constraint on worker reproduction that we call ‘reproductive constraint.’ We show that reproductive constraint is phylogenetically widespread, and is at high levels in most species tested. Reproductive constraint can simultaneously reduce or eliminate the workers'' ability to produce viable eggs for reproduction, while preserving their ability to produce trophic eggs for nutrition, and thus, may have been the basis for the evolutionary retention of worker ovaries in the majority of ants. We propose that high levels of reproductive constraint has most likely evolved as a consequence of selection at the colony level to reduce or eliminate any potential conflict over worker reproduction, therefore maintaining harmony and colony efficiency in advanced ant societies.     

Chronic Bee Paralysis Virus in Honeybee Queens: Evaluating Susceptibility and Infection Routes

Chronic bee paralysis virus (CBPV) is known as a disease of worker honey bees. To investigate pathogenesis of the CBPV on the queen, the sole reproductive individual in a colony, we conducted experiments regarding the susceptibility of queens to CBPV. Results from susceptibility experiment showed a similar disease progress in the queens compared to worker bees after infection. Infected queens exhibit symptoms by Day 6 post infection and virus levels reach 10¹¹ copies per head. In a transmission experiment we showed that social interactions may affect the disease progression. Queens with forced contact to symptomatic worker bees acquired an overt infection with up to 10¹¹ virus copies per head in six days. In contrast, queens in contact with symptomatic worker bees, but with a chance to receive food from healthy bees outside the cage appeared healthy. The virus loads did not exceed 10⁷ in the majority of these queens after nine days. Symptomatic worker bees may transmit sufficient active CBPV particles to the queen through trophallaxis, to cause an overt infection.     

INAUGURAL ARTICLE by a Recently Elected Academy Member:Interrogating an insect society

Insect societies such as those of ants, bees, and wasps consist of 1 or a small number of fertile queens and a large number of sterile or nearly sterile workers. While the queens engage in laying eggs, workers perform all other tasks such as nest building, acquisition and processing of food, and brood care. How do such societies function in a coordinated and efficient manner? What are the rules that individuals follow? How are these rules made and enforced? These questions are of obvious interest to us as fellow social animals but how do we interrogate an insect society and seek answers to these questions? In this article I will describe my research that was designed to seek answers from an insect society to a series of questions of obvious interest to us. I have chosen the Indian paper wasp Ropalidia marginata for this purpose, a species that is abundantly distributed in peninsular India and serves as an excellent model system. An important feature of this species is that queens and workers are morphologically identical and physiologically nearly so. How then does an individual become a queen? How does the queen suppress worker reproduction? How does the queen regulate the nonreproductive activities of the workers? What is the function of aggression shown by different individuals? How and when is the queen''s heir decided? I will show how such questions can indeed be investigated and will emphasize the need for a whole range of different techniques of observation and experimentation.     

Reproduction in worker honey bees is associated with low juvenile hormone titers and rates of biosynthesis.

Three experiments were performed to determine the role of juvenile hormone (JH) in worker reproduction in queenless colonies of honey bees. In Experiment 1, egg-laying workers had low hemolymph titers of JH, as did bees engaged in brood care, while foragers had significantly higher titers. Experiment 2 confirmed these findings by demonstrating that laying workers have significantly lower rates of JH biosynthesis than foragers do. In Experiment 3, ovary development was inhibited slightly by application of the JH analog methoprene to 1-day-old bees, but was not affected by application to older bees, at least some already displaying egg-laying behavior. These results, which are consistent with earlier findings for queen honey bees, are contrary to a common model of insect reproduction, in which elevated JH titers trigger ovary development, which then leads to oviposition. Previous experiments have demonstrated that JH regulates nonreproductive behavior in workers that is associated with colony division of labor; perhaps this function is incompatible with a traditional role for JH in reproduction.