Support for the reproductive ground plan hypothesis in a solitary bee: links between sucrose response and reproductive status.
ABSTRACT: In social bees, foraging behaviour is correlated with reproductive status and sucrose sensitivity via endocrine pathways. This association led to the hypothesis that division of labour in social insect societies is derived from an ancestral ground plan that functions to synchronize dietary preferences with reproductive needs in solitary insects. However, the relationship between these traits is unknown for solitary bees, which represent the ancestral state of social bees. We used the proboscis extension response assay to measure sucrose response among reproductive females of the solitary alkali bee (Nomia melanderi) as a function of acute juvenile hormone (JH) treatments and reproductive physiology. We also tested long-term effects of JH on reproductive development in newly emerged females. JH did not have short-term effects on reproductive physiology or sucrose response, but did have significant long-term effects on ovary and Dufour's gland development. Dufour's gland size, not ovary development, was a significant predictor of sucrose response. This provides support for the reproductive ground plan hypothesis, because the Dufour's gland has conserved reproductive functions in bees. Differing results from this study and honeybees suggest independent origins of division of labour may have evolved via co-option of different components of a conserved ground plan.
Project description:Species-specific biochemistry, morphology, and function of the Dufour's gland have been investigated for social bees and some non-social bee families. Most of the solitary bees previously examined are ground-nesting bees that use Dufour's gland secretions to line brood chambers. This study examines the chemistry of the cuticle and Dufour's gland of cavity-nesting Megachile rotundata and Osmia lignaria, which are species managed for crop pollination. Glandular and cuticular lipid compositions were characterized and compared to each other and according to the nesting experience of adult females. Major lipid classes found were hydrocarbons, free fatty acids, and wax esters. Many components were common to the cuticle and Dufour's glands of each species, yet not identical in number or relative composition. Wax esters and fatty acids were more prevalent in Dufour's glands of M. rotundata than on cuticles. Wax esters were more abundant on cuticles of O. lignaria than in Dufour's glands. In both species, fatty acids were more prevalent in glands of field-collected females compared to any other sample type. Chemical profiles of cuticles and glands were distinct from each other, and, for O. lignaria, profiles of laboratory-maintained bees could be distinguished from those of field-collected bees. Comparison of percentiles of individual components of cuticular and glandular profiles of the same bee showed that the proportions of some cuticular components were predictive of the proportion of the same glandular components, especially for nesting females. Lastly, evidence suggested that Dufour's gland is the major source of nest-marking substances in M. rotundata, but evidence for this role in O. lignaria was less conclusive.
Project description:The evolution of advanced sociality in bees is associated with apparent modifications in juvenile hormone (JH) signaling. By contrast to most insects in which JH is a gonadotropin regulating female fertility, in the highly eusocial honey bee (Apis mellifera) JH has lost its gonadotrophic function in adult females, and instead regulates age-related division of labor among worker bees. In order to shed light on the evolution of JH signaling in bees we performed allatectomy and replacement therapies to manipulate JH levels in workers of the "primitively eusocial" bumblebee Bombus terrestris. Allatectomized worker bees showed remarkable reduction in ovarian development, egg laying, Vitellogenin and Krüppel homolog 1 fat body transcript levels, hemolymph Vitellogenin protein abundance, wax secretion, and egg-cell construction. These effects were reverted, at least partially, by treating allatectomized bees with JH-III, the natural JH of bees. Allatectomy also affected the amount of ester component in Dufour's gland secretion, which is thought to convey a social signal relating to worker fertility. These findings provide a strong support for the hypothesis that in contrast to honey bees, JH is a gonadotropin in bumblebees and lend credence to the hypothesis that the evolution of advanced eusociality in honey bees was associated with major modifications in JH signaling.
Project description:The colony of eusocial bee Apis mellifera has a reproductive queen and sterile workers performing tasks such as brood care and foraging. Chemical communication plays a crucial role in the maintenance of sociability in bees with many compounds released by the exocrine glands. The Dufour's gland is a non-paired gland associated with the sting apparatus with important functions in the communication between members of the colony, releasing volatile chemicals that influence workers roles and tasks. However, the protein content in this gland is not well studied. This study identified differentially expressed proteins in the Dufour's glands of nurse and forager workers of A. mellifera through 2D-gel electrophoresis and mass spectrometry. A total of 131 spots showed different expression between nurse and forager bees, and 28 proteins were identified. The identified proteins were categorized into different functions groups including protein, carbohydrate, energy and lipid metabolisms, cytoskeleton-associated proteins, detoxification, homeostasis, cell communication, constitutive and allergen. This study provides new insights of the protein content in the Dufour's gland contributing to a more complete understanding of the biological functions of this gland in honeybees.
Project description:Pheromones mediate social interactions among individuals in a wide variety of species, from yeast to mammals. In social insects such as honey bees, pheromone communication systems can be extraordinarily complex and serve to coordinate behaviors among many individuals. One of the primary mediators of social behavior and organization in honey bee colonies is queen pheromone, which is produced by multiple glands. The types and quantities of chemicals produced differ significantly between virgin and mated queens, and recent studies have suggested that, in newly mated queens, insemination volume or quantity can affect pheromone production. Here, we examine the long-term impact of different factors involved during queen insemination on the chemical composition of the mandibular and Dufour's glands, two of the major sources of queen pheromone. Our results demonstrate that carbon dioxide (an anesthetic used in instrumental insemination), physical manipulation of genital tract (presumably mimicking the act of copulation), insemination substance (saline vs. semen), and insemination volume (1 vs. 8 µl) all have long-term effects on mandibular gland chemical profiles. In contrast, Dufour's gland chemical profiles were changed only upon insemination and were not influenced by exposure to carbon dioxide, manipulation, insemination substance or volume. These results suggest that the chemical contents of these two glands are regulated by different neuro-physiological mechanisms. Furthermore, workers responded differently to the different mandibular gland extracts in a choice assay. Although these studies must be validated in naturally mated queens of varying mating quality, our results suggest that while the chemical composition of Dufour's gland is associated with mating status, that of the mandibular glands is associated with both mating status and insemination success. Thus, the queen appears to be signaling both status and reproductive quality to the workers, which may impact worker behavior and physiology as well as social organization and productivity of the colony.
Project description:Several insect pheromones are multifunctional and have both releaser and primer effects. In honey bees (Apis mellifera), the queen mandibular pheromone (QMP) and e-beta-ocimene (e?), emitted by young worker larvae, have such dual effects. There is increasing evidence that these multifunctional pheromones profoundly shape honey bee colony dynamics by influencing cooperative brood care, a fundamental aspect of eusocial insect behavior. Both QMP and e? have been shown to affect worker physiology and behavior, but it has not yet been determined if these two key pheromones have interactive effects on hypopharyngeal gland (HPG) development, actively used in caring of larvae, and ovary activation, a component of worker reproductive physiology. Experimental results demonstrate that both QMP and e? significantly suppress ovary activation compared to controls but that the larval pheromone is more effective than QMP. The underlying reproductive anatomy (total ovarioles) of workers influenced HPG development and ovary activation, so that worker bees with more ovarioles were less responsive to suppression of ovary activation by QMP. These bees were more likely to develop their HPG and have activated ovaries in the presence of e?, providing additional links between nursing and reproductive physiology in support of the reproductive ground plan hypothesis.
Project description:Social insect foragers may specialize on certain resource types. Specialization on pollen or nectar among honeybee foragers is hypothesized to result from associations between reproductive physiology and sensory tuning that evolved in ancestral solitary bees (the Reproductive Ground-Plan Hypothesis; RGPH). However, the two non-honeybee species studied showed no association between specialization and ovary activation. Here we investigate the bumblebee B. impatiens because it has the most extensively studied pollen/nectar specialization of any bumblebee. We show that ovary size does not differ between pollen specialist, nectar specialist, and generalist foragers, contrary to the predictions of the RGPH. However, we also found mixed support for the second prediction of the RGPH, that sensory sensitivity, measured through proboscis extension response (PER), is greater among pollen foragers. We also found a correlation between foraging activity and ovary size, and foraging activity and relative nectar preference, but no correlation between ovary size and nectar preference. In one colony non-foragers had larger ovaries than foragers, supporting the reproductive conflict and work hypothesis, but in the other colony they did not.
Project description:Eusociality is characterised by a reproductive division of labour, where some individuals forgo direct reproduction to instead help raise kin. Socially polymorphic sweat bees are ideal models for addressing the mechanisms underlying the transition from solitary living to eusociality, because different individuals in the same species can express either eusocial or solitary behaviour. A key question is whether alternative social phenotypes represent environmentally induced plasticity or predominantly genetic differentiation between populations. In this paper, we focus on the sweat bee Lasioglossum calceatum, in which northern or high-altitude populations are solitary, whereas more southern or low-altitude populations are typically eusocial. To test whether social phenotype responds to local environmental cues, we transplanted adult females from a solitary, northern population, to a southern site where native bees are typically eusocial. Nearly all native nests were eusocial, with foundresses producing small first brood (B1) females that became workers. In contrast, nine out of ten nests initiated by transplanted bees were solitary, producing female offspring that were the same size as the foundress and entered directly into hibernation. Only one of these ten nests became eusocial. Social phenotype was unlikely to be related to temperature experienced by nest foundresses when provisioning B1 offspring, or by B1 emergence time, both previously implicated in social plasticity seen in two other socially polymorphic sweat bees. Our results suggest that social polymorphism in L. calceatum predominantly reflects genetic differentiation between populations, and that plasticity is in the process of being lost by bees in northern populations.Phenotypic plasticity is thought to play a key role in the early stages of the transition from solitary to eusocial behaviour, but may then be lost if environmental conditions become less variable. Socially polymorphic sweat bees exhibit either solitary or eusocial behaviour in different geographic populations, depending on the length of the nesting season. We tested for plasticity in the socially polymorphic sweat bee Lasioglossum calceatum by transplanting nest foundresses from a northern, non-eusocial population to a southern, eusocial population. Plasticity would be detected if transplanted bees exhibited eusocial behaviour. We found that while native bees were eusocial, 90% of transplanted bees and their offspring did not exhibit traits associated with eusociality. Environmental variables such as time of offspring emergence or temperatures experienced by foundresses during provisioning could not explain these differences. Our results suggest that the ability of transplanted bees to express eusociality is being lost, and that social polymorphism predominantly reflects genetic differences between populations.
Project description:In honeybee colonies, food collection is performed by a group of mostly sterile females called workers. After an initial nest phase, workers begin foraging for nectar and pollen, but tend to bias their collection towards one or the other. The foraging choice of honeybees is influenced by vitellogenin (vg), an egg-yolk precursor protein that is expressed although workers typically do not lay eggs. The forager reproductive ground plan hypothesis (RGPH) proposes an evolutionary path in which the behavioural bias toward collecting nectar or pollen on foraging trips is influenced by variation in reproductive physiology, such as hormone levels and vg gene expression. Recently, the connections between vg and foraging behaviour were challenged by Oldroyd and Beekman (2008), who concluded from their study that the ovary, and especially vg, played no role in foraging behaviour of bees. We address their challenge directly by manipulating vg expression by RNA interference- (RNAi) mediated gene knockdown in two honeybee genotypes with different foraging behaviour and reproductive physiology. We show that the effect of vg on the food-loading decisions of the workers occurs only in the genotype where timing of foraging onset (by age) is also sensitive to vg levels. In the second genotype, changing vg levels do not affect foraging onset or bias. The effect of vg on workers' age at foraging onset is explained by the well-supported double repressor hypothesis (DHR), which describes a mutually inhibitory relationship between vg and juvenile hormone (JH) - an endocrine factor that influences development, reproduction, and behaviour in many insects. These results support the RGPH and demonstrate how it intersects with an established mechanism of honeybee behavioural control.
Project description:The interactions between the insulin signaling pathway (ISP) and juvenile hormone (JH) controlling reproductive trade-offs are well documented in insects. JH and insulin regulate reproductive output in mosquitoes; both hormones are involved in a complex regulatory network, in which they influence each other and in which the mosquito's nutritional status is a crucial determinant of the network's output. Previous studies reported that the insulin-TOR (target of rapamacyn) signaling pathway is involved in the nutritional regulation of JH synthesis in female mosquitoes. The present studies further investigate the regulatory circuitry that controls both JH synthesis and reproductive output in response to nutrient availability.We used a combination of diet restriction, RNA interference (RNAi) and insulin treatments to modify insulin signaling and study the cross-talk between insulin and JH in response to starvation. JH synthesis was analyzed using a newly developed assay utilizing fluorescent tags.Our results reveal that starvation decreased JH synthesis via a decrease in insulin signaling in the corpora allata (CA). Paradoxically, starvation-induced up regulation of insulin receptor transcripts and therefore "primed" the gland to respond rapidly to increases in insulin levels. During this response to starvation the synthetic potential of the CA remained unaffected, and the gland rapidly and efficiently responded to insulin stimulation by increasing JH synthesis to rates similar to those of CA from non-starved females.
Project description:In many cooperatively breeding animals, subordinate group members have lower reproductive capacity than dominant group members. Theory suggests subordinates may downregulate their reproductive capacity because dominants punish subordinates who maintain high fertility. However, there is little direct experimental evidence that dominants cause physiological suppression in subordinates. Here, we experimentally test how social interactions influence subordinate reproductive hormones in Polistes dominula paper wasps. Polistes dominula queens commonly found nests in cooperative groups where the dominant queen is more fertile than the subordinate queen. In this study, we randomly assigned wasps to cooperative groups, assessed dominance behaviour during group formation, then measured levels of juvenile hormone (JH), a hormone that mediates Polistes fertility. Within three hours, lowest ranking subordinates had less JH than dominants or solitary controls, indicating that group formation caused rapid JH reduction in low-ranking subordinates. In a second experiment, we measured the behavioural consequences of experimentally increasing subordinate JH. Subordinates with high JH-titres received significantly more aggression than control subordinates or subordinates from groups where the dominant's JH was increased. These results suggest that dominants aggressively punished subordinates who attempted to maintain high fertility. Low-ranked subordinates may rapidly downregulate reproductive capacity to reduce costly social interactions with dominants. Rapid modulation of subordinate reproductive physiology may be an important adaptation to facilitate the formation of stable, cooperative groups.