Dufour’s gland pheromone as a reliable fertility signal among honeybee (Apis mellifera) workers

Differential gene expression underlying the biosynthesis of Dufour's gland signals in Bombus impatiens

Pheromones regulating social behavior are one of the most explored phenomena in social insects. However, compound identity, biosynthesis and their genetic basis are known in only a handful of species. Here we examined the gene expression associated with pheromone biosynthesis of two main chemical classes: esters and terpenes, using the social bee Bombus impatiens. We conducted chemical and RNA-seq analyses of the Dufour's gland, an exocrine gland producing a plethora of pheromones regulating social behavior in hymenopteran species. The Dufour's gland contains mostly long-chained hydrocarbons, terpenes and esters that signal reproductive and social status in several bee species. In bumble bees, the Dufour's gland contains queen- and worker-specific esters, in addition to terpenes and terpene-esters only found in gynes and queens. These compounds are assumed to be synthesized de novo in the gland, however, their genetic basis is unknown. A whole transcriptome gene expression analysis of the gland in queens, gynes, queenless and queenright workers showed distinct transcriptomic profiles, with thousands of differentially expressed genes between the groups. Workers and queens express genes associated with key enzymes in the biosynthesis of wax esters, while queens and gynes preferentially express key genes in terpene biosynthesis. Overall, our data demonstrate gland-specific regulation of chemical signals associated with social behavior and identifies candidate genes and pathways regulating caste-specific chemical signals in social insects.

Dufour's gland analysis reveals caste and physiology specific signals in Bombus impatiens

Reproductive division of labor in insect societies is regulated through multiple concurrent mechanisms, primarily chemical and behavioral. Here, we examined if the Dufour’s gland secretion in the primitively eusocial bumble bee Bombus impatiens signals information about caste, social condition, and reproductive status. We chemically analyzed Dufour’s gland contents across castes, age groups, social and reproductive conditions, and examined worker behavioral and antennal responses to gland extracts. We found that workers and queens each possess caste-specific compounds in their Dufour’s glands. Queens and gynes differed from workers based on the presence of diterpene compounds which were absent in workers, whereas four esters were exclusive to workers. These esters, as well as the total amounts of hydrocarbons in the gland, provided a separation between castes and also between fertile and sterile workers. Olfactometer bioassays demonstrated attraction of workers to Dufour’s gland extracts that did not represent a reproductive conflict, while electroantennogram recordings showed higher overall antennal sensitivity in queenless workers. Our results demonstrate that compounds in the Dufour’s gland act as caste- and physiology-specific signals and are used by workers to discriminate between workers of different social and reproductive status.

Support for the reproductive ground plan hypothesis in a solitary bee: links between sucrose response and reproductive status

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.

Comparison of the Chemical Compositions of the Cuticle and Dufour's Gland of Two Solitary Bee Species from Laboratory and Field Conditions

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.

A conserved class of queen pheromones? Re-evaluating the evidence in bumblebees (Bombus impatiens)

The regulation of reproductive division of labour is a key component in the evolution of social insects. Chemical signals are important mechanisms to regulate worker reproduction, either as queen-produced pheromones that coercively inhibit worker reproduction or as queen signals that honestly advertise her fecundity. A recent study suggested that a conserved class of hydrocarbons serve as queen pheromones across three independent origins of eusociality. In bumblebees (Bombus terrestris), pentacosane (C25) was suggested to serve as a queen pheromone. Here, we repeat these studies using a different species of bumblebee (Bombus impatiens) with a more controlled experimental design. Instead of dequeened colonies, we used same-aged, three-worker queenless groups comprising either experienced or naive workers (with/without adult exposure to queen pheromone). We quantified three hydrocarbons (C23, C25 and C27) on the cuticular surfaces of females and tested their effects on the two worker types. Our results indicate differences in responses of naive and experienced workers, genetic effects on worker reproduction, and general effects of hydrocarbons and duration of egg laying on ovary resorption rates. However, we found no evidence to support the theory that a conserved class of hydrocarbons serve as queen pheromones or queen signals in Bombus impatiens.

Gonadotropic and physiological functions of juvenile hormone in Bumblebee (Bombus terrestris) workers

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.

Dufour's gland secretion, sterility and foraging behavior: correlated behavior traits in bumblebee workers

Bombus terrestris colonies go through two major phases: the "pre-competition phase" in which the queen is the sole reproducer and aggression is rare, and the "competition phase" in which workers aggressively compete over reproduction. Conflicts over reproduction are partially regulated by a group of octyl esters that are produced in Dufour's gland of reproductively subordinate workers and protect them from being aggressed. However, workers possess octyl esters even before overt aggression occurs, raising the question of why produce the ester-signal before it is functionally necessary? In most insect societies, foragers show reduced aggression and low dominance rank. We hypothesize that ester production in B. terrestris is not only correlated with sterility but also with foraging, signaling cooperative behavior by subordinate workers. Such a signal helps to maintain social organization, reduce the cost of fights between reproductives and helpers, and increase colony productivity, enabling subordinates to gain greater inclusive fitness. We demonstrate that foragers produce larger amounts of esters compared to non-foragers, and that their amounts positively correlate with foraging efforts. We further suggest that task performance, potential fecundity, and aggression are interlinked, and that worker-worker interactions are involved in regulating foraging behavior. B. terrestris, being an intermediate phase between primitive and derived eusocial insects, provides an excellent model for understanding the evolution of early phases of eusociality. Our results, combined with those in primitively eusocial wasps, suggest that at early stages of social evolution, reproduction was regulated by a "primordial division of labor", that comprised foragers and reproducers, which further evolved to a more complex division of labor, a hallmark of eusociality.

Virgin honeybee queens fail to suppress worker fertility but not fertility signalling

Queen mating status in social insects is a matter of crucial importance for workers because of its influence on the queen's productivity and consequently their fitness. Behavioural and physiological reactions of workers to the queens mating status have been studied as a proxy to mechanisms maintaining insect sociality. Here we show that unmated honeybee queens have considerably impaired capacity to trigger worker sterility and cooperative behaviour in comparison to mated (and thus more productive) queens and that under unmated queens social harmony in honeybee societies and queen's dominant position are somewhat compromised. Together with this it is shown that honeybee workers exposed to unmated queens despite being active reproductively and behaving accordingly display an impaired ability to advertise their fertility compared to queenless workers. These findings suggest that reproductive development, behavioural reactions and production of fertility signals are differentially regulated and differently influenced by the queen's presence.

Uncoupling fertility from fertility-associated pheromones in worker honeybees (Apis mellifera)

Fertility-associated pheromones, chemical signals delineating ovarian development, were favourably selected in the course of evolution because it is in the best interest of both the signallers (in recruiting help from other colony members) and the receivers (in assisting them to reach an informed decision of how to maximize fitness). Such signals therefore should constitute honest, deception-proof indicators of ovarian development, suggesting, theoretically, that the processes of ovarian development and signal production are irreversibly coupled. Here we demonstrate that these processes can be uncoupled by treating queenless (QL) honeybee callow workers with methoprene, a juvenile hormone (JH) analog. While methoprene effectively inhibited ovarian development, it neither inhibited Dufour's fertility signal nor the mandibular glands' dominance signal. In fact, there was even a slight augmentation of both in the methoprene-treated bees. Thus, although fertility and fertility signals are tightly associated, they can be uncoupled by experimental manipulation. These results are consistent with the hypothesis that ovarian development and fertility-associated signal production are triggered by a common event/signal (e.g. queen pheromone disappearance) but comprise different regulatory systems. The evolutionary implication is that these two traits have evolved independently and may have been co-opted to emphasize the reproductive status of workers in the competition for reproduction.

Reproductive competition in the bumble-bee Bombus terrestris: do workers advertise sterility?

Reproductive competition in social insects is generally mediated through specific fertility pheromones. By analysing Dufour's gland secretion in queens and workers of Bombus terrestris under varying social conditions, we demonstrate here that the volatile constituents of the secretion exhibit a context-dependent composition. The secretion of egg-laying queens is composed of a series of aliphatic hydrocarbons (alkanes and alkenes), while that of sterile workers contains in addition octyl esters, dominated by octyl hexadecanoate and octyl oleate. These esters disappear in workers with developed ovaries, whether queenright (QR) or queenless (QL), rendering their secretion queen-like. This constitutes an unusual case in which the sterile caste, rather than the fertile one, possesses extra components. Individually isolated (socially deprived) workers developed ovaries successfully, but failed to oviposit, and still possessed the octyl esters. Thus, whereas social interactions are not needed in order to develop ovaries, they appear essential for oviposition and compositional changes in Dufour's gland secretion (ester disappearance). The apparent link between high ester levels and an inability to lay eggs lends credence to the hypothesis that these esters signal functional sterility. We hypothesize that by producing a sterility-specific secretion, workers signal that 'I am out of the competition', and therefore are not attacked, either by the queen or by the reproductive workers. This enables proper colony function and brood care, in particular sexual brood, even under the chaotic conditions of the competition phase.

Primer pheromones in social hymenoptera

Social insect are profoundly influenced by primer pheromones (PPhs), which are efficient means for maintaining social harmony in the colony. PPhs act by affecting the physiology of the recipients with a subsequent shift in their behavior, and many PPhs have a releaser effect (i.e., changing the probability of performing a certain behavior upon perception). In this review we try to clarify the interplay between such dual pheromonal effects. Only a few PPhs have been identified, and we provide evidence for their existence in multiple species of social Hymenoptera, which is the most extensively studied of the social insects. We focus on the regulation of reproduction, social policing, and task allocation. Considering PPhs in a broad sense, we also discuss fertility signals and the role of cuticular hydrocarbons as putative PPhs. Identification of the underlying chemistry of PPhs offers insights into insect physiology and the evolution of social behavior. PPhs of the honey bee are used to demonstrate the complexity of pheromonal communication in social insects.

Brain modulation of Dufour's gland ester biosynthesis in vitro in the honeybee (Apis mellifera)

Caste-specific pheromone biosynthesis is a prerequisite for reproductive skew in the honeybee. Nonetheless, this process is not hardwired but plastic, in that egg-laying workers produce a queen-like pheromone. Studies with Dufour's gland pheromone revealed that, in vivo, workers' gland biosynthesis matches the social status of the worker, i.e., sterile workers showed a worker-like pattern whereas fertile workers showed a queen-like pattern (production of the queen-specific esters). However, when incubated in vitro, the gland spontaneously exhibits the queen-like pattern, irrespective of its original worker type, prompting the notion that ester production in workers is under inhibitory control that is queen-dependent. We tested this hypothesis by exposing queen or worker Dufour's glands in vitro to brain extracts of queens, queenright (sterile) workers and males. Unexpectedly, worker brain extracts activated the queen-like esters biosynthesis in workers' Dufour's gland. This stimulation was gender-specific; queen or worker brains demonstrated a stimulatory activity, but male brains did not. Queen gland could not be further stimulated. Bioassays with heated and filtered extracts indicate that the stimulatory brain factor is below 3,000 Da. We suggest that pheromone production in Dufour's gland is under dual, negative-positive control. Under queenright conditions, the inhibitor is released and blocks ester biosynthesis, whereas under queenless conditions, the activator is released, activating ester biosynthesis in the gland. This is consistent with the hypothesis that queenright workers are unequivocally recognized as non-fertile, whereas queenless workers try to become "false queens" as part of the reproductive competition.

Order, disorder, death: lessons from a superorganism

Animal models contribute to the understanding of molecular mechanism of cancer, revealing complex roles of altered cellular-signaling networks and deficient surveillance systems. Analogous pathologies are documented in an unconventional model organism that receives attention in research on systems theory, evolution, and aging. The honeybee (Apis mellifera) colony is an advanced integrative unit, a "superorganism" in which order is controlled via complex signaling cascades and surveillance schemes. A facultatively sterile caste, the workers, regulates patterns of growth, differentiation, homeostasis, and death. Workers differentiate into temporal phenotypes in response to dynamic social cues; chemosensory signals that can translate into dramatic physiological responses, including programmed cell death. Temporal worker forms function together, and effectively identify and terminate abnormal colony members ranging from embryos to adults. As long as this regulatory system is operational at a colony level, the unit survives and propagates. However, if the worker phenotypes that collectively govern order become too few or change into malignant forms that bypass control mechanisms to replicate aberrantly; order is replaced by disorder that ultimately leads to the destruction of the society. In this chapter we describe fundamental properties of honeybee social organization, and explore conditions that lead to states of disorder. Our hope is that this chapter will be an inspirational source for ongoing and future work in the field of cancer research.

Queen pheromones affecting the production of queen-like secretion in workers

The honeybee queen pheromones promote both worker sterility and worker-like pheromone composition; in their absence workers become fertile and express the queen pheromones. Which of the queen pheromones regulate worker pheromone expression and how, is still elusive. Here we investigated how two queen pheromones, the mandibular and Dufour's, singly or combined, affect worker ovarian activation and occurrence of queen-like Dufour's esters. Although queen mandibular pheromone (QMP) alone, or combined with Dufour's secretion, inhibited to some extent worker reproduction, neither was as effective as the queen. The effect of the queen pheromones on worker pheromone expression was limited to workers with developed ovaries. Here too, QMP and Dufour's combined had the greatest inhibitory effect. In contrast, treatment with Dufour's alone resulted in augmentation of esters in the workers. This is another demonstration that a pheromone emitted by one individual affects the rates of its production in another individual. Ester production was tightly coupled to ovarian development. However fertile workers from queenright or QMP-treated colonies had significantly higher amounts of esters in their Dufour's gland than untreated queenless colonies. The fact that the queen or QMP exert greater suppression on signal production than on ovary activation, suggests disparate regulatory pathways, and presents a challenging ultimate as well as proximate questions.