Cuticular and Dufour's Gland Chemistry Reflect Reproductive and Social State in the Facultatively Eusocial Sweat Bee Megalopta genalis (Hymenoptera: Halictidae).

Queen pheromones evolved independently in multiple eusocial insect lineages, in which they mediate reproductive conflict by inhibiting worker ovarian development. Although fundamentally important for reproductive division of labor - the hallmark of eusociality - their evolutionary origins are enigmatic. Here, we analyze cuticular and Dufour's gland chemistries across alternative social and reproductive phenotypes in Megalopta genalis bees (tribe Augochlorini, family Halictidae) that facultatively express simple eusociality. Reproductive bees have distinct overall glandular and cuticular chemical phenotypes compared with non-reproductive workers. On the cuticle, a likely site of signal transmission, reproductives are enriched for certain alkenes, most linear alkanes, and are heavily enriched for all methyl-branched alkanes. Chemicals belonging to these compound classes are known to function as fertility signals in other eusocial insect taxa. Some macrocyclic lactones, compounds that serve as queen pheromones in the other eusocial halictid tribe (Halictini), are also enriched among reproductives relative to workers. The intra-population facultative eusociality of M. genalis permits direct comparisons between individuals expressing alternative reproductive phenotypes - females that reproduce alone (solitary reproductives) and social queens - to highlight traits in the latter that may be important mediators of eusociality. Compared with solitary reproductives, the cuticular chemistries of queens are more strongly differentiated from those of workers, and furthermore are especially enriched for methyl-branched alkanes. Determining the pheromonal function(s) and information content of the candidate signaling compounds we identify will help illuminate the early evolutionary history of queen pheromones, chemical signals central to the organization of insect eusocial behavior.

Chemical Variation among Castes, Female Life Stages and Populations of the Facultative Eusocial Sweat Bee Halictus rubicundus (Hymenoptera: Halictidae).

In eusocial insects, chemical communication is crucial for mediating many aspects of social activities, especially the regulation of reproduction. Though queen signals are known to decrease ovarian activation of workers in highly eusocial species, little is known about their evolution. In contrast, some primitively eusocial species are thought to control worker reproduction through physical aggression by the queen rather than via pheromones, suggesting the evolutionary establishment of chemical signals with more derived sociality. However, studies supporting this hypothesis are largely missing. Socially polymorphic halictid bees, such as Halictus rubicundus, with social and solitary populations in both Europe and North America, offer excellent opportunities to illuminate the evolution of caste-specific signals. Here we compared the chemical profiles of social and solitary populations from both continents and tested whether (i) population or social level affect chemical dissimilarity and whether (ii) caste-specific patterns reflect a conserved queen signal. Our results demonstrate unique odor profiles of European and North American populations, mainly due to different isomers of n-alkenes and macrocyclic lactones; chemical differences may be indicative of phylogeographic drift in odor profiles. We also found common compounds overproduced in queens compared to workers in both populations, indicating a potential conserved queen signal. However, North American populations have a lower caste-specific chemical dissimilarity than European populations which raises the question if both use different mechanisms of regulating reproductive division of labor. Therefore, our study gives new insights into the evolution of eusocial behavior and the role of chemical communication in the inhibition of reproduction.

Evolution of Caste-Specific Chemical Profiles in Halictid Bees.

Chemical communication is crucial for the maintenance of colony organization in eusocial insects and chemical signals are known to mediate important aspects of their social life, including the regulation of reproduction. Sociality is therefore hypothesized to be accompanied by an increase in the complexity of chemical communication. However, little is known about the evolution of odor signals at the transition from solitary living to eusociality. Halictid bees are especially suitable models to study this question as they exhibit considerable variability in social behavior. Here we investigated whether the dissimilarities in cuticle chemical signals in females of different castes and life stages reflect the level of social complexity across halictid bee species. Our hypothesis was that species with a higher social behavior ergo obligate eusocial species possess a more distinct chemical profile between castes or female life stages. We analyzed cuticular chemical profiles of foundresses, breeding females and workers of ancestrally solitary species, facultative and obligate eusocial halictid species. We also tested whether social complexity was associated with a higher investment in chemical signals. Our results revealed higher chemical dissimilarity between castes in obligate than in facultative eusocial species, especially regarding macrocyclic lactones, which were the single common compound class overproduced in queens compared with workers. Chemical dissimilarities were independent of differences in ovarian status in obligate eusocial species but were dependent on ovarian status in facultative eusocial species, which we discuss in an evolutionary framework.

Macrocyclic Lactones Act as a Queen Pheromone in a Primitively Eusocial Sweat Bee.

Eusociality is characterized by the reproductive division of labor between two castes: fertile queens and largely sterile workers. Queen pheromones are known to influence worker behavior and reproductive physiology and are therefore key components in regulating complex eusocial behavior [1]. Recent studies indicate that cuticular hydrocarbons (CHCs) act as queen pheromones in various eusocial hymenopteran species [2-8]. However, almost all species investigated to date are highly eusocial and do not include extant transitory stages from solitary to eusocial behavior [9]. Indeed, primitively eusocial species, which largely lack morphologically distinct castes, are thought to control worker reproduction through the physical aggression of the queen rather than via pheromones [10-12]. Halictid or sweat bees exhibit a high variability of eusociality including solitary and facultatively eusocial species [9, 13-16]. However, the mechanisms controlling worker reproduction in these transitory species are unknown. The results of a recent correlative study based on caste-specific chemical profiles in various halictid bees of different social levels have revealed an overproduction of macrocyclic lactones in queens compared with workers [17]. Using chemical analyses and behavioral experiments in which we simulated below-ground nests of the primitively eusocial sweat bee Lasioglossum malachurum, we identified a queen pheromone and found that macrocyclic lactones, not CHCs, influence worker behavior and decrease ovarian activation in this species. Our data suggest that the evolution of queen pheromones is more complex than previously inferred from highly eusocial species and shed new light on the complexity of the evolution of queen pheromones.

Queen Recognition Signals in Two Primitively Eusocial Halictid Bees: Evolutionary Conservation and Caste-Specific Perception.

Queen signals are known to regulate reproductive harmony within eusocial colonies by influencing worker behavior and ovarian physiology. However, decades of research have resulted in the identification of just a few queen signals, and studies of their mode of action are rare. Our aim was to identify queen recognition signals in the halictid bee Lasioglossum pauxillum and to analyze caste differences in the olfactory perception of queen signals in L. pauxillum and the closely related species L. malachurum. We performed chemical analyses and bioassays to test for caste differences in chemical profiles and worker behavior influenced by queen-specific compounds in L. pauxillum. Our results indicated that caste differences in the chemical profiles were mainly attributable to higher amounts of macrocyclic lactones in queens. Bioassays demonstrated a higher frequency of subordinate behavior in workers elicited by queen-specific amounts of macrocyclic lactones. Thus, macrocyclic lactones function as queen recognition signals in L. pauxillum, as in L. malachurum. Using electrophysiological analyses, we have demonstrated that queens of both tested species lack antennal reactions to certain macrocyclic lactones. Therefore, we assume that this is a mechanism to prevent reproductive self-inhibition in queens. Our results should stimulate debate on the conservation and mode of action of queen signals.

Eleven years' data of grassland management in Germany.

BACKGROUND: The 150 grassland plots were located in three study regions in Germany, 50 in each region. The dataset describes the yearly grassland management for each grassland plot using 116 variables.General information includes plot identifier, study region and survey year. Additionally, grassland plot characteristics describe the presence and starting year of drainage and whether arable farming had taken place 25 years before our assessment, i.e. between 1981 and 2006. In each year, the size of the management unit is given which, in some cases, changed slightly across years.Mowing, grazing and fertilisation were systematically surveyed: Mowing is characterised by mowing frequency (i.e. number of cuts per year), dates of cutting and different technical variables, such as type of machine used or usage of conditioner.For grazing , the livestock species and age (e.g. cattle, horse, sheep), the number of animals, stocking density per hectare and total duration of grazing were recorded. As a derived variable, the mean grazing intensity was then calculated by multiplying the livestock units with the duration of grazing per hectare [LSU days/ha]. Different grazing periods during a year, partly involving different herds, were summed up to an annual grazing intensity for each grassland.For fertilisation , information on the type and amount of different types of fertilisers was recorded separately for mineral and organic fertilisers, such as solid farmland manure, slurry and mash from a bioethanol factory. Our fertilisation measures neglect dung dropped by livestock during grazing. For each type of fertiliser, we calculated its total nitrogen content, derived from chemical analyses by the producer or agricultural guidelines (Table 3).All three management types, mowing, fertilisation and grazing, were used to calculate a combined land use intensity index (LUI) which is frequently used to define a measure for the land use intensity. Here, fertilisation is expressed as total nitrogen per hectare [kg N/ha], but does not consider potassium and phosphorus.Information on additional management practices in grasslands was also recorded including levelling, to tear-up matted grass covers, rolling, to remove surface irregularities, seed addition, to close gaps in the sward. NEW INFORMATION: Investigating the relationship between human land use and biodiversity is important to understand if and how humans affect it through the way they manage the land and to develop sustainable land use strategies. Quantifying land use (the 'X' in such graphs) can be difficult as humans manage land using a multitude of actions, all of which may affect biodiversity, yet most studies use rather simple measures of land use, for example, by creating land use categories such as conventional vs. organic agriculture. Here, we provide detailed data on grassland management to allow for detailed analyses and the development of land use theory. The raw data have already been used for > 100 papers on the effect of management on biodiversity (e.g. Manning et al. 2015).