Age-dependent changes in structure and function of the male labial gland in Bombus terrestris

Imidacloprid reduces the mating success of males in bumblebees

Bumblebees play a vital role in both natural and agricultural environments, but there has been a noticeable decline in their populations. Pesticides, particularly neonicotinoids, are widely regarded as a substantial contributing factor to the decline in bumblebee populations, as evidenced by the detrimental impacts documented across many stages of their life cycle. Mating is vital for the population maintenance of bumblebees. Nevertheless, there is a scarcity of research conducted on the effects of pesticides on the mating process. In this study, we individually examined the impact of imidacloprid on the mating behavior of bumblebee males and queens. A competitive mating experiment was conducted to evaluate the effect on the competitive prowess of male individuals and the mate selection behavior of female individuals. The study revealed that the mating rate of bumblebees exposed to a concentration of 10 ppb of imidacloprid was 3 %. This finding demonstrated a statistically significant impact when compared to the control group, which exhibited a mating rate of 58 % in the normal mating experiment. Furthermore, in the competitive mating experiment, we found that the competitive mating success rate of treated males (1 %) was significantly lower than that of untreated males (35 %). Hence, it provides evidence that neonicotinoid imidacloprid negatively affects bumblebee mating success and cautions us to protect bumblebees from pesticide exposure to prevent a severe impact on their populations.

Sexual attraction: a review of bumblebee male pheromones

Males of many bumblebee species exhibit a conspicuous pre-mating behavior with two distinct behavioral components: scent marking and patrol flying. The marking pheromone is produced by the cephalic part of the labial gland (CLG). As far as is known, the CLG secretion is species specific, and it usually consists of two types of compounds: (i) straight-chain aliphatic alcohols, aldehydes or esters, and (ii) acyclic mono-, sesqui- and diterpenes (alcohols or acetates). Here, we summarize data from the literature reporting chemical composition of the CLG secretions of more than 80 bumblebee species. Similarities and differences within and between subgenera are discussed in the context of biosynthetic pathways and evolution.

Expansion of the fatty acyl reductase gene family shaped pheromone communication in Hymenoptera

Fatty acyl reductases (FARs) are involved in the biosynthesis of fatty alcohols that serve a range of biological roles. Insects typically harbor numerous FAR gene family members. While some FARs are involved in pheromone biosynthesis, the biological significance of the large number of FARs in insect genomes remains unclear.

Using bumble bee (Bombini) FAR expression analysis and functional characterization, hymenopteran FAR gene tree reconstruction, and inspection of transposable elements (TEs) in the genomic environment of FARs, we uncovered a massive expansion of the FAR gene family in Hymenoptera, presumably facilitated by TEs. The expansion occurred in the common ancestor of bumble bees and stingless bees (Meliponini). We found that bumble bee FARs from the expanded FAR-A ortholog group contribute to the species-specific pheromone composition. Our results indicate that expansion and functional diversification of the FAR gene family played a key role in the evolution of pheromone communication in Hymenoptera.

Is the Salivary Gland Associated with Honey Bee Recognition Compounds in Worker Honey Bees (Apis mellifera)?

Cuticular hydrocarbons (CHCs) function as recognition compounds with the best evidence coming from social insects such as ants and honey bees. The major exocrine gland involved in hydrocarbon storage in ants is the post-pharyngeal gland (PPG) in the head. It is still not clearly understood where CHCs are stored in the honey bee. The aim of this study was to investigate the hydrocarbons and esters found in five major worker honey bee (Apis mellifera) exocrine glands, at three different developmental stages (newly emerged, nurse, and forager) using a high temperature GC analysis. We found the hypopharyngeal gland contained no hydrocarbons nor esters, and the thoracic salivary and mandibular glands only contained trace amounts of n-alkanes. However, the cephalic salivary gland (CSG) contained the greatest number and highest quantity of hydrocarbons relative to the five other glands with many of the hydrocarbons also found in the Dufour’s gland, but at much lower levels. We discovered a series of oleic acid wax esters that lay beyond the detection of standard GC columns. As a bee’s activities changed, as it ages, the types of compounds detected in the CSG also changed. For example, newly emerged bees have predominately C₁₉-C₂₃n-alkanes, alkenes and methyl-branched compounds, whereas the nurses’ CSG had predominately C_31:1 and C_33:1 alkene isomers, which are replaced by a series of oleic acid wax esters in foragers. These changes in the CSG were mirrored by corresponding changes in the adults’ CHCs profile. This indicates that the CSG may have a parallel function to the PPG found in ants acting as a major storage gland of CHCs. As the CSG duct opens into the buccal cavity the hydrocarbons can be worked into the comb wax and could help explain the role of comb wax in nestmate recognition experiments.

Gene Expression Dynamics in Major Endocrine Regulatory Pathways along the Transition from Solitary to Social Life in a Bumblebee, Bombus terrestris

Understanding the social evolution leading to insect eusociality requires, among other, a detailed insight into endocrine regulatory mechanisms that have been co-opted from solitary ancestors to play new roles in the complex life histories of eusocial species. Bumblebees represent well-suited models of a relatively primitive social organization standing on the mid-way to highly advanced eusociality and their queens undergo both, a solitary and a social phase, separated by winter diapause. In the present paper, we characterize the gene expression levels of major endocrine regulatory pathways across tissues, sexes, and life-stages of the buff-tailed bumblebee, Bombus terrestris, with special emphasis on critical stages of the queen's transition from solitary to social life. We focused on fundamental genes of three pathways: (1) Forkhead box protein O and insulin/insulin-like signaling, (2) Juvenile hormone (JH) signaling, and (3) Adipokinetic hormone signaling. Virgin queens were distinguished by higher expression of forkhead box protein O and downregulated insulin-like peptides and JH signaling, indicated by low expression of methyl farnesoate epoxidase (MFE) and transcription factor Krüppel homolog 1 (Kr-h1). Diapausing queens showed the expected downregulation of JH signaling in terms of low MFE and vitellogenin (Vg) expressions, but an unexpectedly high expression of Kr-h1. By contrast, reproducing queens revealed an upregulation of MFE and Vg together with insulin signaling. Surprisingly, the insulin growth factor 1 (IGF-1) turned out to be a queen-specific hormone. Workers exhibited an expression pattern of MFE and Vg similar to that of reproducing queens. Males were characterized by high Kr-h1 expression and low Vg level. The tissue comparison unveiled an unexpected resemblance between the fat body and hypopharyngeal glands across all investigated genes, sexes, and life stages.

Exploring complex pheromone biosynthetic processes in the bumblebee male labial gland by RNA sequencing

Male marking pheromones (MPs) are used by the majority of bumblebee species (Hymenoptera: Apidae), including a commercially important greenhouse pollinator, the buff-tailed bumblebee (Bombus terrestris), to attract conspecific females. MP biosynthetic processes in the cephalic part of the bumblebee male labial gland (LG) are of extraordinary complexity, involving enzymes of fatty acid and isoprenoid biosynthesis, which jointly produce more than 50 compounds. We employed a differential transcriptomic approach to identify candidate genes involved in MP biosynthesis by sequencing Bombus terrestris LG and fat body (FB) transcriptomes. We identified 12 454 abundantly expressed gene products (reads per kilobase of exon model per million mapped reads value > 1) that had significant hits in the GenBank nonredundant database. Of these, 876 were upregulated in the LG (> 4-fold difference). We identified more than 140 candidate genes potentially involved in MP biosynthesis, including esterases, fatty acid reductases, lipases, enzymes involved in limited fatty acid chain shortening, neuropeptide receptors and enzymes involved in biosynthesis of triacylglycerols, isoprenoids and fatty acids. For selected candidates, we confirmed their abundant expression in LG using quantitative real-time reverse transcription-PCR (qRT-PCR). Our study shows that the Bombus terrestris LG transcriptome reflects both fatty acid and isoprenoid MP biosynthetic processes and identifies rational gene targets for future studies to disentangle the molecular basis of MP biosynthesis. Additionally, LG and FB transcriptomes enrich the available transcriptomic resources for Bombus terrestris.

Regulation of Isoprenoid Pheromone Biosynthesis in Bumblebee Males

Males of the closely related species Bombus terrestris and Bombus lucorum attract conspecific females by completely different marking pheromones. MP of B. terrestris and B. lucorum pheromones contain mainly isoprenoid (ISP) compounds and fatty acid derivatives, respectively. Here, we studied the regulation of ISP biosynthesis in both bumblebees. RNA-seq and qRT-PCR analyses indicated that acetoacetyl-CoA thiolase (AACT), 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), and farnesyl diphosphate synthase (FPPS) transcripts are abundant in the B. terrestris labial gland. Maximal abundance of these transcripts correlated well with AACT enzymatic activity detected in the LG extracts. In contrast, transcript abundances of AACT, HMGR, and FPPS in B. lucorum were low, and AACT activity was not detected in LGs. These results suggest that transcriptional regulation plays a key role in the control of ISP biosynthetic gene expression and ISP pheromone biosynthesis in bumblebee males.

Seasonal Dynamics in the Chemistry and Structure of the Fat Bodies of Bumblebee Queens

Insects’ fat bodies are responsible for nutrient storage and for a significant part of intermediary metabolism. Thus, it can be expected that the structure and content of the fat body will adaptively change, if an insect is going through different life stages. Bumblebee queens belong to such insects as they dramatically change their physiology several times over their lives in relation to their solitary overwintering, independent colony foundation stage, and during the colony life-cycle ending in the senescent stage. Here, we report on changes in the ultrastructure and lipid composition of the peripheral fat body of Bombus terrestris queens in relation to seasonal changes in the queens’ activity. Six life stages are defined and evaluated in particular: pharate, callow, before and after hibernation, egg-laying, and senescence. Transmission electron microscopy revealed that the fat body contained two main cell types–adipocytes and oenocytes. Only adipocytes reveal important changes related to the life phase, and mostly the ration between inclusion and cytoplasm volume varies among particular stages. Both electron microscopy and chemical analyses of lipids highlighted seasonal variability in the quantity of the stored lipids, which peaked prior to hibernation. Triacylglycerols appeared to be the main energy source during hibernation, while the amount of glycogen before and after hibernation remained unchanged. In addition, we observed that the representation of some fatty acids within the triacylglycerols change during the queen’s life. Last but not least, we show that fat body cell membranes do not undergo substantial changes concerning phospholipid composition in relation to overwintering. This finding supports the hypothesis that the cold-adaptation strategy of bumblebee queens is more likely to be based on polyol accumulation than on the restructuring of lipid membranes.

Changes in the chemical profile of cephalic salivary glands of Scaptotrigona postica (Hymenoptera, Meliponini) workers are phase related

Most advanced eusocial bees recruit their nest mates to food resources. Recent studies in Meliponini species have revealed that the cephalic salivary (labial) glands (CSGs) are responsible for the production of scent trail pheromones. Studies on CSGs have shown that changes occur in worker glandular cell morphology from emergence from brood combs until forager phase, which may be correlated to changes in the composition of the CSG secretion. However, the composition of the CSG secretion and the chemical changes that occur in it according to the worker's life phase or tasks performed are unknown for many species, including Scaptotrigona postica. In this study, the chemical profile of CSG secretion in S. postica workers was studied. Glands were taken from specimens that were newly emerged (NE), working in the brood comb area (CA) and foraging (FO), and were analyzed by gas chromatography-mass spectrometry. The results showed that the glandular secretion consists of oxygenated compounds of middle volatility (acids, alcohols, aldehydes, ketones, esters and ether), and their quantity varies among the different life phases, increasing as the individual moves from intra- to extra-colonial activities. The NE phase contained the smallest variety and quantity of compounds. Because of the variability of compounds, the CA workers were separated into three subgroups according to the chemical constitution of their secretion. Forager workers showed the largest quantity and variety of chemical compounds. The major compounds in forager gland secretion were 7-hexadecen-1-yl acetate and 5-tetradecen-1-yl acetate. Statistical analysis indicates that the chemical composition of glandular secretion is phase related.

Biosynthetic Studies of the Male Marking Pheromone in Bumblebees by Using Labelled Fatty Acids and Two-Dimensional Gas Chromatography with Mass Detection

Two-dimensional comprehensive gas chromatography (GC×GC) coupled with mass detection was used as a tool for biosynthetic studies of bumblebee pheromones. Prior to biosynthetic experiments, the chromatographic behaviour of isotopically modified esters in the GC×GC system as well as their behaviour in mass detection was studied. The male marking pheromones of Bombus lucorum, Bombus lapidarius and Bombus terrestris were investigated. Main pheromonal components are ethyl tetradec-9-enoate (53 %) and ethyl dodecanoate (6 %) in B. lucorum, hexadec-9-en-1-ol (52 %) and hexadecan-1-ol (31 %) in B. lapidarius, and 2,3-dihydrofarnesol (58 %) and ethyl dodecanoate (15 %) in B. terrestris. The research strategy was based on 1) in vivo incubation of isotopically (² H, ¹³ C) modified fatty acids (FAs) and analysis of their metabolites and 2) feeding experiments with ² H- and ¹³ C-labelled FAs mixed with food. It was observed that labelled FAs were modified into the most abundant aliphatic compounds present in labial gland secretions. In feeding experiments, the labelled FAs were transformed into pheromone components. Transport of the FA precursors from the fat body through haemolymph was confirmed. The results show that FAs, stored in the form of triacylglycerols in the fat body, are likely to participate in the biosynthesis of some aliphatic pheromone components.

The effect of caste and reproductive state on the chemistry of the cephalic labial glands secretion of Bombus terrestris

The cephalic labial glands are well developed in many bee species. In bumble bee males, they cover most of the head volume, and their secretion is used in marking reproductive territories and attracting virgin queens. In females, however, they are poorly studied. Here, we present chemical analyses of their secretion in queens and workers of Bombus terrestris under various social conditions. The secretion revealed a context-dependent composition with sterile females possessing large amounts of fatty acid dodecyl esters, ranging from dodecyl hexanoate to dodecyl oleate, compared to small amounts in fertile females. Significant reduction in the dodecyl esters also was found in queens at the competition phase, where worker reproduction, aggression, and gyne differentiation occur. The exclusive production of esters by sterile individuals also is typical of Dufour's gland secretion in this species, albeit in this case these are octyl rather than dodecyl esters, and the differences between sterile and fertile individuals are qualitative rather than quantitative. We propose that the labial gland esters provide yet another signal of reproductive inactivity. In virgin queens, it may signal that egg laying is deferred to the next season, while in workers it reinforces the message "I am sterile and out of the reproductive competition". Whether the reduction in dodecyl esters in fertile queens as a function of colony social development is involved in regulating gyne production and the onset of the competition phase is yet to be deciphered.

Chemical ecology of bumble bees

Bumble bees are of major importance, ecologically and economically as pollinators in cool and temperate biomes and as model organisms for scientific research. Chemical signals and cues have been shown to play an outstanding role in intraspecific and interspecific communication systems within and outside of a bumble bee colony. In the present review we compile and critically assess the literature on the chemical ecology of bumble bees, including cuckoo bumble bees. The development of new and more sensitive analytical tools and improvements in sociogenetic methods significantly enhanced our knowledge about chemical compounds that mediate the regulation of reproduction in the social phase of colony development, about the interactions between host bumble bees and their social parasites, about pheromones involved in mating behavior, as well as about the importance of signals, cues and context-dependent learning in foraging behavior. Our review intends to stimulate new studies on the many unresolved questions concerning the chemical ecology of these fascinating insects.

Characterization of neutral lipase BT-1 isolated from the labial gland of Bombus terrestris males

Background

In addition to their general role in the hydrolysis of storage lipids, bumblebee lipases can participate in the biosynthesis of fatty acids that serve as precursors of pheromones used for sexual communication.

Results

We studied the temporal dynamics of lipolytic activity in crude extracts from the cephalic part of Bombus terrestris labial glands. Extracts from 3-day-old males displayed the highest lipolytic activity. The highest lipase gene expression level was observed in freshly emerged bumblebees, and both gene expression and lipase activity were lower in bumblebees older than 3 days. Lipase was purified from labial glands, further characterized and named as BT-1. The B. terrestris orthologue shares 88% sequence identity with B. impatiens lipase HA. The molecular weight of B. terrestris lipase BT-1 was approximately 30 kDa, the pH optimum was 8.3, and the temperature optimum was 50°C. Lipase BT-1 showed a notable preference for C8-C10 p-nitrophenyl esters, with the highest activity toward p-nitrophenyl caprylate (C8). The Michaelis constant (K_m) and maximum reaction rate (V_max) for p-nitrophenyl laurate hydrolysis were K_m = 0.0011 mM and V_max = 0.15 U/mg.

Conclusion

This is the first report describing neutral lipase from the labial gland of B. terrestris. Our findings help increase understanding of its possible function in the labial gland.

The role of desaturases in the biosynthesis of marking pheromones in bumblebee males

Bumblebee males (Hymenoptera) produce species-specific labial gland secretions called marking pheromones (MPs). MPs generally consist of terpenoids and fatty-acid-derived aliphatic compounds with various chain lengths predominantly containing one or no double bonds. The unsaturated fatty-acid-derived MP components were hypothesized to be produced by fatty acid desaturases (FADs) that exhibit diverse substrate specificities. To address this hypothesis, we isolated and functionally characterized FADs from three bumblebee species: Bombus lucorum, Bombus terrestris, and Bombus lapidarius. By employing RNA sequencing of the male labial glands and fat bodies of B. lucorum and B. terrestris, we identified five paralogous FAD-like sequences but only two FAD lineages were abundant and differentially expressed in the labial glands. We found that abundant FAD lineages were also expressed in the labial gland and fat body of Bombus lapidarius. Functional characterization of FADs in a yeast expression system confirmed that Δ4-FADs exhibited a unique Δ4-desaturase activity exclusively on 14-carbon fatty acyls and Δ9-FADs displayed Δ9-desaturase activity on 14- to 18-carbon fatty acyls. These results indicate that Δ9-FADs are involved in the biosynthesis of major unsaturated components of MPs in B. lucorum and B. lapidarius despite the diverse MP composition of these bumblebee species. The contribution of lipases, acyltransferases, esterases, and fatty acid reductases to production of the species-specific MP composition is also discussed in light of the transcriptomic data obtained in this study.

Changes in the composition of triacylglycerols in the fat bodies of bumblebee males during their lifetime

The age-dependent changes in the composition of triacylglycerols (TAG) in the fat bodies of bumblebee males were studied using HPLC/MS. Two related species (Bombus terrestris and B. lucorum) were compared, with the age of the males being 0-30 days. The total amount of TAG in B. lucorum was about 2.7 times higher than that in B. terrestris for all of the ages studied. One to three-day-old males had the highest content of TAG in their fat bodies (1.6-2.3 mg/individual in B. terrestris and 3.8-4.2 mg/individual in B. lucorum). The analytical data show different patterns in both species. The qualitative composition of fatty acids in TAG was similar, but the mean relative abundance between B. terrestris and B. lucorum differed: 14:0, 7 and 14%; 16:0, 20 and 44%; 18:3, 62 and 23%; 18:1, 3 and 8%, respectively (the data is based on a GC/MS integration). A statistical evaluation of the dynamic changes in the TAG composition revealed that in B. terrestris different age classes were well separated according to their TAG composition while in B. lucorum the TAG did not change substantially during the male's life. The TAG analyses provide more precise information on the differences between the classes studied than the FA composition alone.

Pheromone gland development and pheromone production in lutzomyia longipalpis (Diptera: Psychodidae: Phlebotominae)

The sand fly Lutzomyia longipalpis (Lutz & Neiva) (Diptera: Psychodidae: Phlebotominae) is the main vector of American visceral leishmaniasis. Adult males produce a terpenoid sex pheromone that in some cases also acts as male aggregation pheromone. We have analyzed the correlation between male pheromone production levels and pheromone gland cell morphogenesis after adult emergence from pupae. The abdominal tergites of L. longipalpis males were dissected and fixed in glutaraldehyde for transmission electron microscopy, or the pheromone was extracted in analytical grade hexane. Pheromone chemical analysis was carried out at 3- to 6-h intervals during the first 24 h after emergence and continued daily until the seventh day. All extracts were analyzed by gas chromatography. For the morphological analysis, we used insects collected at 0-6, 9-12, 12-14, and 96 h after emergence. Ultrastructural data from 0- to 6-h-old adult males revealed smaller pheromone gland cells with small microvilli at the end apparatus. Lipid droplets and peroxisomes were absent or very rare, but a large number of mitochondria could be seen. Lipid droplets started to appear in the gland cells cytoplasm approximately 9 h after adult emergence, and their number and size increased with age, together with the presence of several peroxisomes, suggesting a role for these organelles in pheromone biosynthesis. At 12-15 h after emergence, the lipid droplets were mainly distributed near the microvilli but were smaller than those in mature older males (4 d old). Pheromone biosynthesis started around 12 h after emergence and increased continuously during the first 3 d, stabilizing thereafter, coinciding with the period when males are more able to attract females.

Comparison of age-dependent quantitative changes in the male labial gland secretion of Bombus terrestris and Bombus lucorum

Age-related changes of antennal-active components of male labial gland extracts were studied in two closely related bumblebee species, Bombus terrestris and B. lucorum. In B. terrestris, compounds eliciting electroantennogram (EAG) responses of virgin queens were ethyl dodecanoate, 2,3-dihydrofarnesal, 2,3-dihydrofarnesol, hexadecan-1-ol, octadeca-9,12,15-trien-1-ol, and geranylcitronellol. Compounds that elicited EAG responses from queens of B. lucorum were ethyl dodecanoate, ethyl tetradec-7-enoate, ethyl tetradec-9-enoate, ethyl hexadec-9-enoate, hexadecan-1-ol, hexadec-7-enal, octadeca-9,12-dien-1-ol, octadeca-9,12,15-trien-1-ol, and octadecan-1-ol. Quantities of these compounds in the labial glands changed significantly over the lifetime of the respective males of the two species. In both species, concentrations of the respective compounds reached their maximum within seven days after eclosion. Subsequently, a rapid decrease in the amount of EAG-active compounds occurred in B. terrestris, whereas in B. lucorum the amount of active compounds stayed approximately constant or decreased at a slow rate. Microscopy showed that in B. terrestris secretory cells of the labial glands undergo apoptosis from the fifth to the tenth day of life, whilst in B. lucorum labial gland cells remain unchanged throughout the life of the males.