Juvenile hormone regulates brain-reproduction tradeoff in bumble bees but not in honey bees

Tracking Existing Factors Directly Affecting the Reproduction of Bumblebees: Current Knowledge

Bumblebees are primary social insects and a vital class of pollinating insects. Their distinctive reproductive mode is characterized by the independent initiation and construction of the nest by the queen and the subsequent production of sufficient workers, males, and gynes following colony development. After successful mating, the queen transitions to the first phase of its annual life cycle. The reproductive processes are directly influenced by environmental factors, including floral resources and pesticides. Moreover, the reproductive level is regulated by biological factors, particularly the role of workers, who participate in egg laying and pass on their genetic material to the next generation of queens. Successful reproduction can only be achieved by maintaining colony development under natural or artificial breeding conditions. Consequently, understanding the known factors that influence bumblebee reproduction is essential for developing conservation strategies for wild bumblebees and for successfully breeding diverse bumblebee species. Breeding various bumblebee species is crucial for in-depth research into known factors and for further exploration of other potential factors, which will also help to meet the demand for pollination in agricultural facilities globally.

Precise Regulation of Juvenile Hormone III R-Stereoisomer Synthesis by Apis mellifera through Specifically Binding Methyl-(2E,6E)-farnesoate and Strictly Controlling Its Titer

Juvenile hormone III (JH III) is a crucial hormone synthesized exclusively as R-stereoisomer in most insects. Herein, we established a mature Tris-HCl culture system for essential biochemical reactions and applied stable instrumental detection methods to analyze JH III, methyl farnesoate (MF) and juvenile hormone acid (JHA) using UPLC-MS/MS. Our results revealed that the R-JH III terminal synthesis pathway in Apis mellifera follows the "esterify then epoxidize" sequence, with precise methyl-(2E,6E)-farnesoate titer regulation and its spatial cis-trans isomerism, achieving selective R-JH III synthesis. Furthermore, we observed that the preferred generation of S/R-JH III chiral enantiomers varied depending on the spatial cis-trans isomerism of different MFs. Our results suggest that S-JH III could theoretically exist in insects, offering a novel perspective for understanding the synthesis mechanism of diverse complex juvenile hormones in different insect species.

Integration of information from multiple sources drives and maintains the division of labor in bumble bee colonies

Bumble bees are eusocial bees in which the division of labor (DoL) in reproduction and in task performance changes during their annual lifecycle. The queen monopolizes reproduction in young colonies, but at later stages, some workers start to challenge the queen and lay their own unfertilized eggs. The division of colony maintenance and growth tasks relates to worker body size. Reproduction and task performance are regulated by multiple social signals of the queen, the workers, and the brood. Here, we review recent studies suggesting that bumble bees use multiple sources of information to establish and maintain DoL in both reproduction and in task performance. Juvenile hormone (JH) is an important neuroendocrine signal involved in the regulation of DoL in reproduction but not in worker task performance. The reliance on multiple signals facilitates flexibility in face of changes in the social and geophysical environment.

Uncovering the role of juvenile hormone in ovary development and egg laying in bumble bees

Juvenile hormone (JH) regulates developmental and physiological processes in insects. In bumble bees, the hormone acts as a gonadotropin that mediates ovary development, but the exact physiological pathways involved in ovary activation and subsequent egg laying are poorly understood. In this study, we examine how queen hibernation state, caste, and species impact the gonadotropic effect of JH in bumble bee queens through methoprene (JH analogue) application. We extend previous research by assessing queen egg laying and colony initiation, alongside ovary development. Furthermore, we compared sensitivity of workers of both species to the juvenile hormone's gonadotropic effect. In both bumble bee species, the ovaries of hibernated queens were developed five to six days after breaking diapause, regardless of methoprene treatment. By contrast, methoprene did have a stimulatory effect on ovary development in non-hibernated queens. The dose needed to obtain this effect was higher in B. impatiens. Methoprene did not have gonadotropic effects in callow workers of both species. These results indicate that the physiological effect of exogenous methoprene application varies according to species, caste and hibernation status. Interestingly, despite gonadotropic effects in non-hibernated queens, oviposition was not accelerated by JH. This suggests that JH alone is insufficient to induce egg laying and that an additional stimulus, which is naturally present in hibernated queens, is required. Consequently, our findings indicate that other physiological processes, beyond a rise in JH alone, are required for oviposition and colony initiation.

Honeybee as a food nutrition analysis model of neural development and gut microbiota

Research on the relationships between the gut microbiota and the neurophysiology and behavior of animals has grown exponentially in just a few years. Insect behavior may be controlled by molecular mechanisms that are partially homologous to those in mammals, and swarming insects may be suitable as experiment models in these types of investigations. All core gut bacteria in honeybees can be cultivated in vitro. Certain gut microflora of bees can be genetically engineered or sterilized and colonized. The bee gut bacteria model is established more rapidly and has a higher flux than other sterile animal models. It may help elucidate the pathogenesis of intestinal diseases and identify effective molecular therapeutic targets against them. In the present review, we focused on the contributions of the honeybee model in learning cognition and microbiome research. We explored the relationship between honeybee behavior and neurodevelopment and the factors determining the mechanisms by which the gut microbiota affects the host. In particular, we concentrated on the correlation between gut microbiota and brain development. Finally, we examined strategies for the effective use of simple animal models in animal cognition and microbiome research.

Positive selection in cytochrome P450 genes is associated with gonad phenotype and mating strategy in social bees

The honey bee, Apis mellifera differs from all other social bees in its gonad phenotype and mating strategy. Honey bee queens and drones have tremendously enlarged gonads, and virgin queens mate with several males. In contrast, in all the other bees, the male and female gonads are small, and the females mate with only one or very few males, thus, suggesting an evolutionary and developmental link between gonad phenotype and mating strategy. RNA-seq comparisons of A. mellifera larval gonads revealed 870 genes as differentially expressed in queens versus workers and drones. Based on Gene Ontology enrichment we selected 45 genes for comparing the expression levels of their orthologs in the larval gonads of the bumble bee Bombus terrestris and the stingless bee, Melipona quadrifasciata, which revealed 24 genes as differentially represented. An evolutionary analysis of their orthologs in 13 solitary and social bee genomes revealed four genes with evidence of positive selection. Two of these encode cytochrome P450 proteins, and their gene trees indicated a lineage-specific evolution in the genus Apis, indicating that cytochrome P450 genes may be involved in the evolutionary association of polyandry and the exaggerated gonad phenotype in social bees.

Convergent and complementary selection shaped gains and losses of eusociality in sweat bees

Sweat bees have repeatedly gained and lost eusociality, a transition from individual to group reproduction. Here we generate chromosome-length genome assemblies for 17 species and identify genomic signatures of evolutionary trade-offs associated with transitions between social and solitary living. Both young genes and regulatory regions show enrichment for these molecular patterns. We also identify loci that show evidence of complementary signals of positive and relaxed selection linked specifically to the convergent gains and losses of eusociality in sweat bees. This includes two pleiotropic proteins that bind and transport juvenile hormone (JH)-a key regulator of insect development and reproduction. We find that one of these proteins is primarily expressed in subperineurial glial cells that form the insect blood-brain barrier and that brain levels of JH vary by sociality. Our findings are consistent with a role of JH in modulating social behaviour and suggest that eusocial evolution was facilitated by alteration of the proteins that bind and transport JH, revealing how an ancestral developmental hormone may have been co-opted during one of life's major transitions. More broadly, our results highlight how evolutionary trade-offs have structured the molecular basis of eusociality in these bees and demonstrate how both directional selection and release from constraint can shape trait evolution.

Efficiency of ant-control agents in colony-level oral toxicity tests using Tetramorium tsushimae (Hymenoptera: Formicidae) for post-establishment control of the red imported fire ant, Solenopsis invicta (Hymenoptera: Formicidae)

The red imported fire ant Solenopsis invicta Buren (Hymenoptera: Formicidae) causes serious damage worldwide as an invasive alien species. The species has expanded its range to the Pacific Rim since 2000s and Japan has faced its multiple introductions since 2017. While colony-level control methods are urgently needed, testing living colonies of the unestablished species is challenging especially due to various restrictions under the COVID-19 pandemic. Here, we proposed alternative long-term toxicity assays using artificial colonies of Tetramorium tsushimae Emery (Hymenoptera: Formicidae), a Japanese native species belonging to the same subfamily (Myrmicinae) as S. invicta. We conducted an acute toxicity test to determine if T. tsushimae is a suitable substitute for S. invicta using fipronil and found the LD₅₀ value in T. tsushimae was close to that in S. invicta. Then, we conducted the long-term toxicity test with fipronil and two insect growth regulators (pyriproxyfen and etoxazole) using artificial colonies of T. tsushimae. All workers and larvae in the fipronil-treated colonies died within 3 days of treatment initiation. Emergence of new workers was observed after 18 days in the etoxazole-treated and control colonies, but not in the pyriproxyfen-treated colonies. We concluded that fipronil was the most promising insecticide for post-establishment control, and pyriproxyfen was effective as a toxic-bait agent for colony-level control.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13355-022-00800-x.

Conserved and convergent mechanisms underlying performance-life-history trade-offs

Phenotypic trade-offs are inevitable in nature, but the mechanisms driving them are poorly understood. Movement and oxygen are essential to all animals, and as such, the common ancestor to all living animals passed on mechanisms to acquire oxygen and contract muscle, sometimes at the expense of other activities or expression of traits. Nevertheless, convergent pathways have also evolved to deal with critical trade-offs that are necessary to survive ubiquitous environmental challenges. We discuss how whole-animal performance traits, such as locomotion, are important to fitness, yet costly, resulting in trade-offs with other aspects of the phenotype via specific conserved and convergent mechanistic pathways across all animals. Specifically, we discuss conserved pathways involved in muscle structure and signaling, insulin/insulin-like signaling, sirtuins, mitochondria and hypoxia-inducible factors, as well as convergent pathways involved in energy regulation, development, reproductive investment and energy storage. The details of these mechanisms are only known from a few model systems, and more comparative studies are needed. We make two main recommendations as a framework for future studies of animal form and function. First, studies of performance should consider the broader life-history context of the organism, and vice versa, as performance expression can require a large portion of acquired resources. Second, studies of life histories or mechanistic pathways that measure performance should do so in meaningful and standardized ways. Understanding proximate mechanisms of phenotypic trade-offs will not only better explain the phenotypes of the organisms we study, but also allow predictions about phenotypic variation at the evolutionary scale.

Bumble Bee (Bombus vosnesenskii) Queen Nest Searching Occurs Independent of Ovary Developmental Status

Studies on the physiological states of wild-caught organisms are essential to uncovering the links between ecological and physiological processes. Bumble bee queens emerge from overwintering in the spring. At this time, queens develop their ovaries and search for a nest site in which to start a colony. Whether these two processes, ovary development and nest-searching, interact with or influence one another remains an unresolved question in behavioral physiology. We explored the hypothesis that ovary development and nest-searching might be mechanistically connected, by testing whether (1) ovary development precedes nest-searching behavior; (2) nest occupation precedes ovary development; or (3) ovary development and nest-searching occur independently, in bumble bee (Bombus vosnesenskii) queens. We collected queens either nest-searching (and thus prior to occupying a nest) or pollen-collecting (and thus provisioning an occupied nest) and measured their degree of ovary activation. We further screened these queens for parasites or other symbionts, to identify additional factors that may impact their reproductive success at this time. We found that queens searched for and occupied nests at all stages of ovary development, indicating that these processes occur independently in this system. Nest-searching queens were more likely to have substantial mite loads than pollen-collecting queens, who had already located and occupied a nest. However, mite loads did not significantly predict ovary developmental status. Collectively, our work shows that nesting status and symbionts alone are insufficient to explain the variation in spring bumble bee queen ovary development. We propose that ovary development and nest-searching occur opportunistically, which may enable queens to begin laying eggs earlier in the season than if these processes occurred in discrete succession.

Krüppel-homologue 1 Mediates Hormonally Regulated Dominance Rank in a Social Bee

Dominance hierarchies are ubiquitous in invertebrates and vertebrates, but little is known on how genes influence dominance rank. Our gaps in knowledge are specifically significant concerning female hierarchies, particularly in insects. To start filling these gaps, we studied the social bumble bee Bombus terrestris, in which social hierarchies among females are common and functionally significant. Dominance rank in this bee is influenced by multiple factors, including juvenile hormone (JH) that is a major gonadotropin in this species. We tested the hypothesis that the JH responsive transcription factor Krüppel homologue 1 (Kr-h1) mediates hormonal influences on dominance behavior. We first developed and validated a perfluorocarbon nanoparticles-based RNA interference protocol for knocking down Kr-h1 expression. We then used this procedure to show that Kr-h1 mediates the influence of JH, not only on oogenesis and wax production, but also on aggression and dominance rank. To the best of our knowledge, this is the first study causally linking a gene to dominance rank in social insects, and one of only a few such studies on insects or on female hierarchies. These findings are important for determining whether there are general molecular principles governing dominance rank across gender and taxa.

Juvenile hormone functions as a metabolic rate accelerator in bumble bees (Bombus terrestris)

Juvenile hormone (JH) is a modulator of many physiological transitions in insects, including molting, metamorphosis, diapause, and reproduction. These processes often include metabolic changes. Here we show that JH accelerates metabolic rate in bumble bees (Bombus terrestris). We reduced JH levels in worker bumble bees by removing their corpora allata (allatectomy) and elevated JH levels in queens through a topical application of JH-III. Natural and high JH levels increased the metabolic rate in both workers and queens and triggered an increased protein turnover rate. Following the treatments, JH also caused an increase in food consumption and a reduction in lipid levels and flight muscle mass of queens, and a reduction in lipids levels in workers. Furthermore, the topical application of a JH analog to queens prior to their diapause caused a decline in their survival of diapause. These findings support the hypothesis that JH acts as a metabolic rate accelerator, initiating a resource shift in bumble bees, and thereby reducing diapause survival in queens. Based on previous studies on JH we suggest that, additional to its behavioral or physiological effects, JH's function as an accelerator of metabolic processes is conserved across different life stages and insect species.

Role of juvenile hormone receptor Methoprene-tolerant 1 in silkworm larval brain development and domestication

The insect brain is the central part of the neurosecretory system, which controls morphology, physiology, and behavior during the insect's lifecycle. Lepidoptera are holometabolous insects, and their brains develop during the larval period and metamorphosis into the adult form. As the only fully domesticated insect, the Lepidoptera silkworm Bombyx mori experienced changes in larval brain morphology and certain behaviors during the domestication process. Hormonal regulation in insects is a key factor in multiple processes. However, how juvenile hormone (JH) signals regulate brain development in Lepidoptera species, especially in the larval stage, remains elusive. We recently identified the JH receptor Methoprene tolerant 1 ( Met1) as a putative domestication gene. How artificial selection on Met1 impacts brain and behavioral domestication is another important issue addressing Darwin's theory on domestication. Here, CRISPR/Cas9-mediated knockout of Bombyx Met1 caused developmental retardation in the brain, unlike precocious pupation of the cuticle. At the whole transcriptome level, the ecdysteroid (20-hydroxyecdysone, 20E) signaling and downstream pathways were overactivated in the mutant cuticle but not in the brain. Pathways related to cell proliferation and specialization processes, such as extracellular matrix (ECM)-receptor interaction and tyrosine metabolism pathways, were suppressed in the brain. Molecular evolutionary analysis and in vitro assay identified an amino acid replacement located in a novel motif under positive selection in B. mori, which decreased transcriptional binding activity. The B. mori MET1 protein showed a changed structure and dynamic features, as well as a weakened co-expression gene network, compared with B. mandarina. Based on comparative transcriptomic analyses, we proposed a pathway downstream of JH signaling (i.e., tyrosine metabolism pathway) that likely contributed to silkworm larval brain development and domestication and highlighted the importance of the biogenic amine system in larval evolution during silkworm domestication.

Independent variations in genome-wide expression, alternative splicing, and DNA methylation in brain tissues among castes of the buff-tailed bumblebee, Bombus terrestris

Caste differentiation in social hymenopterans is an intriguing example of phenotypic plasticity. However, the co-ordination among gene regulatory factors to mediate caste differentiation remains inconclusive. In this study, we determined the role of gene regulation and related epigenetic processes in pre-imaginal caste differentiation in the primitively eusocial bumblebee Bombus terrestris. By combining RNA-Seq data from Illumina and PacBio and accurately quantifying methylation at whole-genomic base pair resolution, we found that queens, workers, and drones mainly differentiate in gene expression but not in alternative splicing and DNA methylation. Gynes are the most distinct with the lowest global level of whole-genomic methylation and with the largest number of caste-specific transcripts and alternative splicing events. By contrast, workers exhibit few uniquely expressed or alternatively spliced genes. Moreover, several genes involved in hormone and neurotransmitter metabolism are related to caste differentiation, whereas several neuropeptides are linked with sex differentiation. Despite little genome-wide association among differential gene expression, splicing, and differential DNA methylation, the overlapped gene ontology (GO) terms point to nutrition-related activity. Therefore, variations in gene regulation correlate with the behavioral differences among castes and highlight the specialization of toolkit genes in bumblebee gynes at the beginning of the adult stage.

Transcriptomic Signatures of Ageing Vary in Solitary and Social Forms of an Orchid Bee

Eusocial insect queens are remarkable in their ability to maximize both fecundity and longevity, thus escaping the typical trade-off between these two traits. Several mechanisms have been proposed to underlie the remolding of the trade-off, such as reshaping of the juvenile hormone (JH) pathway, or caste-specific susceptibility to oxidative stress. However, it remains a challenge to disentangle the molecular mechanisms underlying the remolding of the trade-off in eusocial insects from caste-specific physiological attributes that have subsequently arisen. The socially polymorphic orchid bee Euglossa viridissima represents an excellent model to address the role of sociality per se in longevity as it allows direct comparisons of solitary and social individuals within a common genetic background. We investigated gene expression and JH levels in young and old bees from both solitary and social nests. We found 902 genes to be differentially expressed with age in solitary females, including genes involved in oxidative stress, versus only 100 genes in social dominant females, and 13 genes in subordinate females. A weighted gene coexpression network analysis further highlights pathways related to ageing in this species, including the target of rapamycin pathway. Eleven genes involved in translation, apoptosis, and DNA repair show concurrent age-related expression changes in solitary but not in social females, representing potential differences based on social status. JH titers did not vary with age or social status. Our results represent an important step in understanding the proximate mechanisms underlying the remodeling of the fecundity/longevity trade-off that accompanies the evolutionary transition from solitary life to eusociality.