Support for the reproductive ground plan hypothesis of social evolution and major QTL for ovary traits of Africanized worker honey bees (Apis mellifera L.)

Effects of microRNA-305 knockdown on brain gene expression associated with division of labor in honey bee colonies (Apis mellifera)

Division of labor in honey bee colonies is based on the behavioral maturation of adult workers that involves a transition from working in the hive to foraging. This behavioral maturation is associated with distinct task-related transcriptomic profiles in the brain and abdominal fat body that are related to multiple regulatory factors including juvenile hormone (JH) and queen mandibular pheromone (QMP). A prominent physiological feature associated with behavioral maturation is a loss of abdominal lipid mass as bees transition to foraging. We used transcriptomic and physiological analyses to study whether microRNAs (miRNAs) are involved in the regulation of division of labor. We first identified two miRNAs that showed patterns of expression associated with behavioral maturation, ame-miR-305-5p and ame-miR-375-3p. We then downregulated the expression of these two miRNAs with sequence-specific antagomirs. Neither ame-miR-305-5p nor ame-miR-375-3p knockdown in the abdomen affected abdominal lipid mass on their own. Similarly, knockdown of ame-miR-305-5p in combination with JH or QMP also did not affect lipid mass. By contrast, ame-miR-305-5p knockdown in the abdomen caused substantial changes in gene expression in the brain. Brain gene expression changes included genes encoding transcription factors previously implicated in behavioral maturation. The results of these functional genomic experiments extend previous correlative associations of microRNAs with honey bee division of labor and point to specific roles for ame-miR-305-5p.

Quantitative trait loci mapping for survival of virus infection and virus levels in honey bees

Israeli acute paralysis virus (IAPV) is a highly virulent, Varroa-vectored virus that is of global concern for honey bee health. Little is known about the genetic basis of honey bees to withstand infection with IAPV or other viruses. We set up and analyzed a backcross between preselected honey bee colonies of low and high IAPV susceptibility to identify quantitative trait loci (QTL) associated with IAPV susceptibility. Experimentally inoculated adult worker bees were surveyed for survival and selectively sampled for QTL analysis based on SNPs identified by whole-genome resequencing and composite interval mapping. Additionally, natural titers of other viruses were quantified in the abdomen of these workers via qPCR and also used for QTL mapping. In addition to the full dataset, we analyzed distinct subpopulations of susceptible and non-susceptible workers separately. These subpopulations are distinguished by a single, suggestive QTL on chromosome 6, but we identified numerous other QTL for different abdominal virus titers, particularly in the subpopulation that was not susceptible to IAPV. The pronounced QTL differences between the susceptible and non-susceptible subpopulations indicate either an interaction between IAPV infection and the bees' interaction with other viruses or heterogeneity among workers of a single cohort that manifests itself as IAPV susceptibility and results in distinct subgroups that differ in their interaction with other viruses. Furthermore, our results indicate that low susceptibility of honey bees to viruses can be caused by both, virus tolerance and virus resistance. QTL were partially overlapping among different viruses, indicating a mixture of shared and specific processes that control viruses. Some functional candidate genes are located in the QTL intervals, but their genomic co-localization with numerous genes of unknown function delegates any definite characterization of the underlying molecular mechanisms to future studies.

Societies to genes: can we get there from here?

Understanding the organization and evolution of social complexity is a major task because it requires building an understanding of mechanisms operating at different levels of biological organization from genes to social interactions. I discuss here, a unique forward genetic approach spanning more than 30 years beginning with human-assisted colony-level selection for a single social trait, the amount of pollen honey bees (Apis mellifera L.) store. The goal was to understand a complex social trait from the social phenotype to genes responsible for observed trait variation. The approach combined the results of colony-level selection with detailed studies of individual behavior and physiology resulting in a mapped, integrated phenotypic architecture composed of correlative relationships between traits spanning anatomy, physiology, sensory response systems, and individual behavior that affect individual foraging decisions. Colony-level selection reverse engineered the architecture of an integrated phenotype of individuals resulting in changes in the social trait. Quantitative trait locus (QTL) studies combined with an exceptionally high recombination rate (60 kb/cM), and a phenotypic map, provided a genotype-phenotype map of high complexity demonstrating broad QTL pleiotropy, epistasis, and epistatic pleiotropy suggesting that gene pleiotropy or tight linkage of genes within QTL integrated the phenotype. Gene expression and knockdown of identified positional candidates revealed genes affecting foraging behavior and confirmed one pleiotropic gene, a tyramine receptor, as a target for colony-level selection that was under selection in two different tissues in two different life stages. The approach presented here has resulted in a comprehensive understanding of the structure and evolution of honey bee social organization.

The genomic basis of evolutionary differentiation among honey bees

In contrast to the western honey bee, Apis mellifera, other honey bee species have been largely neglected despite their importance and diversity. The genetic basis of the evolutionary diversification of honey bees remains largely unknown. Here, we provide a genome-wide comparison of three honey bee species, each representing one of the three subgenera of honey bees, namely the dwarf (Apis florea), giant (A. dorsata), and cavity-nesting (A. mellifera) honey bees with bumblebees as an outgroup. Our analyses resolve the phylogeny of honey bees with the dwarf honey bees diverging first. We find that evolution of increased eusocial complexity in Apis proceeds via increases in the complexity of gene regulation, which is in agreement with previous studies. However, this process seems to be related to pathways other than transcriptional control. Positive selection patterns across Apis reveal a trade-off between maintaining genome stability and generating genetic diversity, with a rapidly evolving piRNA pathway leading to genomes depleted of transposable elements, and a rapidly evolving DNA repair pathway associated with high recombination rates in all Apis species. Diversification within Apis is accompanied by positive selection in several genes whose putative functions present candidate mechanisms for lineage-specific adaptations, such as migration, immunity, and nesting behavior.

Reproductive activation in honeybee (Apis mellifera) workers protects against abiotic and biotic stress

Social insect reproductives exhibit exceptional longevity instead of the classic trade-off between somatic maintenance and reproduction. Even normally sterile workers experience a significant increase in life expectancy when they assume a reproductive role. The mechanisms that enable the positive relation between the antagonistic demands of reproduction and somatic maintenance are unclear. To isolate the effect of reproductive activation, honeybee workers were induced to activate their ovaries. These reproductively activated workers were compared to controls for survival and gene expression patterns after exposure to Israeli Acute Paralysis Virus or the oxidative stressor paraquat. Reproductive activation increased survival, indicating better immunity and oxidative stress resistance. After qPCR analysis confirmed our experimental treatments at the physiological level, whole transcriptome analysis revealed that paraquat treatment significantly changed the expression of 1277 genes in the control workers but only two genes in reproductively activated workers, indicating that reproductive activation preemptively protects against oxidative stress. Significant overlap between genes that were upregulated by reproductive activation and in response to paraquat included prominent members of signalling pathways and anti-oxidants known to affect ageing. Thus, while our results confirm a central role of vitellogenin, they also point to other mechanisms to explain the molecular basis of the lack of a cost of reproduction and the exceptional longevity of social insect reproductives. Thus, socially induced reproductive activation preemptively protects honeybee workers against stressors, explaining their longevity. This article is part of the theme issue 'Ageing and sociality: why, when and how does sociality change ageing patterns?'

Association between the Microsatellite Ap243, AC117 and SV185 Polymorphisms and Nosema Disease in the Dark Forest Bee Apis mellifera mellifera

The microsporidian Nosema parasites, primarily Nosema ceranae, remain critical threats to the health of the honey bee Apis mellifera. One promising intervention approach is the breeding of Nosema-resistant honey bee colonies using molecular technologies, for example marker-assisted selection (MAS). For this, specific genetic markers used in bee selection should be developed. The objective of the paper is to search for associations between some microsatellite markers and Nosema disease in a dark forest bee Apis mellifera mellifera. For the dark forest bee, the most promising molecular genetic markers for determining resistance to nosemosis are microsatellite loci AC117, Ap243 and SV185, the alleles of which (“177”, “263” and “269”, respectively) were associated with a low level of Nosema infection. This article is the first associative study aimed at finding DNA loci of resistance to nosemosis in the dark forest bee. Nevertheless, microsatellite markers identified can be used to predict the risk of developing the Nosema disease.

Individual differences in honey bee behavior enabled by plasticity in brain gene regulatory networks

Understanding the regulatory architecture of phenotypic variation is a fundamental goal in biology, but connections between gene regulatory network (GRN) activity and individual differences in behavior are poorly understood. We characterized the molecular basis of behavioral plasticity in queenless honey bee (Apis mellifera) colonies, where individuals engage in both reproductive and non-reproductive behaviors. Using high-throughput behavioral tracking, we discovered these colonies contain a continuum of phenotypes, with some individuals specialized for either egg-laying or foraging and 'generalists' that perform both. Brain gene expression and chromatin accessibility profiles were correlated with behavioral variation, with generalists intermediate in behavior and molecular profiles. Models of brain GRNs constructed for individuals revealed that transcription factor (TF) activity was highly predictive of behavior, and behavior-associated regulatory regions had more TF motifs. These results provide new insights into the important role played by brain GRN plasticity in the regulation of behavior, with implications for social evolution.

Tyramine and its receptor TYR1 linked behavior QTL to reproductive physiology in honey bee workers (Apis mellifera)

Honey bees (Apis mellifera) provide an excellent model for studying how complex social behavior evolves and is regulated. Social behavioral traits such as the division of labor have been mapped to specific genomic regions in quantitative trait locus (QTL) studies. However, relating genomic mapping to gene function and regulatory mechanism remains a big challenge for geneticists. In honey bee workers, division of labor is known to be regulated by reproductive physiology, but the genetic basis of this regulation remains unknown. In this case, QTL studies have identified tyramine receptor 1 (TYR1) as a candidate gene in region pln2, which is associated with multiple worker social traits and reproductive anatomy. Tyramine (TA), a neurotransmitter, regulates physiology and behavior in diverse insect species including honey bees. Here, we examine directly the effects of TYR1 and TA on worker reproductive physiology, including ovariole number, ovary function and the production of vitellogenin (VG, an egg yolk precursor). First, we used a pharmacology approach to demonstrate that TA affects ovariole number during worker larval development and increases ovary maturation during the adult stage. Second, we used a gene knockdown approach to show that TYR1 regulates vg transcription in adult workers. Finally, we estimated correlations in gene expression and propose that TYR1 may regulate vg transcription by coordinating hormonal and nutritional signals. Taken together, our results suggest TYR1 and TA play important roles in regulating worker reproductive physiology, which in turn regulates social behavior. Our study exemplifies a successful forward-genetic strategy going from QTL mapping to gene function.

Caste-based differential transcriptional expression of hexamerins in response to a juvenile hormone analog in the red imported fire ant (Solenopsis invicta)

The reproductive ground plan hypothesis proposes that gene networks regulating foraging behavior and reproductive female physiology in social insects emerged from ancestral gene and endocrine factor networks. Expression of storage proteins such as vitellogenins and hexamerins is an example of this co-option. Hexamerins, through their role modulating juvenile hormone availability, are involved in caste determination in termites. The genome of the fire ant (Solenopsis invicta) encodes four hexamerin genes, hexamerin-like (LOC105192919, hereafter called hexamerin 1), hexamerin (LOC105204474, hereafter called hexamerin 2), arylphorin subunit alpha-like, and arylphorin subunit beta. In this study, a phylogenetic analysis of the S. invicta hexamerins determined that each predicted protein clustered with one of the orthologous Apis mellifera hexamerins. Gene expression analyses by RT-qPCR revealed differential expression of the hexamerins between queens and workers, and between specific task-allocated workers (nurses and foragers). Queens and nurses had significantly higher expression of all genes when compared to foragers. Hexamerin 1 was expressed at higher levels in queens, while hexamerin 2 and arylphorin subunit beta were expressed at significantly higher levels in nurses. Arylphorin subunit alpha-like showed no significant difference in expression between virgin queens and nurses. Additionally, we analyzed the relationship between the expression of hexamerin genes and S-hydroprene, a juvenile hormone analog. Significant changes in hexamerin expression were recorded in nurses, virgin queens, and foragers 12 h after application of the analog. Hexamerin 1 and arylphorin subunit alpha-like expression were significantly lower after analog application in virgin queens. In foragers, hexamerin 2 and arylphorin subunit beta were significantly lower after analog application, while in nurses expression of all genes were significantly lower after analog application. Our results suggest that in S. invicta hexamerin genes could be associated with reproductive division of labor and task-allocation of workers.

The dynamic association between ovariole loss and sterility in adult honeybee workers

In the social insects, ovary state (the presence or absence of mature oocytes) and ovary size (the number of ovarioles) are often used as proxies for the reproductive capacity of an individual worker. Ovary size is assumed to be fixed post-eclosion whereas ovary state is demonstrably plastic post-eclosion. Here, we show that in fact ovary size declines as honeybee workers age. This finding is robust across two honeybee species: Apis mellifera and A. cerana The ovariole loss is likely to be due to the regression of particular ovarioles via programmed cell death. We also provide further support for the observation that honeybee workers with activated ovaries (mature oocytes present) most commonly have five ovarioles rather than a greater or smaller number. This result suggests that workers with more than five ovarioles are unable to physiologically support more than five activated ovarioles and that workers with fewer than five ovarioles are below a threshold necessary for ovary activation. As a worker's ovariole number declines with age, studies on worker ovariole number need to take this plasticity into account.

Integration of lncRNA-miRNA-mRNA reveals novel insights into oviposition regulation in honey bees

BACKGROUND

The honey bee (Apis mellifera) is a highly diverse species commonly used for honey production and pollination services. The oviposition of the honey bee queen affects the development and overall performance of the colony. To investigate the ovary activation and oviposition processes on a molecular level, a genome-wide analysis of lncRNAs, miRNAs and mRNA expression in the ovaries of the queens was performed to screen for differentially expressed coding and noncoding RNAs. Further analysis identified relevant candidate genes or RNAs.

RESULTS

The analysis of the RNA profiles in different oviposition phase of the queens revealed that 740 lncRNAs, 81 miRNAs and 5,481 mRNAs were differently expressed during the ovary activation; 88 lncRNAs, 13 miRNAs and 338 mRNAs were differently expressed during the oviposition inhibition process; and finally, 100 lncRNAs, four miRNAs and 497 mRNAs were differently expressed during the oviposition recovery process. In addition, functional annotation of differentially expressed RNAs revealed several pathways that are closely related to oviposition, including hippo, MAPK, notch, Wnt, mTOR, TGF-beta and FoxO signaling pathways. Furthermore, in the QTL region for ovary size, 73 differentially expressed genes and 14 differentially expressed lncRNAs were located, which are considered as candidate genes affecting ovary size and oviposition. Moreover, a core set of genes served as bridges among different miRNAs were identified through the integrated analysis of lncRNA-miRNA-mRNA network.

CONCLUSION

The observed dramatic expression changes of coding and noncoding RNAs suggest that they may play a critical role in honey bee queens' oviposition. The identified candidate genes for oviposition activation and regulation could serve as a resource for further studies of genetic markers of oviposition in honey bees.

Molecular Evolution of Insect Sociality: An Eco-Evo-Devo Perspective

The evolution of eusociality is a perennial issue in evolutionary biology, and genomic advances have fueled steadily growing interest in the genetic changes underlying social evolution. Along with a recent flurry of research on comparative and evolutionary genomics in different eusocial insect groups (bees, ants, wasps, and termites), several mechanistic explanations have emerged to describe the molecular evolution of eusociality from solitary behavior. These include solitary physiological ground plans, genetic toolkits of deeply conserved genes, evolutionary changes in protein-coding genes, cis regulation, and the structure of gene networks, epigenetics, and novel genes. Despite this proliferation of ideas, there has been little synthesis, even though these ideas are not mutually exclusive and may in fact be complementary. We review available data on molecular evolution of insect sociality and highlight key biotic and abiotic factors influencing social insect genomes. We then suggest both phylogenetic and ecological evolutionary developmental biology (eco-evo-devo) perspectives for a more synthetic view of molecular evolution in insect societies.

Evolution of Social Insect Polyphenism Facilitated by the Sex Differentiation Cascade

The major transition to eusociality required the evolution of a switch to canalize development into either a reproductive or a helper, the nature of which is currently unknown. Following predictions from the ‘theory of facilitated variation’, we identify sex differentiation pathways as promising candidates because of their pre-adaptation to regulating development of complex phenotypes. We show that conserved core genes, including the juvenile hormone-sensitive master sex differentiation gene doublesex (dsx) and a krüppel homolog 2 (kr-h2) with putative regulatory function, exhibit both sex and morph-specific expression across life stages in the ant Cardiocondyla obscurior. We hypothesize that genes in the sex differentiation cascade evolved perception of alternative input signals for caste differentiation (i.e. environmental or genetic cues), and that their inherent switch-like and epistatic behavior facilitated signal transfer to downstream targets, thus allowing them to control differential development into morphological castes.

Division of labor into reproductive queens and helper workers in the societies of ants, bees and wasps is achieved by phenotypic plasticity, which allows individuals to embark on discrete developmental trajectories in response to variable signals. These signals can be genetic, epigenetic or environmental, thereby resembling the extreme variation in signals for sex determination across multicellular animals. We show that common developmental pathways downstream of these input signals, including the conserved sex differentiation gene doublesex, regulate sex and caste-specific phenotypic differentiation in the ant species Cardiocondyla obscurior. Many different mechanisms of gene regulation have been implicated in controlling caste-specific development in social insects but these all depend on a higher-level genetic switch. We propose that highly conserved hub genes such as dsx, which can translate variable input signals into large transcription differences using intermediate-level regulators, are tightly linked with the repeated evolutionary transition to eusociality and caste polyphenism.

Climbing the social ladder: the molecular evolution of sociality

Genomic tools are allowing us to dissect the roles of genes and genetic architecture in social evolution, and eusocial insects are excellent models. Numerous hypotheses for molecular evolution of eusociality have been proposed, ranging from regulatory shifts in 'old' genes to rapid evolution of 'new' genes. A broad model to explain this major transition in evolution has been lacking. We provide a synthetic framework centered on the idea that different evolutionary processes dominate during different transitional stages, beginning with changes in gene regulation and culminating in novel genes later on. By considering multiple mechanisms as we 'climb the social ladder', we can test whether the transitions from solitary to simple sociality to complex sociality represent incremental changes or genetic revolutions.

Genomic correlates of recombination rate and its variability across eight recombination maps in the western honey bee (Apis mellifera L.)

Background

Meiotic recombination has traditionally been explained based on the structural requirement to stabilize homologous chromosome pairs to ensure their proper meiotic segregation. Competing hypotheses seek to explain the emerging findings of significant heterogeneity in recombination rates within and between genomes, but intraspecific comparisons of genome-wide recombination patterns are rare. The honey bee (Apis mellifera) exhibits the highest rate of genomic recombination among multicellular animals with about five cross-over events per chromatid.

Results

Here, we present a comparative analysis of recombination rates across eight genetic linkage maps of the honey bee genome to investigate which genomic sequence features are correlated with recombination rate and with its variation across the eight data sets, ranging in average marker spacing ranging from 1 Mbp to 120 kbp. Overall, we found that GC content explained best the variation in local recombination rate along chromosomes at the analyzed 100 kbp scale. In contrast, variation among the different maps was correlated to the abundance of microsatellites and several specific tri- and tetra-nucleotides.

Conclusions

The combined evidence from eight medium-scale recombination maps of the honey bee genome suggests that recombination rate variation in this highly recombining genome might be due to the DNA configuration instead of distinct sequence motifs. However, more fine-scale analyses are needed. The empirical basis of eight differing genetic maps allowed for robust conclusions about the correlates of the local recombination rates and enabled the study of the relation between DNA features and variability in local recombination rates, which is particularly relevant in the honey bee genome with its exceptionally high recombination rate.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1281-2) contains supplementary material, which is available to authorized users.

Genetic architecture of a hormonal response to gene knockdown in honey bees

Variation in endocrine signaling is proposed to underlie the evolution and regulation of social life histories, but the genetic architecture of endocrine signaling is still poorly understood. An excellent example of a hormonally influenced set of social traits is found in the honey bee (Apis mellifera): a dynamic and mutually suppressive relationship between juvenile hormone (JH) and the yolk precursor protein vitellogenin (Vg) regulates behavioral maturation and foraging of workers. Several other traits cosegregate with these behavioral phenotypes, comprising the pollen hoarding syndrome (PHS) one of the best-described animal behavioral syndromes. Genotype differences in responsiveness of JH to Vg are a potential mechanistic basis for the PHS. Here, we reduced Vg expression via RNA interference in progeny from a backcross between 2 selected lines of honey bees that differ in JH responsiveness to Vg reduction and measured JH response and ovary size, which represents another key aspect of the PHS. Genetic mapping based on restriction site-associated DNA tag sequencing identified suggestive quantitative trait loci (QTL) for ovary size and JH responsiveness. We confirmed genetic effects on both traits near many QTL that had been identified previously for their effect on various PHS traits. Thus, our results support a role for endocrine control of complex traits at a genetic level. Furthermore, this first example of a genetic map of a hormonal response to gene knockdown in a social insect helps to refine the genetic understanding of complex behaviors and the physiology that may underlie behavioral control in general.

At the brink of eusociality: transcriptomic correlates of worker behaviour in a small carpenter bee

Background

There is great interest in understanding the genomic underpinnings of social evolution, in particular, the evolution of eusociality (caste-containing societies with non-reproductives that care for siblings). Subsociality is a key precursor for the evolution of eusociality and characterized by prolonged parental care and parent-offspring interaction. Here, we provide the first transcriptomic data for the small carpenter bee, Ceratina calcarata. This species is of special interest because it is subsocial and in the same family as the highly eusocial honey bee, Apis mellifera. In addition, some C. calcarata females demonstrate alloparental care without reproduction, which provides a unique opportunity to study worker behaviour in a non-eusocial species.

Results

We uncovered similar gene expression patterns related to maternal care and sibling care in different groups of females. This agrees with the maternal heterochrony hypothesis, specifically, that changes in timing of offspring care gene expression are related to worker behaviour in incipient insect societies. In addition, we also detected some similarity to caste-related gene expression patterns in highly eusocial honey bees, and uncovered large lifetime changes in gene expression that accompany shifts in reproductive and maternal care behaviour.

Conclusions

For Ceratina calcarata, we found that transcript expression profiles were most similar between sibling care and maternal care females. The maternal care behaviour exhibited post-reproductively by Ceratina mothers is concordant in terms of transcript expression with the alloparental care exhibited by workers. In line with theoretical predictions, our data are consistent with the maternal heterochrony hypothesis for the evolutionary development of worker behaviour in subsocial bees.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-014-0260-6) contains supplementary material, which is available to authorized users.

Queen and young larval pheromones impact nursing and reproductive physiology of honey bee (Apis mellifera) workers

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 (eβ), 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 eβ 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 eβ 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 eβ, providing additional links between nursing and reproductive physiology in support of the reproductive ground plan hypothesis.

The Architecture of the Pollen Hoarding Syndrome in Honey Bees: Implications for Understanding Social Evolution, Behavioral Syndromes, and Selective Breeding

Social evolution has influenced every aspect of contemporary honey bee biology, but the details are difficult to reconstruct. The reproductive ground plan hypothesis of social evolution proposes that central regulators of the gonotropic cycle of solitary insects have been coopted to coordinate social complexity in honey bees, such as the division of labor among workers. The predicted trait associations between reproductive physiology and social behavior have been identified in the context of the pollen hoarding syndrome, a larger suite of interrelated traits. The genetic architecture of this syndrome is characterized by a partially overlapping genetic architecture with several consistent, pleiotropic QTL. Despite these central QTL and an integrated hormonal regulation, separate aspects of the pollen hoarding syndrome may evolve independently due to peripheral QTL and additionally segregating genetic variance. The characterization of the pollen hoarding syndrome has also demonstrated that this syndrome involves many non-behavioral traits, which may be the case for numerous "behavioral" syndromes. Furthermore, the genetic architecture of the pollen hoarding syndrome has implications for breeding programs for improving honey health and other desirable traits: If these traits are comparable to the pollen hoarding syndrome, consistent pleiotropic QTL will enable marker assisted selection, while sufficient additional genetic variation may permit the dissociation of trade-offs for efficient multiple trait selection.

Genome-wide mapping of gene-microbiota interactions in susceptibility to autoimmune skin blistering

Susceptibility to chronic inflammatory diseases is determined by immunogenetic and environmental risk factors. Resident microbial communities often differ between healthy and diseased states, but whether these differences are of primary aetiological importance or secondary to the altered inflammatory environment remains largely unknown. Here we provide evidence for host gene–microbiota interactions contributing to disease risk in a mouse model of epidermolysis bullosa acquisita, an autoantibody-induced inflammatory skin disease. Using an advanced intercross, we identify genetic loci contributing to skin microbiota variability, susceptibility to skin blistering and their overlap. Furthermore, by treating bacterial species abundances as covariates with disease we reveal a novel disease locus. The majority of the identified covariate taxa are characterized by reduced abundance being associated with increased disease risk, providing evidence of a primary role in protection from disease. Further characterization of these putative probiotic species or species assemblages offers promising potential for preventative and therapeutic treatment development.

The pathogenesis of inflammatory disorders afflicting the skin is multifactorial. Srinivas et al. show that diversity of the skin microbiota is a critical factor determining the susceptibility to epidermolysis bullosa acquisita, a chronic mucocutaneous autoimmune skin blistering disease.

Reproduction, social behavior, and aging trajectories in honeybee workers

While a negative correlation between reproduction and life span is commonly observed, specialized reproductive individuals outlive their non-reproductive nestmates in all eusocial species, including the honeybee, Apis mellifera (L). The consequences of reproduction for individual life expectancy can be studied directly by comparing reproductive and non-reproductive workers. We quantified the life span consequences of reproduction in honeybee workers by removal of the queen to trigger worker reproduction. Furthermore, we observed the social behavior of large cohorts of workers under experimental and control conditions to test for associations with individual life expectancy. Worker life expectancy was moderately increased by queen removal. Queenless colonies contained a few long-lived workers, and oviposition behavior was associated with a strong reduction in mortality risk, indicating that a reproductive role confers a significant survival advantage. This finding is further substantiated by an association between brood care behavior and worker longevity that depends on the social environment. In contrast, other in-hive activities, such as fanning, trophallaxis, and allogrooming did not consistently affect worker life expectancy. The influence of foraging varied among replicates. An earlier age of transitioning from in-hive tasks to outside foraging was always associated with shorter life spans, in accordance with previous studies. In sum, our studies quantify how individual mortality is affected by particular social roles and colony environments and demonstrate interactions between the two. The exceptional, positive association between reproduction and longevity in honeybees extends to within-caste plasticity, which may be exploited for mechanistic studies.

Sequence and expression characteristics of long noncoding RNAs in honey bee caste development--potential novel regulators for transgressive ovary size

Division of labor in social insect colonies relies on a strong reproductive bias that favors queens. Although the ecological and evolutionary success attained through caste systems is well sketched out in terms of ultimate causes, the molecular and cellular underpinnings driving the development of caste phenotypes are still far from understood. Recent genomics approaches on honey bee developmental biology revealed a set of genes that are differentially expressed genes in larval ovaries and associated with transgressive ovary size in queens and massive cell death in workers. Amongst these, two contigs called special attention, both being over 200 bp in size and lacking apparent coding potential. Herein, we obtained their full cDNA sequences. These and their secondary structure characteristics placed in evidence that they are bona fide long noncoding RNAs (lncRNA) differentially expressed in larval ovaries, thus named lncov1 and lncov2. Genomically, both map within a previously identified QTL on chromosome 11, associated with transgressive ovary size in honey bee workers. As lncov1 was over-expressed in worker ovaries we focused on this gene. Real-time qPCR analysis on larval worker ovaries evidenced an expression peak coinciding with the onset of autophagic cell death. Cellular localization analysis through fluorescence in situ hybridization revealed perinuclear spots resembling omega speckles known to regulate trafficking of RNA-binding proteins. With only four lncRNAs known so far in honey bees, two expressed in the ovaries, these findings open a novel perspective on regulatory factors acting in the fine tuning of developmental processes underlying phenotypic plasticity related to social life histories.

Behavioural syndromes and social insects: personality at multiple levels

Animal personalities or behavioural syndromes are consistent and/or correlated behaviours across two or more situations within a population. Social insect biologists have measured consistent individual variation in behaviour within and across colonies for decades. The goal of this review is to illustrate the ways in which both the study of social insects and of behavioural syndromes has overlapped, and to highlight ways in which both fields can move forward through the synergy of knowledge from each. Here we, (i) review work to date on behavioural syndromes (though not always referred to as such) in social insects, and discuss mechanisms and fitness effects of maintaining individual behavioural variation within and between colonies; (ii) summarise approaches and principles from studies of behavioural syndromes, such as trade-offs, feedback, and statistical methods developed specifically to study behavioural consistencies and correlations, and discuss how they might be applied specifically to the study of social insects; (iii) discuss how the study of social insects can enhance our understanding of behavioural syndromes-research in behavioural syndromes is beginning to explore the role of sociality in maintaining or developing behavioural types, and work on social insects can provide new insights in this area; and (iv) suggest future directions for study, with an emphasis on examining behavioural types at multiple levels of organisation (genes, individuals, colonies, or groups of individuals).

Multiple mating but not recombination causes quantitative increase in offspring genetic diversity for varying genetic architectures

Explaining the evolution of sex and recombination is particularly intriguing for some species of eusocial insects because they display exceptionally high mating frequencies and genomic recombination rates. Explanations for both phenomena are based on the notion that both increase colony genetic diversity, with demonstrated benefits for colony disease resistance and division of labor. However, the relative contributions of mating number and recombination rate to colony genetic diversity have never been simultaneously assessed. Our study simulates colonies, assuming different mating numbers, recombination rates, and genetic architectures, to assess their worker genotypic diversity. The number of loci has a strong negative effect on genotypic diversity when the allelic effects are inversely scaled to locus number. In contrast, dominance, epistasis, lethal effects, or limiting the allelic diversity at each locus does not significantly affect the model outcomes. Mating number increases colony genotypic variance and lowers variation among colonies with quickly diminishing returns. Genomic recombination rate does not affect intra- and inter-colonial genotypic variance, regardless of mating frequency and genetic architecture. Recombination slightly increases the genotypic range of colonies and more strongly the number of workers with unique allele combinations across all loci. Overall, our study contradicts the argument that the exceptionally high recombination rates cause a quantitative increase in offspring genotypic diversity across one generation. Alternative explanations for the evolution of high recombination rates in social insects are therefore needed. Short-term benefits are central to most explanations of the evolution of multiple mating and high recombination rates in social insects but our results also apply to other species.

Genetics of reproduction and regulation of honeybee (Apis mellifera L.) social behavior

Honeybees form complex societies with a division of labor for reproduction, nutrition, nest construction and maintenance, and defense. How does it evolve? Tasks performed by worker honeybees are distributed in time and space. There is no central control over behavior and there is no central genome on which selection can act and effect adaptive change. For 22 years, we have been addressing these questions by selecting on a single social trait associated with nutrition: the amount of surplus pollen (a source of protein) that is stored in the combs of the nest. Forty-two generations of selection have revealed changes at biological levels extending from the society down to the level of the gene. We show how we constructed this vertical understanding of social evolution using behavioral and anatomical analyses, physiology, genetic mapping, and gene knockdowns. We map out the phenotypic and genetic architectures of food storage and foraging behavior and show how they are linked through broad epistasis and pleiotropy affecting a reproductive regulatory network that influences foraging behavior. This is remarkable because worker honeybees have reduced reproductive organs and are normally sterile; however, the reproductive regulatory network has been co-opted for behavioral division of labor.

Complex pleiotropy characterizes the pollen hoarding syndrome in honey bees (Apis mellifera L.)

The pollen hoarding syndrome consists of a large suite of correlated traits in honey bees that may have played an important role in colony organization and consequently the social evolution of honey bees. The syndrome was first discovered in two strains that have been artificially selected for high and low pollen hoarding. These selected strains are used here to further investigate the phenotypic and genetic links between two central aspects of the pollen hoarding syndrome, sucrose responsiveness and pollen hoarding. Sons of hybrid queen offspring of these two strains were tested for sucrose responsiveness and used to produce colonies with either a highly responsive or an unresponsive father. These two colony groups differed significantly in the amount of pollen stored on brood combs and with regards to their relationship between brood and pollen amounts. Additionally, four quantitative trait loci (QTL) for pollen hoarding behavior were assessed for their effect on sucrose responsiveness. Drone offspring of two hybrid queens were phenotyped for responsiveness and genotyped at marker loci for these QTL, identifying some pleiotropic effects of the QTL with significant QTL interactions. Both experiments thus provided corroborating evidence that the distinct traits of the pollen hoarding syndrome are mechanistically and genetically linked, and that these links are complex and dependent on background genotype. The study demonstrates genetic worker-drone correlations within the context of the pollen hoarding syndrome and establishes that an indirect selection response connects pollen hoarding and sucrose responsiveness, regardless of which trait is directly selected.

Pleiotropy of segregating genetic variants that affect honey bee worker life expectancy

In contrast to many other complex traits, the natural genetic architecture of life expectancy has not been intensely studied, particularly in non-model organisms, such as the honey bee (Apis mellifera L.). Multiple factors that determine honey bee worker lifespan have been identified and genetic analyses have been performed on some of those traits. Several of the traits are included in a suite of correlated traits that form the pollen hoarding syndrome, which was named after the behavior to store surplus pollen in the nest and is tied to social evolution. Here, seven quantitative trait loci that had previously been identified for their effects on different aspects of the pollen hoarding syndrome were studied for their genetic influence on the survival of adult honey bee workers. To gain a more comprehensive understanding of the genetic architecture of worker longevity, a panel of 280 additional SNP markers distributed across the genome was also tested. Allelic distributions were compared between young and old bees in two backcross populations of the bi-directionally selected high- and low-pollen hoarding strain. Our results suggest a pleiotropic effect of at least one of the behavioral quantitative trait loci on worker longevity and one significant and several other putative genetic effects in other genomic regions. At least one locus showed evidence for strong antagonistic pleiotropy and several others suggested genetic factors that influence pre-emergence survival of worker honey bees. Thus, the predicted association between worker lifespan and the pollen hoarding syndrome was supported at the genetic level and the magnitude of the identified effects also strengthened the view that naturally segregating genetic variation can have major effects on age-specific survival probability in the wild.

Regulation of behaviorally associated gene networks in worker honey bee ovaries

Several lines of evidence support genetic links between ovary size and division of labor in worker honey bees. However, it is largely unknown how ovaries influence behavior. To address this question, we first performed transcriptional profiling on worker ovaries from two genotypes that differ in social behavior and ovary size. Then, we contrasted the differentially expressed ovarian genes with six sets of available brain transcriptomes. Finally, we probed behavior-related candidate gene networks in wild-type ovaries of different sizes. We found differential expression in 2151 ovarian transcripts in these artificially selected honey bee strains, corresponding to approximately 20.3% of the predicted gene set of honey bees. Differences in gene expression overlapped significantly with changes in the brain transcriptomes. Differentially expressed genes were associated with neural signal transmission (tyramine receptor, TYR) and ecdysteroid signaling; two independently tested nuclear hormone receptors (HR46 and ftz-f1) were also significantly correlated with ovary size in wild-type bees. We suggest that the correspondence between ovary and brain transcriptomes identified here indicates systemic regulatory networks among hormones (juvenile hormone and ecdysteroids), pheromones (queen mandibular pheromone), reproductive organs and nervous tissues in worker honey bees. Furthermore, robust correlations between ovary size and neuraland endocrine response genes are consistent with the hypothesized roles of the ovaries in honey bee behavioral regulation.

Cross-species correlation between queen mating numbers and worker ovary sizes suggests kin conflict may influence ovary size evolution in honeybees

During social evolution, the ovary size of reproductively specialized honey bee queens has dramatically increased while their workers have evolved much smaller ovaries. However, worker division of labor and reproductive competition under queenless conditions are influenced by worker ovary size. Little comparative information on ovary size exists in the different honey bee species. Here, we report ovariole numbers of freshly dissected workers from six Apis species from two locations in Southeast Asia. The average number of worker ovarioles differs significantly among species. It is strongly correlated with the average mating number of queens, irrespective of body size. Apis dorsata, in particular, is characterized by numerous matings and very large worker ovaries. The relation between queen mating number and ovary size across the six species suggests that individual selection via reproductive competition plays a role in worker ovary size evolution. This indicates that genetic diversity, generated by multiple mating, may bear a fitness cost at the colony level.