Specific developmental gene silencing in the honey bee using a homeobox motif

Reminiscences on the honeybee genome project and the rise of epigenetic concepts in insect science

The sequencing of the honeybee genome in 2006 was an important technological and logistic achievement experience. But what benefits have flown from the honeybee genome project? What does the annotated genomic assembly mean for the study of behavioural complexity and organismal function in honeybees? Here, I discuss several lines of research that have arisen from this project and highlight the rapidly expanding studies on insect epigenomics, emergent properties of royal jelly, the mechanism of nutritional control of development and the contribution of epigenomic regulation to the evolution of sociality. I also argue that the term 'insect epigenetics' needs to be carefully redefined to reflect the diversity of epigenomic toolkits in insects and the impact of lineage-specific innovations on organismal outcomes. The honeybee genome project helped pioneer advances in social insect molecular biology, and fuelled breakthrough research into the role of flexible epigenomic control systems in linking genotype to phenotype.

The function and evolution of a genetic switch controlling sexually dimorphic eye differentiation in honeybees

Animals develop sex-specific morphological structures that are diverse between organisms. However, understanding the developmental and evolutionary mechanisms governing these traits is still limited and largely restricted to DM domain genes, which are conserved, sex-specific developmental regulators identified in genetic models. Here, we report a sex-specific developmental regulator gene, glubschauge (glu) that selectively regulates sexually dimorphic eye differentiation in honeybees. We found that the sex determination gene feminizer (fem) controls sex-specific splicing of glu transcripts, establishing a genetic switch in which Glu proteins with a zinc finger (ZnF) domain are only expressed in females. We showed that female coding sequence was essential and sufficient for partial feminization. Comparative sequence and functional studies revealed that the evolutionary origination of the genetic switch was followed by the mutational origin of the essential ZnF domain. Our results demonstrate that glu is a newly evolved sex-specific genetic switch for region-specific regulation of a dimorphic character.

Highly efficient site-specific integration of DNA fragments into the honeybee genome using CRISPR/Cas9

Functional genetic studies in honeybees have been limited to transposon mediated transformation and site directed mutagenesis tools. However, site- and sequence-specific manipulations that insert DNA fragments or replace sequences at specific target sites are lacking. Such tools would enable the tagging of proteins, the expression of reporters and site-specific amino acid changes, which are all gold standard manipulations for physiological, organismal, and genetic studies. However, such manipulations must be very efficient in honeybees since screening and crossing procedures are laborious due to their social organization. Here, we report an accurate and remarkably efficient site-specific integration of DNA-sequences into the honeybee genome using clustered regularly interspaced short palindromic repeat/clustered regularly interspaced short palindromic repeat-associated protein 9-mediated homology-directed repair. We employed early embryonic injections and selected a highly efficient sgRNA in order to insert 294 and 729 bp long DNA sequences into a specific locus at the dsx gene. These sequences were locus-specifically integrated in 57% and 59% of injected bees. Most importantly, 21% and 25% of the individuals lacked the wildtype sequence demonstrating that we generated homozygous mutants in which all cells are affected (no mosaicism). The highly efficient, locus-specific insertions of nucleotide sequences generating homozygous mutants demonstrate that systematic molecular studies for honeybees are in hand that allow somatic mutation approaches via workers or studies in the next generation using queens with their worker progeny. The employment of early embryonic injections and screenings of highly efficient sgRNAs may offer the prospect of highly successful sequence- and locus-specific mutations also in other organisms.

A simple method for ex vivo honey bee cell culture capable of in vitro gene expression analysis

Cultured cells are a very powerful tool for investigating biological events in vitro; therefore, cell lines have been established not only in model insect species, but also in non-model species. However, there are few reports on the establishment of stable cell lines and development of systems to introduce genes into the cultured cells of the honey bee (Apis mellifera). We describe a simple ex vivo cell culture system for the honey bee. Hemocyte cells obtained from third and fourth instar larvae were cultured in commercial Grace’s insect medium or MGM-450 insect medium for more than two weeks maintaining a normal morphology without deterioration. After an expression plasmid vector bearing the enhanced green fluorescent protein (egfp) gene driven by the immediate early 2 (IE2) viral promoter was transfected into cells, EGFP fluorescence was detected in cells for more than one week from one day after transfection. Furthermore, double-stranded RNA corresponding to a part of the egfp gene was successfully introduced into cells and interfered with egfp gene expression. A convenient and reproducible method for an ex vivo cell culture that is fully practicable for gene expression assays was established for the honey bee.

Debugging: Strategies and Considerations for Efficient RNAi-Mediated Control of the Whitefly Bemisia tabaci

The whitefly Bemisia tabaci is a globally important pest that is difficult to control through insecticides, transgenic crops, and natural enemies. Post-transcriptional gene silencing through RNA interference (RNAi) has shown potential as a pest management strategy against B. tabaci. While genomic data and other resources are available to create highly effective customizable pest management strategies with RNAi, current applications do not capitalize on species-specific biology. This lack of specificity has the potential to have substantial ecological impacts. Here, we discuss both short- and long-term considerations for sustainable RNAi pest management strategies for B. tabaci, focusing on the need for species specificity incorporating both life history and population genetic considerations. We provide a conceptual framework for selecting sublethal target genes based on their involvement in physiological pathways, which has the greatest potential to ameliorate unintended negative consequences. We suggest that these considerations allow an integrated pest management approach, with fewer negative ecological impacts and reduced likelihood of the evolution of resistant populations.

RNAi in Insects: A Revolution in Fundamental Research and Pest Control Applications

In this editorial for the Special Issue on 'RNAi in insect pest control', three important applications of RNA interference (RNAi) in insects are briefly discussed and linked to the different studies published in this Special Issue. The discovery of the RNAi mechanism revolutionized entomological research, as it presented researchers with a tool to knock down genes, which is easily applicable in a wide range of insect species. Furthermore, RNAi also provides crop protection with a novel and promising pest control mode-of-action. The sequence-dependent nature allows RNAi-based control strategies to be highly species selective and the active molecule, a natural biological molecule known as double-stranded RNA (dsRNA), has a short environmental persistence. However, more research is needed to investigate different cellular and physiological barriers, such as cellular uptake and dsRNA degradation in the digestive system in insects, in order to provide efficient control methods against a wide range of insect pest species. Finally, the RNAi pathway is an important part of the innate antiviral immune defence of insects, and could even lead to applications targeting viruses in beneficial insects such as honeybees in the future.

A high quality method for hemolymph collection from honeybee larvae

The drastic decline of bees is associated with several factors, including the immune system suppression due to the increased exposure to pesticides. A widely used method to evaluate these effects on these insects' immune systems is the counting of circulating hemocytes in the hemolymph. However, the extraction of hemolymph from larvae is quite difficult, and the collected material is frequently contaminated with other tissues and gastrointestinal fluids, which complicates counting. Therefore, the present work established a high quality and easily reproducible method of extracting hemolymph from honeybee larvae (Apis mellifera), the extraction with ophthalmic scissors. Extraction methods with the following tools also were tested: 30G needle, fine-tipped forceps, hypodermic syringe, and capillaries tubes. The hemolymph was obtained via an incision on the larvae’s right side for all methods, except for the extraction with ophthalmic scissors, in which the hemolymph was extracted from the head region. To assess the purity of the collected material, turbidity analyses of the samples using a turbidimeter were proposed, tested, and evaluated. The results showed that the use of ophthalmic scissors provided the clearest samples and was free from contamination. A reference range between 22,432.35 and 24,504.87 NTU (nephelometric turbidity units) was established, in which the collected samples may be considered of high quality and free from contamination.

Ferritin RNA interference inhibits the formation of iron granules in the trophocytes of worker honey bees (Apis mellifera)

Iron granules containing superparamagnetic magnetite act as magnetoreceptor for magnetoreception in honey bees. Biomineralization of iron granules occurs in the iron deposition vesicles of trophocytes and requires the participation of actin, myosin, ferritin2, and ATP synthase. The mechanism of magnetoreception in honey bees can be explored by suppressing the formation of iron granules. Toward this goal, we injected double-stranded RNA of ferritin2 and ferritin1 into newly emerged worker honey bees to knock down these genes via RNA interference. We confirmed that mRNA and protein production of the ferritins was inhibited, leading to immature iron granules. Downregulating ferritin2 and ferritin1, moreover, leads to different deposition morphology of 7.5-nm diameter iron particles, indicating that the two genes play different roles in the formation of iron granules in worker honey bees.

High-Efficiency CRISPR/Cas9-Mediated Gene Editing in Honeybee (Apis mellifera) Embryos

The honeybee (Apis mellifera) is an important insect pollinator of wild flowers and crops, playing critical roles in the global ecosystem. Additionally, the honeybee serves as an ideal social insect model. Therefore, functional studies on honeybee genes are of great interest. However, until now, effective gene manipulation methods have not been available in honeybees. Here, we reported an improved CRISPR/Cas9 gene-editing method by microinjecting sgRNA and Cas9 protein into the region of zygote formation within 2 hr after queen oviposition, which allows one-step generation of biallelic knockout mutants in honeybee with high efficiency. We first targeted the Mrjp1 gene. Two batches of honeybee embryos were collected and injected with Mrjp1 sgRNA and Cas9 protein at the ventral cephalic side and the dorsal posterior side of the embryos, respectively. The gene-editing rate at the ventral cephalic side was 93.3%, which was much higher than that (11.8%) of the dorsal-posterior-side injection. To validate the high efficiency of our honeybee gene-editing system, we targeted another gene, Pax6, and injected Pax6 sgRNA and Cas9 protein at the ventral cephalic side in the third batch. A 100% editing rate was obtained. Sanger sequencing of the TA clones showed that 73.3% (for Mrjp1) and 76.9% (for Pax6) of the edited current-generation embryos were biallelic knockout mutants. These results suggest that the CRISPR/Cas9 method we established permits one-step biallelic knockout of target genes in honeybee embryos, thereby demonstrating an efficient application to functional studies of honeybee genes. It also provides a useful reference to gene editing in other insects with elongated eggs.

A genetic switch for worker nutrition-mediated traits in honeybees

Highly social insects are characterized by caste dimorphism, with distinct size differences of reproductive organs between fertile queens and the more or less sterile workers. An abundance of nutrition or instruction via diet-specific compounds has been proposed as explanations for the nutrition-driven queen and worker polyphenism. Here, we further explored these models in the honeybee (Apis mellifera) using worker nutrition rearing and a novel mutational screening approach using the clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) method. The worker nutrition-driven size reduction of reproductive organs was restricted to the female sex, suggesting input from the sex determination pathway. Genetic screens on the sex determination genes in genetic females for size polyphenism revealed that doublesex (dsx) mutants display size-reduced reproductive organs irrespective of the sexual morphology of the organ tissue. In contrast, feminizer (fem) mutants lost the response to worker nutrition-driven size control. The first morphological worker mutants in honeybees demonstrate that the response to nutrition relies on a genetic program that is switched “ON” by the fem gene. Thus, the genetic instruction provided by the fem gene provides an entry point to genetically dissect the underlying processes that implement the size polyphenism.

In honeybees, nutrition drives dimorphic size development of reproductive organs in fertile queens and sterile workers. A study using the first induced morphological mutants in honeybees demonstrates that this developmental plasticity requires a genetic program that is switched on by the “feminizer” gene.

In honeybees, nutrition drives dimorphic size development of reproductive organs in fertile queens and sterile workers. The first induced morphological mutants in honeybees demonstrate that this developmental plasticity requires a genetic program that is switched “ON” by the feminizer (fem) gene.

Honey Bee and Bumble Bee Antiviral Defense

Bees are important plant pollinators in both natural and agricultural ecosystems. Managed and wild bees have experienced high average annual colony losses, population declines, and local extinctions in many geographic regions. Multiple factors, including virus infections, impact bee health and longevity. The majority of bee-infecting viruses are positive-sense single-stranded RNA viruses. Bee-infecting viruses often cause asymptomatic infections but may also cause paralysis, deformity or death. The severity of infection is governed by bee host immune responses and influenced by additional biotic and abiotic factors. Herein, we highlight studies that have contributed to the current understanding of antiviral defense in bees, including the Western honey bee (Apis mellifera), the Eastern honey bee (Apis cerana) and bumble bee species (Bombus spp.). Bee antiviral defense mechanisms include RNA interference (RNAi), endocytosis, melanization, encapsulation, autophagy and conserved immune pathways including Jak/STAT (Janus kinase/signal transducer and activator of transcription), JNK (c-Jun N-terminal kinase), MAPK (mitogen-activated protein kinases) and the NF-κB mediated Toll and Imd (immune deficiency) pathways. Studies in Dipteran insects, including the model organism Drosophila melanogaster and pathogen-transmitting mosquitos, provide the framework for understanding bee antiviral defense. However, there are notable differences such as the more prominent role of a non-sequence specific, dsRNA-triggered, virus limiting response in honey bees and bumble bees. This virus-limiting response in bees is akin to pathways in a range of organisms including other invertebrates (i.e., oysters, shrimp and sand flies), as well as the mammalian interferon response. Current and future research aimed at elucidating bee antiviral defense mechanisms may lead to development of strategies that mitigate bee losses, while expanding our understanding of insect antiviral defense and the potential evolutionary relationship between sociality and immune function.

Linking gene regulation to cell behaviors in the posterior growth zone of sequentially segmenting arthropods

Virtually all arthropods all arthropods add their body segments sequentially, one by one in an anterior to posterior progression. That process requires not only segment specification but typically growth and elongation. Here we review the functions of some of the key genes that regulate segmentation: Wnt, caudal, Notch pathway, and pair-rule genes, and discuss what can be inferred about their evolution. We focus on how these regulatory factors are integrated with growth and elongation and discuss the importance and challenges of baseline measures of growth and elongation. We emphasize a perspective that integrates the genetic regulation of segment patterning with the cellular mechanisms of growth and elongation.

The honeybee as a model insect for developmental genetics

Honeybees are an important component of modern agricultural systems, and a fascinating and scientifically engrossing insect. Honeybees are not commonly used as model systems for understanding development in insects despite their importance in agriculture. Honeybee embryogenesis, while being superficially similar to Drosophila, is molecularly very different, especially in axis formation and sex determination. In later development, much of honeybee biology is modified by caste development, an as yet poorly understood, but excellent, system to study developmental plasticity. In adult stages, developmental plasticity of the ovaries, related to reproductive constraint exhibits another aspect of plasticity. Here they review the tools, current knowledge and opportunities in honeybee developmental biology, and provide an updated embryonic staging scheme to support future studies.

Negligible uptake and transfer of diet-derived pollen microRNAs in adult honey bees

The putative transfer and gene regulatory activities of diet-derived miRNAs in ingesting animals are still debated. Importantly, no study to date has fully examined the role of dietary uptake of miRNA in the honey bee, a critical pollinator in both agricultural and natural ecosystems. After controlled pollen feeding experiments in adult honey bees, we observed that midguts demonstrated robust increases in plant miRNAs after pollen ingestion. However, we found no evidence of biologically relevant delivery of these molecules to proximal or distal tissues of recipient honey bees. Our results, therefore, support the premise that pollen miRNAs ingested as part of a typical diet are not robustly transferred across barrier epithelia of adult honey bees under normal conditions. Key future questions include whether other small RNA species in honey bee diets behave similarly and whether more specialized and specific delivery mechanisms exist for more efficient transport, particularly in the context of stressed barrier epithelia.

The high-throughput production of dsRNA against sacbrood virus for use in the honey bee Apis cerana (Hymenoptera: Apidae)

Sacbrood virus (SBV) is a serious threat to honey bees. Currently, there is no specific drug available for the treatment of SBV that does not affect the quality of the bee product. RNA interference (RNAi) is an important antiviral strategy for disease control. To effectively utilize this technology, the large-scale production and purification of double-stranded RNA (dsRNA) is necessary. Here, a dsRNA-expressing plasmid targeting the VP1 gene of Chinese sacbrood virus (CSBV) was constructed and expressed in Escherichia coli (E. coli) HT115 (DE3). After lysing and ethanol precipitation from E. coli, dsRNA VP1 was purified with an anion exchange chromatography column. Second instar larvae of Apis cerana were fed the purified dsRNA VP1. A significant decrease in larval mortality and the level of expression of the VP1 gene after CSBV infection was demonstrated after the ingestion of dsRNA VP1. This result provides a potential method for the large-scale production of dsRNA to protect A. cerana from CSBV infection.

RNAi and Antiviral Defense in the Honey Bee

Honey bees play an important agricultural and ecological role as pollinators of numerous agricultural crops and other plant species. Therefore, investigating the factors associated with high annual losses of honey bee colonies in the US is an important and active area of research. Pathogen incidence and abundance correlate with Colony Collapse Disorder- (CCD-) affected colonies in the US and colony losses in the US and in some European countries. Honey bees are readily infected by single-stranded positive sense RNA viruses. Largely dependent on the host immune response, virus infections can either remain asymptomatic or result in deformities, paralysis, or death of adults or larvae. RNA interference (RNAi) is an important antiviral defense mechanism in insects, including honey bees. Herein, we review the role of RNAi in honey bee antiviral defense and highlight some parallels between insect and mammalian immune systems. A more thorough understanding of the role of pathogens on honey bee health and the immune mechanisms bees utilize to combat infectious agents may lead to the development of strategies that enhance honey bee health and result in the discovery of additional mechanisms of immunity in metazoans.

Laboratory Assay of Brood Care for Quantitative Analyses of Individual Differences in Honey Bee (Apis mellifera) Affiliative Behavior

Care of offspring is a form of affiliative behavior that is fundamental to studies of animal social behavior. Insects do not figure prominently in this topic because Drosophila melanogaster and other traditional models show little if any paternal or maternal care. However, the eusocial honey bee exhibits cooperative brood care with larvae receiving intense and continuous care from their adult sisters, but this behavior has not been well studied because a robust quantitative assay does not exist. We present a new laboratory assay that enables quantification of group or individual honey bee brood “nursing behavior” toward a queen larva. In addition to validating the assay, we used it to examine the influence of the age of the larva and the genetic background of the adult bees on nursing performance. This new assay also can be used in the future for mechanistic analyses of eusociality and comparative analyses of affilative behavior with other animals.

RNA interference technology used for the study of aquatic virus infections

Aquaculture is one of the most important economic activities in Asia and is presently the fastest growing sector of food production in the world. Explosive increases in global fish farming have been accompanied by an increase in viral diseases. Viral infections are responsible for huge economic losses in fish farming, and control of these viral diseases in aquaculture remains a serious challenge. Recent advances in biotechnology have had a significant impact on disease reduction in aquaculture. RNAi is one of the most important technological breakthroughs in modern biology, allowing us to directly observe the effects of the loss of specific genes in living systems. RNA interference technology has emerged as a powerful tool for manipulating gene expression in the laboratory. This technology represents a new therapeutic approach for treating aquatic diseases, including viral infections. RNAi technology is based on a naturally occurring post-transcriptional gene silencing process mediated by the formation of dsRNA. RNAi has been proven widely effective for gene knockdown in mammalian cultured cells, but its utility in fish remains unexplored. This review aims to highlight the RNAi technology that has made significant contributions toward the improvement of aquatic animal health and will also summarize the current status and future strategies concerning the therapeutic applications of RNAi to combat viral disease in aquacultured organisms.

Epigenomics and the control of fate, form and function in social insects

Phenotypic plasticity is central to the success of social insects. The ability to form functionally and behaviourally diverse phenotypes from a common genome enables synthesis of highly specialised castes that carry out unique roles essential for colony survival. There is accumulating evidence that the epigenome may underlie some of this diversity in social insects. Here we discuss recent research into the role of epigenomic control of behavioural and developmental caste determination in social insects. Furthermore we suggest future strategies for unravelling the complex mechanisms by which the epigenome may shape these diverse societies.

Highly efficient integration and expression of piggyBac-derived cassettes in the honeybee (Apis mellifera)

Honeybees (Apis mellifera), which are important pollinators of plants, display remarkable individual behaviors that collectively contribute to the organization of a complex society. Advances in dissecting the complex processes of honeybee behavior have been limited in the recent past due to a lack of genetic manipulation tools. These tools are difficult to apply in honeybees because the unit of reproduction is the colony, and many interesting phenotypes are developmentally specified at later stages. Here, we report highly efficient integration and expression of piggyBac-derived cassettes in the honeybee. We demonstrate that 27 and 20% of queens stably transmitted two different expression cassettes to their offspring, which is a 6- to 30-fold increase in efficiency compared with those generally reported in other insect species. This high efficiency implies that an average beekeeping facility with a limited number of colonies can apply this tool. We demonstrated that the cassette stably and efficiently expressed marker genes in progeny under either an artificial or an endogenous promoter. This evidence of efficient expression encourages the use of this system to inhibit gene functions through RNAi in specific tissues and developmental stages by using various promoters. We also showed that the transgenic marker could be used to select transgenic offspring to be employed to facilitate the building of transgenic colonies via the haploid males. We present here the first to our knowledge genetic engineering tool that will efficiently allow for the systematic detection and better understanding of processes underlying the biology of honeybees.

Biased gene expression in early honeybee larval development

Background

Female larvae of the honeybee (Apis mellifera) develop into either queens or workers depending on nutrition. This nutritional stimulus triggers different developmental trajectories, resulting in adults that differ from each other in physiology, behaviour and life span.

Results

To understand how these trajectories are established we have generated a comprehensive atlas of gene expression throughout larval development. We found substantial differences in gene expression between worker and queen-destined larvae at 6 hours after hatching. Some of these early changes in gene expression are maintained throughout larval development, indicating that caste-specific developmental trajectories are established much earlier than previously thought. Within our gene expression data we identified processes that potentially underlie caste differentiation. Queen-destined larvae have higher expression of genes involved in transcription, translation and protein folding early in development with a later switch to genes involved in energy generation. Using RNA interference, we were able to demonstrate that one of these genes, hexamerin 70b, has a role in caste differentiation. Both queen and worker developmental trajectories are associated with the expression of genes that have alternative splice variants, although only a single variant of a gene tends to be differentially expressed in a given caste.

Conclusions

Our data, based on the biases in gene expression early in development together with published data, supports the idea that caste development in the honeybee consists of two phases; an initial biased phase of development, where larvae can still switch to the other caste by differential feeding, followed by commitment to a particular developmental trajectory.

Characterisation of the RNA interference response against the long-wavelength receptor of the honeybee

Targeted knock-down is the method of choice to advance the study of sensory and brain functions in the honeybee by using molecular techniques. Here we report the results of a first attempt to interfere with the function of a visual receptor, the long-wavelength-sensitive (L-) photoreceptor. RNA interference to inhibit this receptor led to a reduction of the respective mRNA and protein. The interference effect was limited in time and space, and its induction depended on the time of the day most probably because of natural daily variations in opsin levels. The inhibition did not effectively change the physiological properties of the retina. Possible constraints and implications of this method for the study of the bee's visual system are discussed. Overall this study underpins the usefulness and feasibility of RNA interference as manipulation tool in insect brain research.

Honey bee promoter sequences for targeted gene expression

The honey bee, Apis mellifera, displays a rich behavioural repertoire, social organization and caste differentiation, and has an interesting mode of sex determination, but we still know little about its underlying genetic programs. We lack stable transgenic tools in honey bees that would allow genetic control of gene activity in stable transgenic lines. As an initial step towards a transgenic method, we identified promoter sequences in the honey bee that can drive constitutive, tissue-specific and cold shock-induced gene expression. We identified the promoter sequences of Am-actin5c, elp2l, Am-hsp83 and Am-hsp70 and showed that, except for the elp2l sequence, the identified sequences were able to drive reporter gene expression in Sf21 cells. We further demonstrated through electroporation experiments that the putative neuron-specific elp2l promoter sequence can direct gene expression in the honey bee brain. The identification of these promoter sequences is an important initial step in studying the function of genes with transgenic experiments in the honey bee, an organism with a rich set of interesting phenotypes.

RNAi-mediated double gene knockdown and gustatory perception measurement in honey bees (Apis mellifera)

This video demonstrates novel techniques of RNA interference (RNAi) which downregulate two genes simultaneously in honey bees using double-stranded RNA (dsRNA) injections. It also presents a protocol of proboscis extension response (PER) assay for measuring gustatory perception. RNAi-mediated gene knockdown is an effective technique downregulating target gene expression. This technique is usually used for single gene manipulation, but it has limitations to detect interactions and joint effects between genes. In the first part of this video, we present two strategies to simultaneously knock down two genes (called double gene knockdown). We show both strategies are able to effectively suppress two genes, vitellogenin (vg) and ultraspiracle (usp), which are in a regulatory feedback loop. This double gene knockdown approach can be used to dissect interrelationships between genes and can be readily applied in different insect species. The second part of this video is a demonstration of proboscis extension response (PER) assay in honey bees after the treatment of double gene knockdown. The PER assay is a standard test for measuring gustatory perception in honey bees, which is a key predictor for how fast a honey bee's behavioral maturation is. Greater gustatory perception of nest bees indicates increased behavioral development which is often associated with an earlier age at onset of foraging and foraging specialization in pollen. In addition, PER assay can be applied to identify metabolic states of satiation or hunger in honey bees. Finally, PER assay combined with pairing different odor stimuli for conditioning the bees is also widely used for learning and memory studies in honey bees.

Origin and function of the major royal jelly proteins of the honeybee (Apis mellifera) as members of the yellow gene family

In the honeybee, Apis mellifera, the queen larvae are fed with a diet exclusively composed of royal jelly (RJ), a secretion of the hypopharyngeal gland of young worker bees that nurse the brood. Up to 15% of RJ is composed of proteins, the nine most abundant of which have been termed major royal jelly proteins (MRJPs). Although it is widely accepted that RJ somehow determines the fate of a female larva and in spite of considerable research efforts, there are surprisingly few studies that address the biochemical characterisation and functions of these MRJPs. Here we review the research on MRJPs not only in honeybees but in hymenopteran insects in general and provide metadata analyses on genome organisation of mrjp genes, corroborating previous reports that MRJPs have important functions for insect development and not just a nutritional value for developing honeybee larvae.

A survey of the effectiveness of non-cell autonomous RNAi throughout development in the sawfly, Athalia rosae (Hymenoptera)

RNA interference (RNAi) is a powerful and convenient tool not only for functional analysis of specific genes, but also for large-scale screening of gene function in insects; however, reports on its efficiency throughout development in a single species are limited. We demonstrate here that non-cell autonomous RNAi by injection of double-stranded RNA (dsRNA) knocks down targeting genes in most developmental stages in the sawfly, Athalia rosae. Injection of dsRNA targeting the green fluorescence protein (gfp) gene into eggs of a transgenic strain carrying the constitutively expressing gfp gene resulted in the absence of GFP fluorescence during embryogenesis, while a portion of the gfp dsRNA-injected embryos began exhibiting GFP fluorescence at late embryogenesis. When gfp dsRNA was injected into parental female pupae, the RNAi effect was carried over to all embryos of the next generation and the effect lasted until mid-larval stages. Parental injection of dsRNA was more efficient than embryonic injection in terms of penetrance of the effect and the survival rate. After injection of gfp dsRNA into last instar larvae, the RNAi effect was sustained during prepupal and pupal stages and in adults. The gfp gene transcript markedly decreased in these knockdown phenotypes. It was revealed by employing fluorescence-labeled dsRNA that injected dsRNA was taken up in internal organs. Knockdown of an endogenous gene, Distal-less (Dll), resulted in typical phenotypes represented by the lack and malformation of Dll-expressing organs, such as distal parts of the appendages and wing edges without showing off-target effects. In contrast, RNAi by dsRNA injection seems to be hardly effective in mid- to late-larval stages.

In vitro and in vivo application of RNA interference for targeting genes involved in peritrophic matrix synthesis in a lepidopteran system

The midgut of most insects is lined with a semipermeable acellular tube, the peritrophic matrix (PM), composed of chitin and proteins. Although various genes encoding PM proteins have been characterized, our understanding of their roles in PM structure and function is very limited. One promising approach for obtaining functional information is RNA interference, which has been used to reduce the levels of specific mRNAs using double-stranded RNAs administered to larvae by either injection or feeding. Although this method is well documented in dipterans and coleopterans, reports of its success in lepidopterans are varied. In the current study, the silencing midgut genes encoding PM proteins (insect intestinal mucin 1, insect intestinal mucin 4, PM protein 1) and the chitin biosynthetic or modifying enzymes (chitin synthase-B and chitin deacetylase 1) in a noctuid lepidopteran, Mamestra configurata, was examined in vitro and in vivo. In vitro studies in primary midgut epithelial cell preparations revealed an acute and rapid silencing (by 24 h) for the gene encoding chitin deacetylase 1 and a slower rate of silencing (by 72 h) for the gene encoding PM protein 1. Genes encoding insect intestinal mucins were slightly silenced by 72 h, whereas no silencing was detected for the gene encoding chitin synthase-B. In vivo experiments focused on chitin deacetylase 1, as the gene was silenced to the greatest extent in vitro. Continuous feeding of neonates and fourth instar larvae with double-stranded RNA resulted in silencing of chitin deacetylase 1 by 24 and 36 h, respectively. Feeding a single dose to neonates also resulted in silencing by 24 h. The current study demonstrates that genes encoding PM proteins can be silenced and outlines conditions for RNA interference by per os feeding in lepidopterans.

Non-Target Effects of Green Fluorescent Protein (GFP)-Derived Double-Stranded RNA (dsRNA-GFP) Used in Honey Bee RNA Interference (RNAi) Assays

RNA interference has been frequently applied to modulate gene function in organisms where the production and maintenance of mutants is challenging, as in our model of study, the honey bee, Apis mellifera. A green fluorescent protein (GFP)-derived double-stranded RNA (dsRNA-GFP) is currently commonly used as control in honey bee RNAi experiments, since its gene does not exist in the A. mellifera genome. Although dsRNA-GFP is not expected to trigger RNAi responses in treated bees, undesirable effects on gene expression, pigmentation or developmental timing are often observed. Here, we performed three independent experiments using microarrays to examine the effect of dsRNA-GFP treatment (introduced by feeding) on global gene expression patterns in developing worker bees. Our data revealed that the expression of nearly 1,400 genes was altered in response to dsRNA-GFP, representing around 10% of known honey bee genes. Expression changes appear to be the result of both direct off-target effects and indirect downstream secondary effects; indeed, there were several instances of sequence similarity between putative siRNAs generated from the dsRNA-GFP construct and genes whose expression levels were altered. In general, the affected genes are involved in important developmental and metabolic processes associated with RNA processing and transport, hormone metabolism, immunity, response to external stimulus and to stress. These results suggest that multiple dsRNA controls should be employed in RNAi studies in honey bees. Furthermore, any RNAi studies involving these genes affected by dsRNA-GFP in our studies should use a different dsRNA control.

Oral delivery of double-stranded RNA in larvae of the yellow fever mosquito, Aedes aegypti: implications for pest mosquito control

RNA interference has already proven itself to be a highly versatile molecular biology tool for understanding gene function in a limited number of insect species, but its widespread use in other species will be dependent on the development of easier methods of double-stranded RNA (dsRNA) delivery. This study demonstrates that RNA interference can be induced in the mosquito Aedes aegypti L. (Diptera: Culicidae) simply by soaking larvae in a solution of dsRNA for two hours. The mRNA transcripts for β-tubulin, chitin synthase-1 and -2, and heat shock protein 83 were reduced between 30 and 50% three days post-dsRNA treatment. The dsRNA was mixed with a visible dye to identify those individuals that fed on the dsRNA, and based on an absence of RNA interference in those individuals that contained no dye within their guts, the primary route of entry of dsRNA is likely through the gut epithelium. RNA interference was systemic in the insects, inducing measurable knock down of gene expression in tissues beyond the gut. Silencing of the β-tubulin and chitin synthase-1 genes resulted in reduced growth and/or mortality of the larvae, demonstrating the utility of dsRNA as a potential mosquito larvicide. Silencing of chitin synthase-2 did not induce mortality in the larvae, and silencing of heat shock protein 83 only induced mortality in the insects if they were subsequently subjected to a heat stress. Drosophila melanogaster Meigen (Diptera: Drosophilidae) larvae were also soaked in dsRNA designed to specifically target either their own β-tubulin gene, or that of A. aegypti, and significant mortality was only seen in larvae treated with dsRNA targeting their own gene, which suggests that dsRNA pesticides could be designed to be species-limited.

The Am-tra2 gene is an essential regulator of female splice regulation at two levels of the sex determination hierarchy of the honeybee

Heteroallelic and homo- or hemiallelic Complementary sex determiner (Csd) proteins determine sexual fate in the honeybee (Apis mellifera) by controlling the alternative splicing of the downstream gene fem (feminizer). Thus far, we have little understanding of how heteroallelic Csd proteins mediate the splicing of female fem messenger RNAs (mRNAs) or how Fem proteins direct the splicing of honeybee dsx (Am-dsx) pre-mRNAs. Here, we report that Am-tra2, which is an ortholog of Drosophila melanogaster tra2, is an essential component of female splicing of the fem and Am-dsx transcripts in the honeybee. The Am-tra2 transcripts are alternatively (but non-sex-specifically) spliced, and they are translated into six protein isoforms that all share the basic RNA-binding domain/RS (arginine/serine) domain structure. Knockdown studies showed that the Am-tra2 gene is required to splice fem mRNAs into the productive female and nonproductive male forms. We suggest that the Am-Tra2 proteins are essential regulators of fem pre-mRNA splicing that, together with heteroallelic Csd proteins and/or Fem proteins, implement the female pathway. In males, the Am-Tra2 proteins may enhance the switch of fem transcripts into the nonproductive male form when heteroallelic Csd proteins are absent. This dual function of Am-Tra2 proteins possibly enhances and stabilizes the binary decision process of male/female splicing. Our knockdown studies also imply that the Am-Tra2 protein is an essential regulator for Am-dsx female splice regulation, suggesting an ancestral role in holometabolous insects. We also provide evidence that the Am-tra2 gene has an essential function in honeybee embryogenesis that is unrelated to sex determination.

RNA interference with special reference to combating viruses of crustacea

RNA interference has evolved from being a nuisance biological phenomenon to a valuable research tool to determine gene function and as a therapeutic agent. Since pioneering observations regarding RNA interference were first reported in the 1990s from the nematode worm, plants and Drosophila, the RNAi phenomenon has since been reported in all eukaryotic organisms investigated from protozoans, plants, arthropods, fish and mammals. The design of RNAi therapeutics has progressed rapidly to designing dsRNA that can specifically and effectively silence disease related genes. Such technology has demonstrated the effective use of short interfering as therapeutics. In the absence of a B cell lineage in arthropods, and hence no long term vaccination strategy being available, the introduction of using RNA interference in crustacea may serve as an effective control and preventative measure for viral diseases for application in aquaculture.

RNAi: future in insect management

RNA interference is a post- transcriptional, gene regulation mechanism found in virtually all plants and animals including insects. The demonstration of RNAi in insects and its successful use as a tool in the study of functional genomics opened the door to the development of a variety of novel, environmentally sound approaches for insect pest management. Here the current understanding of the biogenesis of the two RNAi classes in insects is reviewed. These are microRNAs (miRNAs) and short interfering RNAs (siRNAs). Several other key approaches in RNAi -based for insect control, as well as for the prevention of diseases in insects are also reviewed. The problems and prospects for the future use of RNAi in insects are presented.

Honey bee PTEN--description, developmental knockdown, and tissue-specific expression of splice-variants correlated with alternative social phenotypes

Background

Phosphatase and TENsin (PTEN) homolog is a negative regulator that takes part in IIS (insulin/insulin-like signaling) and Egfr (epidermal growth factor receptor) activation in Drosophila melanogaster. IIS and Egfr signaling events are also involved in the developmental process of queen and worker differentiation in honey bees (Apis mellifera). Here, we characterized the bee PTEN gene homologue for the first time and begin to explore its potential function during bee development and adult life.

Results

Honey bee PTEN is alternatively spliced, resulting in three splice variants. Next, we show that the expression of PTEN can be down-regulated by RNA interference (RNAi) in the larval stage, when female caste fate is determined. Relative to controls, we observed that RNAi efficacy is dependent on the amount of PTEN dsRNA that is delivered to larvae. For larvae fed queen or worker diets containing a high amount of PTEN dsRNA, PTEN knockdown was significant at a whole-body level but lethal. A lower dosage did not result in a significant gene down-regulation. Finally, we compared same-aged adult workers with different behavior: nursing vs. foraging. We show that between nurses and foragers, PTEN isoforms were differentially expressed within brain, ovary and fat body tissues. All isoforms were expressed at higher levels in the brain and ovaries of the foragers. In fat body, isoform B was expressed at higher level in the nurse bees.

Conclusion

Our results suggest that PTEN plays a central role during growth and development in queen- and worker-destined honey bees. In adult workers, moreover, tissue-specific patterns of PTEN isoform expression are correlated with differences in complex division of labor between same-aged individuals. Therefore, we propose that knowledge on the roles of IIS and Egfr activity in developmental and behavioral control may increase through studies of how PTEN functions can impact bee social phenotypes.

Systemic RNA-interference in the honeybee Apis mellifera: tissue dependent uptake of fluorescent siRNA after intra-abdominal application observed by laser-scanning microscopy

RNA interference has been successfully used in adult honeybees, but there are only few reports about abdominal application of dsRNA/siRNA which have reached more distant tissues than the fat body. We studied systemic RNAi in honeybees by injecting fluorescent siRNA of the ubiquitously expressed honeybee homologue of the Glycerol-3-Phosphate Dehydrogenase (amGpdh) into the abdomens of adult bees and followed them by laser scanning microscopy and qPCR. The fat body was the sole tissue emitting fluorescence and showing a decreased gene expression, whereas the siRNA had apparently not reached the other tissues. Therefore, we conclude that certain genes in other tissues than the fat body cannot be easily reached by injecting siRNA into the body cavity. In particular, the lack of amGpdh knock down in ovaries after amGpdh dsRNA injection, supports that in some cases it may be particularly difficult to interfere with gene expression in ovaries by intra-abdominal injection. In these cases alternative inhibition techniques may be required to achieve an organismic non-lethal disruption of transcription.

Proteomic analysis of honeybee (Apis mellifera L.) pupae head development

The honeybee pupae development influences its future adult condition as well as honey and royal jelly productions. However, the molecular mechanism that regulates honeybee pupae head metamorphosis is still poorly understood. To further our understand of the associated molecular mechanism, we investigated the protein change of the honeybee pupae head at 5 time-points using 2-D electrophoresis, mass spectrometry, bioinformatics, quantitative real-time polymerase chain reaction and Western blot analysis. Accordingly, 58 protein spots altered their expression across the 5 time points (13–20 days), of which 36 proteins involved in the head organogenesis were upregulated during early stages (13–17 days). However, 22 proteins involved in regulating the pupae head neuron and gland development were upregulated at later developmental stages (19–20 days). Also, the functional enrichment analysis further suggests that proteins related to carbohydrate metabolism and energy production, development, cytoskeleton and protein folding were highly involved in the generation of organs and development of honeybee pupal head. Furthermore, the constructed protein interaction network predicted 33 proteins acting as key nodes of honeybee pupae head growth of which 9 and 4 proteins were validated at gene and protein levels, respectively. In this study, we uncovered potential protein species involved in the formation of honeybee pupae head development along with their specific temporal requirements. This first proteomic result allows deeper understanding of the proteome profile changes during honeybee pupae head development and provides important potential candidate proteins for future reverse genetic research on honeybee pupae head development to improve the performance of related organs.

In vivo identification of Bacillus thuringiensis Cry4Ba toxin receptors by RNA interference knockdown of glycosylphosphatidylinositol-linked aminopeptidase N transcripts in Aedes aegypti larvae

Bacillus thuringiensis Cry4Ba toxin selectively kills Aedes aegypti mosquito larvae as it is in part due to the presence of specific membrane-bound protein receptors. In this study, using data mining approach, we initially identified three potential glycosylphosphatidylinositol-linked aminopeptidase N (GPI-APN) isoforms, APN2778, APN2783 and APN5808, which are believed to act as Cry4Ba toxin receptors. These three isoforms that are functionally expressed in the larval midgut can be sequence-specific knocked down (ranging from ∼80 % to 95 %) by soaking the Aedes aegypti larvae in buffer of long double-stranded GPI-APN RNAs (∼300-680 bp). Finally, to see the physiological effect of APN knockdowns, the larvae were fed with Escherichia coli expressing Cry4Ba toxin. The results revealed that all the three identified GPI-APN isoforms may possibly function as a Cry4Ba receptor, particularly for APN2783 as those larvae with this transcript knockdown showed a dramatic increase in resistance to Cry4Ba toxicity.

Differential protein expression in honeybee (Apis mellifera L.) larvae: underlying caste differentiation

Honeybee (Apis mellifera) exhibits divisions in both morphology and reproduction. The queen is larger in size and fully developed sexually, while the worker bees are smaller in size and nearly infertile. To better understand the specific time and underlying molecular mechanisms of caste differentiation, the proteomic profiles of larvae intended to grow into queen and worker castes were compared at 72 and 120 hours using two dimensional electrophoresis (2-DE), network, enrichment and quantitative PCR analysis. There were significant differences in protein expression between the two larvae castes at 72 and 120 hours, suggesting the queen and the worker larvae have already decided their fate before 72 hours. Specifically, at 72 hours, queen intended larvae over-expressed transketolase, aldehyde reductase, and enolase proteins which are involved in carbohydrate metabolism and energy production, imaginal disc growth factor 4 which is a developmental related protein, long-chain-fatty-acid CoA ligase and proteasome subunit alpha type 5 which metabolize fatty and amino acids, while worker intended larvae over-expressed ATP synthase beta subunit, aldehyde dehydrogenase, thioredoxin peroxidase 1 and peroxiredoxin 2540, lethal (2) 37 and 14-3-3 protein epsilon, fatty acid binding protein, and translational controlled tumor protein. This differential protein expression between the two caste intended larvae was more pronounced at 120 hours, with particular significant differences in proteins associated with carbohydrate metabolism and energy production. Functional enrichment analysis suggests that carbohydrate metabolism and energy production and anti-oxidation proteins play major roles in the formation of caste divergence. The constructed network and validated gene expression identified target proteins for further functional study. This new finding is in contrast to the existing notion that 72 hour old larvae has bipotential and can develop into either queen or worker based on epigenetics and can help us to gain new insight into the time of departure as well as caste trajectory influencing elements at the molecular level.

Sex determination in honeybees: two separate mechanisms induce and maintain the female pathway

Sex determination in honeybees is realized by the csd and the fem gene that establish and maintain, throughout development, sexual fates via the control of alternative splicing.

Organisms have evolved a bewildering diversity of mechanisms to generate the two sexes. The honeybee (Apis mellifera) employs an interesting system in which sex is determined by heterozygosity at a single locus (the Sex Determination Locus) harbouring the complementary sex determiner (csd) gene. Bees heterozygous at Sex Determination Locus are females, whereas bees homozygous or hemizygous are males. Little is known, however, about the regulation that links sex determination to sexual differentiation. To investigate the control of sexual development in honeybees, we analyzed the functions and the regulatory interactions of genes involved in the sex determination pathway. We show that heterozygous csd is only required to induce the female pathway, while the feminizer (fem) gene maintains this decision throughout development. By RNAi induced knockdown we show that the fem gene is essential for entire female development and that the csd gene exclusively processes the heterozygous state. Fem activity is also required to maintain the female determined pathway throughout development, which we show by mosaic structures in fem-repressed intersexuals. We use expression of Fem protein in males to demonstrate that the female maintenance mechanism is controlled by a positive feedback splicing loop in which Fem proteins mediate their own synthesis by directing female fem mRNA splicing. The csd gene is only necessary to induce this positive feedback loop in early embryogenesis by directing splicing of fem mRNAs. Finally, fem also controls the splicing of Am-doublesex transcripts encoding conserved male- and female-specific transcription factors involved in sexual differentiation. Our findings reveal how the sex determination process is realized in honeybees differing from Drosophila melanogaster.

Sexual differentiation is a fundamental process in the animal kingdom, and different species have evolved a bewildering diversity of mechanisms to generate the two sexes in the proper proportions. Sex determination in honeybees (Apis mellifera) provides an interesting and unusual system to study, as it is governed by heterozygosity of a single locus harbouring the complementary sex determiner gene (csd), in contrast to the well-studied sex chromosome system of Drosophila melanogaster. We show that the female sex determination pathway is exclusively induced by the csd gene in early embryogenesis. Later on and throughout development this inductive signal is maintained via a positive feedback loop of the feminizer (fem) gene, in which the Fem protein mediates its own synthesis. The findings reveal how the sex determination process in honeybees is realized by the regulation and function of two genes differing from Drosophila.

A non-invasive method for silencing gene transcription in honeybees maintained under natural conditions

In the Apis mellifera post-genomic era, RNAi protocols have been used in functional approaches. However, sample manipulation and invasive methods such as injection of double-stranded RNA (dsRNA) can compromise physiology and survival. To circumvent these problems, we developed a non-invasive method for honeybee gene knockdown, using a well-established vitellogenin RNAi system as a model. Second instar larvae received dsRNA for vitellogenin (dsVg-RNA) in their natural diet. For exogenous control, larvae received dsRNA for GFP (dsGFP-RNA). Untreated larvae formed another control group. Around 60% of the treated larvae naturally developed until adult emergence when 0.5 microg of dsVg-RNA or dsGFP-RNA was offered while no larvae that received 3.0 microg of dsRNA reached pupal stages. Diet dilution did not affect the removal rates. Viability depends not only on the delivered doses but also on the internal conditions of colonies. The weight of treated and untreated groups showed no statistical differences. This showed that RNAi ingestion did not elicit drastic collateral effects. Approximately 90% of vitellogenin transcripts from 7-day-old workers were silenced compared to controls. A large number of samples are handled in a relatively short time and smaller quantities of RNAi molecules are used compared to invasive methods. These advantages culminate in a versatile and a cost-effective approach.

Identification of the proteome composition occurring during the course of embryonic development of bees (Apis mellifera)

To investigate the proteome during embryonic development of honeybees, Apis mellifera, proteins were identified by two-dimensional gel electrophoresis, mass spectrometry and protein engine identification tools that were applied to MASCOT and Xproteo search engines. 312, 320, 315 proteins were detected in 24, 48 and 72 h embryos. Thirty-eight highly abundant proteins were identified at the three time points by MS fingerprinting. All 21 proteins could be identified as products of annotated genes of the honeybee. Identified proteins included six proteins related to the metabolism of carbohydrates and energy production, six proteins belonging to the heat shock protein family, three cytoskeletal proteins, four proteins related to the antioxidant system of the embryo and two proteins related to growth regulation of the embryo. Quantitative proteomics was applied to analyze differences in amounts of these proteins during the three above mentioned developmental stages. Our data present an initial molecular picture of honeybee embryos, and will hopefully pave the way for future research on this animal.

RNA interference in infectious tropical diseases

Introduction of double-stranded RNA (dsRNA) into some cells or organisms results in degradation of its homologous mRNA, a process called RNA interference (RNAi). The dsRNAs are processed into short interfering RNAs (siRNAs) that subsequently bind to the RNA-induced silencing complex (RISC), causing degradation of target mRNAs. Because of this sequence-specific ability to silence target genes, RNAi has been extensively used to study gene functions and has the potential to control disease pathogens or vectors. With this promise of RNAi to control pathogens and vectors, this paper reviews the current status of RNAi in protozoans, animal parasitic helminths and disease-transmitting vectors, such as insects. Many pathogens and vectors cause severe parasitic diseases in tropical regions and it is difficult to control once the host has been invaded. Intracellularly, RNAi can be highly effective in impeding parasitic development and proliferation within the host. To fully realize its potential as a means to control tropical diseases, appropriate delivery methods for RNAi should be developed, and possible off-target effects should be minimized for specific gene suppression. RNAi can also be utilized to reduce vector competence to interfere with disease transmission, as genes critical for pathogenesis of tropical diseases are knockdowned via RNAi.

Hedgehog signaling pathway function conserved in Tribolium segmentation

In Drosophila, maintenance of parasegmental boundaries and formation of segmental grooves depend on interactions between segment polarity genes. Wingless and Engrailed appear to have similar roles in both short and long germ segmentation, but relatively little is known about the extent to which Hedgehog signaling is conserved. In a companion study to the Tribolium genome project, we analyzed the expression and function of hedgehog, smoothened, patched, and cubitus interruptus orthologs during segmentation in Tribolium. Their expression was largely conserved between Drosophila and Tribolium. Parental RNAi analysis of positive regulators of the pathway (Tc-hh, Tc-smo, or Tc-ci) resulted in small spherical cuticles with little or no evidence of segmental grooves. Segmental Engrailed expression in these embryos was initiated but not maintained. Wingless-independent Engrailed expression in the CNS was maintained and became highly compacted during germ band retraction, providing evidence that derivatives from every segment were present in these small spherical embryos. On the other hand, RNAi analysis of a negative regulator (Tc-ptc) resulted in embryos with ectopic segmental grooves visible during germband elongation but not discernible in the first instar larval cuticles. These transient grooves formed adjacent to Engrailed expressing cells that encircled wider than normal wg domains in the Tc-ptc RNAi embryos. These results suggest that the en–wg–hh gene circuit is functionally conserved in the maintenance of segmental boundaries during germ band retraction and groove formation in Tribolium and that the segment polarity genes form a robust genetic regulatory module in the segmentation of this short germ insect.