Uncovering the immune responses of Apis mellifera ligustica larval gut to Ascosphaera apis infection utilizing transcriptome sequencing

Dynamics and regulatory role of circRNAs in Asian honey bee larvae following fungal infection

Non-coding RNA (ncRNA) plays a vital part in the regulation of immune responses, growth, and development in plants and animals. Here, the identification, characteristic analysis, and molecular verification of circRNAs in Apis cerana cerana worker larval guts were conducted, followed by in-depth investigation of the expression pattern of larval circRNAs during Ascosphaera apis infection and exploration of the potential regulatory part of differentially expressed circRNAs (DEcircRNAs) in host immune responses. A total of 3178 circRNAs in the larval guts of A. c. cerana were identified, with a length distribution ranging from 15 to 96,007 nt. Additionally, 155, 95, and 86 DEcircRNAs were identified in the in the 4-, 5-, and 6-day-old larval guts following A. apis infection. These DEcircRNAs were predicted to target 29, 25, and 18 parental genes relevant to 12, 20, and 17 GO terms as well as 144, 114, and 61 KEGG pathways, including 5 cellular and 4 humoral immune pathways. Complex competing endogenous RNA (ceRNA) regulatory networks were detected as being formed among DEcircRNAs, DEmiRNAs, and DEmRNAs. The target DEmRNAs were engaged in 36, 47, and 47 GO terms as well as 331, 332, and 331 pathways, including 6 cellular and 6 humoral immune pathways. Further, 19 DEcircRNAs, 5 DEmiRNAs, and 3 mRNAs were included in the sub-networks relative to 3 antioxidant enzymes. Finally, back-splicing sites within 15 circRNAs and the difference in the 15 DEcircRNAs' expression between uninoculated and A. apis-inoculated larval guts were confirmed based on molecular methods. These findings not only enrich our understanding of bee host-fungal pathogen interactions but also lay a foundation for illuminating the mechanism underlying the DEcircRNA-mediated immune defense of A. c. cerana larvae against A. apis invasion. KEY POINTS: • The expression pattern of circRNAs was altered in the A. cerana worker larval guts following A. apis infection. • Back-splicing sites within 15 A. cerana circRNAs were verified using molecular approaches. DEcircRNAs potentially modulated immune responses and antioxidant enzymes in A. apis-challenged host guts.

Glyphosate exposure affected longevity-related pathways and reduced survival in asian honey bees (Apis cerana)

Glyphosate (N-(phosphonomethyl)glycine, GLY) ranks among the most extensively used and effective herbicides globally. However, excessive GLY utilization poses a substantial threat to the survival of honey bees (Apis cerana). Here we monitored the survival status of A. cerana treated with GLY, and conducted transcriptome sequencing of the bee gut and head to further explore potential GLY influences at the molecular level. We observed that the mortality rate of bees increased as GLY concentration escalated. Pivotal pathways emerged in response to the GLY treatment, with a substantial number of differentially expressed genes enriched in the longevity regulating pathway - multiple species. This strongly suggested that GLY may influence the physiological behavior of bees by impacting this particular pathway. Moreover, our analysis revealed a notable reduction in the enzymatic activities of CYP450 and AChE in both the bee head and intestines of when exposed to GLY. Conversely, the enzymatic activity of superoxide dismutase (SOD) in the head remained unaffected, whereas in the intestines, it exhibited a significant increase. Additionally, prophenol oxidase (PPO) and glutathione-S-transferases (GSTs) displayed contrasting trends in enzymatic activity in both organs. This study offers valuable insights into how GLY impacted the survival of A. cerana.

Regulatory Roles of Long Non-Coding RNAs Relevant to Antioxidant Enzymes and Immune Responses of Apis cerana Larvae Following Ascosphaera apis Invasion

Long non-coding RNAs (lncRNAs) play an essential part in controlling gene expression and a variety of biological processes such as immune defense and stress-response. However, whether and how lncRNAs regulate responses of Apis cerana larvae to Ascosphaera apis invasion has remained unclear until now. Here, the identification and structural analysis of lncRNAs in the guts of A. cerana worker larvae were conducted, and the expression profile of larval lncRNAs during the A. apis infection process was then analyzed, followed by an investigation of the regulatory roles of differentially expressed lncRNAs (DElncRNAs) in the host response. In total, 76 sense lncRNAs, 836 antisense lncRNAs, 184 intron lncRNAs, 362 bidirectional lncRNAs, and 2181 intron lncRNAs were discovered in the larval guts. Additionally, 30 known and 9 novel lncRNAs were potential precursors for 36 and 11 miRNAs, respectively. In the three comparison groups, 386, 351, and 272 DElncRNAs were respectively identified, indicating the change in the overall expression pattern of host lncRNAs following the A. apis invasion. Analysis of cis-acting effect showed that DElncRNAs in the 4-, 5-, and 6-day-old comparison groups putatively regulated 55, 30, and 20 up- and down-stream genes, respectively, which were involved in a series of crucial functional terms and pathways, such as MAPK signaling pathway, and cell process. Analysis showed that 31, 8, and 11 DElncRNAs as potential antisense lncRNAs may interact with 26, 8, and 9 sense-strand mRNAs. Moreover, investigation of the competing endogenous RNA (ceRNA) network indicated that 148, 283, and 257 DElncRNAs were putatively regulated. The expression of target genes by targeting corresponding DEmiRNAs included those associated with antioxidant enzymes and immune responses. These results suggested that DElncRNAs played a potential part in the larval guts responding to the A. apis infection through a cis-acting manner and ceRNA mechanisms. Our findings deepen our understanding of interactions between A. cerana larvae and A. apis and offer a basis for clarifying the DElncRNA-mediated mechanisms underlying the host response to fungal invasion.

Systematic Characterization and Regulatory Role of lncRNAs in Asian Honey Bees Responding to Microsporidian Infestation

Long noncoding RNAs (lncRNAs) are pivotal regulators in gene expression and diverse biological processes, such as immune defense and host-pathogen interactions. However, little is known about the roles of lncRNAs in the response of the Asian honey bee (Apis cerana) to microsporidian infestation. Based on our previously obtained high-quality transcriptome datasets from the midgut tissues of Apis cerana cerana workers at 7 days post inoculation (dpi) and 10 dpi with Nosema ceranae (AcT7 and AcT10 groups) and the corresponding un-inoculated midgut tissues (AcCK7 and AcCK10 groups), the transcriptome-wide identification and structural characterization of lncRNAs were conducted, and the differential expression pattern of lncRNAs was then analyzed, followed by investigation of the regulatory roles of differentially expressed lncRNAs (DElncRNAs) in host response. Here, 2365, 2322, 2487, and 1986 lncRNAs were, respectively, identified in the AcCK7, AcT7, AcCK7, and AcT10 groups. After removing redundant ones, a total of 3496 A. c. cerana lncRNAs were identified, which shared similar structural characteristics with those discovered in other animals and plants, such as shorter exons and introns than mRNAs. Additionally, 79 and 73 DElncRNAs were screened from the workers' midguts at 7 dpi and 10 dpi, respectively, indicating the alteration of the overall expression pattern of lncRNAs in host midguts after N. ceranae infestation. These DElncRNAs could, respectively, regulate 87 and 73 upstream and downstream genes, involving a suite of functional terms and pathways, such as metabolic process and Hippo signaling pathway. Additionally, 235 and 209 genes co-expressed with DElncRNAs were found to enrich in 29 and 27 terms, as well as 112 and 123 pathways, such as ABC transporters and the cAMP signaling pathway. Further, it was detected that 79 (73) DElncRNAs in the host midguts at 7 (10) dpi could target 321 (313) DEmiRNAs and further target 3631 (3130) DEmRNAs. TCONS_00024312 and XR_001765805.1 were potential precursors for ame-miR-315 and ame-miR-927, while TCONS_00006120 was the putative precursor for both ame-miR-87-1 and ame-miR-87-2. These results together suggested that DElncRNAs are likely to play regulatory roles in the host response to N. ceranae infestation through the regulation of neighboring genes via a cis-acting effect, modulation of co-expressed mRNAs via trans-acting effect, and control of downstream target genes' expression via competing endogenous RNA networks. Our findings provide a basis for disclosing the mechanism underlying DElncRNA-mediated host N. ceranae response and a new perspective into the interaction between A. c. cerana and N. ceranae.

Honey bee functional genomics using symbiont-mediated RNAi

Honey bees are indispensable pollinators and model organisms for studying social behavior, development and cognition. However, their eusociality makes it difficult to use standard forward genetic approaches to study gene function. Most functional genomics studies in bees currently utilize double-stranded RNA (dsRNA) injection or feeding to induce RNAi-mediated knockdown of a gene of interest. However, dsRNA injection is laborious and harmful, and dsRNA feeding is difficult to scale cheaply. Further, both methods require repeated dsRNA administration to ensure a continued RNAi response. To fill this gap, we engineered the bee gut bacterium Snodgrassella alvi to induce a sustained host RNA interference response that reduces expression of a targeted gene. To employ this functional genomics using engineered symbionts (FUGUES) procedure, a dsRNA expression plasmid is cloned in Escherichia coli using Golden Gate assembly and then transferred to S. alvi. Adult worker bees are then colonized with engineered S. alvi. Finally, gene knockdown is verified through qRT-PCR, and bee phenotypes of interest can be further assessed. Expression of targeted genes is reduced by as much as 50-75% throughout the entire bee body by 5 d after colonization. This protocol can be accomplished in 4 weeks by bee researchers with microbiology and molecular cloning skills. FUGUES currently offers a streamlined and scalable approach for studying the biology of honey bees. Engineering other microbial symbionts to influence their hosts in ways that are similar to those described in this protocol may prove useful for studying additional insect and animal species in the future.

Effect of Ascosphaera apis Infestation on the Activities of Four Antioxidant Enzymes in Asian Honey Bee Larval Guts

Ascosphaera apis infects exclusively bee larvae and causes chalkbrood, a lethal fungal disease that results in a sharp reduction in adult bees and colony productivity. However, little is known about the effect of A. apis infestation on the activities of antioxidant enzymes in bee larvae. Here, A. apis spores were purified and used to inoculate Asian honey bee (Apis cerana) larvae, followed by the detection of the host survival rate and an evaluation of the activities of four major antioxidant enzymes. At 6 days after inoculation (dpi) with A. apis spores, obvious symptoms of chalkbrood disease similar to what occurs in Apis mellifera larvae were observed. PCR identification verified the A. apis infection of A. cerana larvae. Additionally, the survival rate of larvae inoculated with A. apis was high at 1−2 dpi, which sharply decreased to 4.16% at 4 dpi and which reached 0% at 5 dpi, whereas that of uninoculated larvae was always high at 1~8 dpi, with an average survival rate of 95.37%, indicating the negative impact of A. apis infection on larval survival. As compared with those in the corresponding uninoculated groups, the superoxide dismutase (SOD) and catalase (CAT) activities in the 5- and 6-day-old larval guts in the A. apis−inoculated groups were significantly decreased (p < 0.05) and the glutathione S-transferase (GST) activity in the 4- and 5-day-old larval guts was significantly increased (p < 0.05), which suggests that the inhibition of SOD and CAT activities and the activation of GST activity in the larval guts was caused by A. apis infestation. In comparison with that in the corresponding uninoculated groups, the polyphenol oxidase (PPO) activity was significantly increased (p < 0.05) in the 5-day-old larval gut but significantly reduced (p < 0.01) in the 6-day-old larval gut, indicating that the PPO activity in the larval guts was first enhanced and then suppressed. Our findings not only unravel the response of A. cerana larvae to A. apis infestation from a biochemical perspective but also offer a valuable insight into the interaction between Asian honey bee larvae and A. apis.

ame-miR-34 Modulates the Larval Body Weight and Immune Response of Apis mellifera Workers to Ascosphara apis Invasion

MiRNAs are critical regulators of numerous physiological and pathological processes. Ascosphaera apis exclusively infects bee larvae and causes chalkbrood disease. However, the function and mechanism of miRNAs in the bee larval response to A. apis infection is poorly understood. Here, ame-miR-34, a previously predicted miRNA involved in the response of Apis mellifera larvae to A. apis invasion, was subjected to molecular validation, and overexpression and knockdown were then conducted to explore the regulatory functions of ame-miR-34 in larval body weight and immune response. Stem-loop RT-PCR and Sanger sequencing confirmed the authenticity of ame-miR-34 in the larval gut of A. mellifera. RT-qPCR results demonstrated that compared with that in the uninfected larval guts, the expression level of ame-miR-34 was significantly downregulated (p < 0.001) in the guts of A. apis-infected 4-, 5-, and 6-day-old larvae, indicative of the remarkable suppression of host ame-miR-34 due to A. apis infection. In comparison with the corresponding negative control (NC) groups, the expression level of ame-miR-34 in the larval guts in the mimic-miR-34 group was significantly upregulated (p < 0.001), while that in the inhibitor-miR-34 group was significantly downregulated (p < 0.01). Similarly, effective overexpression and knockdown of ame-miR-34 were achieved. In addition, the body weights of 5- and 6-day-old larvae were significantly increased compared with those in the mimic-NC group; the weights of 5-day-old larvae in the inhibitor-miR-34 group were significantly decreased in comparison with those in the inhibitor-NC group, while the weights of 4- and 6-day-old larvae in the inhibitor-miR-34 group were significantly increased, indicating the involvement of ame-miR-34 in modulating larval body weight. Furthermore, the expression levels of both hsp and abct in the guts of A. apis-infected 4-, 5-, and 6-day-old larvae were significantly upregulated after ame-miR-34 overexpression. In contrast, after ame-miR-34 knockdown, the expression levels of the aforementioned two key genes in the A. apis-infected 4-, 5-, and 6-day-old larval guts were significantly downregulated. Together, the results demonstrated that effective overexpression and knockdown of ame-miR-34 in both noninfected and A. apis-infected A. mellifera larval guts could be achieved by the feeding method, and ame-miR-34 exerted a regulatory function in the host immune response to A. apis invasion through positive regulation of the expression of hsp and abct. Our findings not only provide a valuable reference for the functional investigation of bee larval miRNAs but also reveal the regulatory role of ame-miR-34 in A. mellifera larval weight and immune response. Additionally, the results of this study may provide a promising molecular target for the treatment of chalkbrood disease.

Comprehensive investigation and regulatory function of lncRNAs engaged in western honey bee larval immune response to Ascosphaera apis invasion

Ascosphaera apis is a fungal pathogen that exclusively infects bee larvae, causing chalkbrood disease, which results in severe damage for beekeeping industry. Long non-coding RNAs (lncRNAs) are versatile regulators in various biological processes such as immune defense and host-pathogen interaction. However, expression pattern and regulatory role of lncRNAs involved in immune response of bee host to A. apis invasion is still very limited. Here, the gut tissues of Apis mellifera ligustica 4-, 5-, and 6-day-old larvae inoculated by A. apis spores (AmT1, AmT2, and AmT3 groups) and corresponding un-inoculated larval guts (AmCK1, AmCK2, and AmCK3 groups) were prepared and subjected to deep sequencing, followed by identification of lncRNAs, analysis of differentially expressed lncRNAs (DElncRNAs), and investigation of competing endogenous RNA (ceRNA) network. In total, 3,746 A. m. ligustica lncRNAs were identified, including 78 sense lncRNAs, 891 antisense lncRNAs, 1,893 intergenic lncRNAs, 346 bidirectional lncRNAs, and 210 intronic lncRNAs. In the 4-, 5-, and 6- comparison groups, 357, 236, and 505 DElncRNAs were discovered. Additionally, 217, 129, and 272 DElncRNAs were respectively predicted to regulate neighboring genes via cis-acting manner, and these targets were associated with a series of GO terms and KEGG pathways of great importance, such as response to stimulus and Jak-STAT signaling pathway. Moreover, 197, 95, and 356 DElncRNAs were observed to target 10, eight, and 21 DEmiRNAs and further target 147, 79, and 315 DEmRNAs, forming complex regulatory networks. Further investigation suggested that these targets were engaged in several key cellular and humoral immune pathways, such as phagosome and MAPK signaling pathway. Ultimately, the expression trends of nine randomly selected DElncRNAs were verified by RT-qPCR, confirming the authenticity and reliability of our transcriptome data. Findings in this current work not only provide candidate DElncRNAs for functional study, but also lay a foundation for unclosing the mechanism underlying DElncRNA-regulated larval immune responses to A. apis invasion.

Current Knowledge on Bee Innate Immunity Based on Genomics and Transcriptomics

As important pollinators, bees play a critical role in maintaining the balance of the ecosystem and improving the yield and quality of crops. However, in recent years, the bee population has significantly declined due to various pathogens and environmental stressors including viruses, bacteria, parasites, and increased pesticide application. The above threats trigger or suppress the innate immunity of bees, their only immune defense system, which is essential to maintaining individual health and that of the colony. In addition, bees can be divided into solitary and eusocial bees based on their life traits, and eusocial bees possess special social immunities, such as grooming behavior, which cooperate with innate immunity to maintain the health of the colony. The omics approach gives us an opportunity to recognize the distinctive innate immunity of bees. In this regard, we summarize innate bee immunity from a genomic and transcriptomic perspective. The genetic characteristics of innate immunity were revealed by the multiple genomes of bees with different kinds of sociality, including honeybees, bumblebees, wasps, leaf-cutter bees, and so on. Further substantial transcriptomic data of different tissues from diverse bees directly present the activation or suppression of immune genes under the infestation of pathogens or toxicity of pesticides.

Genome-wide identification and comparative analysis of Cry toxin receptor families in 7 insect species with a focus on Spodoptera litura

Cadherin, aminopeptidase N (APN) and alkaline phosphatase (ALP) have been characterized as Cry receptors. In this study, comparative genomic analysis of the 3 receptor families was performed in 7 insects. ALPs and APNs are divided into three and eight clades in phylogenetic trees, respectively. ALPs in clade 3 and APNs in clade 1 contain multiple paralogs within each species and most paralogs are located closely in chromosomes. Drosophila melanogaster has expanded APNs in clade 5 and were lowly expressed in midgut. Cadherins are divided into 16 clades; they may diverge before holometabolous insect speciation except for BtR and Cad89D-like clades. Eight insects from different orders containing BtR orthologs are sensitive to Cry1A or Cry3A, while five species without BtR are insensitive to both toxins. Most APNs in clade 1, several ALPs in clade 3, BtR and Cad89D-like genes were highly or moderately expressed in larval midgut of Spodoptera litura and the other six species, and several members in these clades have been identified as Cry receptors. Expressions of putative S. litura Cry receptors in the midgut after exposing to Bt toxins were also analyzed.

Transcriptomic Responses of the Honey Bee Brain to Infection with Deformed Wing Virus

Managed colonies of European honey bees (Apis mellifera) are under threat from Varroa destructor mite infestation and infection with viruses vectored by mites. In particular, deformed wing virus (DWV) is a common viral pathogen infecting honey bees worldwide that has been shown to induce behavioral changes including precocious foraging and reduced associative learning. We investigated how DWV infection of bees affects the transcriptomic response of the brain. The transcriptomes of individual brains were analyzed using RNA-Seq after experimental infection of newly emerged adult bees with DWV. Two analytical methods were used to identify differentially expressed genes from the ~15,000 genes in the Apis mellifera genome. The 269 genes that had increased expression in DWV infected brains included genes involved in innate immunity such as antimicrobial peptides (AMPs), Ago2, and Dicer. Single bee brain NMR metabolomics methodology was developed for this work and indicates that proline is strongly elevated in DWV infected brains, consistent with the increased presence of the AMPs abaecin and apidaecin. The 1361 genes with reduced expression levels includes genes involved in cellular communication including G-protein coupled, tyrosine kinase, and ion-channel regulated signaling pathways. The number and function of the downregulated genes suggest that DWV has a major impact on neuron signaling that could explain DWV related behavioral changes.

Reconstruction and functional annotation of Ascosphaera apis full-length transcriptome utilizing PacBio long reads combined with Illumina short reads

Ascosphaera apis is a widespread fungal pathogen of honeybee larvae that results in chalkbrood disease, leading to heavy losses for the beekeeping industry in China and many other countries. This work was aimed at generating a full-length transcriptome of A. apis using PacBio single-molecule real-time (SMRT) sequencing. Here, more than 23.97 Gb of clean reads was generated from long-read sequencing of A. apis mycelia, including 464,043 circular consensus sequences (CCS) and 394,142 full-length non-chimeric (FLNC) reads. In total, we identified 174,095 high-confidence transcripts covering 5141 known genes with an average length of 2728 bp. We also discovered 2405 genic loci and 11,623 isoforms that have not been annotated yet within the current reference genome. Additionally, 16,049, 10,682, 4520 and 7253 of the discovered transcripts have annotations in the Non-redundant protein (Nr), Clusters of Eukaryotic Orthologous Groups (KOG), Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. Moreover, 1205 long non-coding RNAs (lncRNAs) were identified, which have less exons, shorter exon and intron lengths, shorter transcript lengths, lower GC percent, lower expression levels, and fewer alternative splicing (AS) evens, compared with protein-coding transcripts. A total of 253 members from 17 transcription factor (TF) families were identified from our transcript datasets. Finally, the expression of A. apis isoforms was validated using a molecular approach. Overall, this is the first report of a full-length transcriptome of entomogenous fungi including A. apis. Our data offer a comprehensive set of reference transcripts and hence contributes to improving the genome annotation and transcriptomic study of A. apis.

Comparative Transcriptome and Proteome Analysis of Heat Acclimation in Predatory Mite Neoseiulus barkeri

In our previous study, we reported a high temperature adapted strain (HTAS) of the predatory mite Neoseiulus barkeri was artificially selected via a long-term heat acclimation (35°C) and frequent heat hardenings. To understand the molecular basis of heat acclimation, 'omics' analyses were performed to compare the differences between HTAS female adults to conventional strain (CS) at transcriptional and translational levels. We obtained a total of 5,374 differentially expressed genes and 500 differentially expressed proteins. Among them, 119 transcripts had concurrent transcription and translation profiles. It's conserved that some processes, such as high expression of heat shock protein (HSP) genes, involved in heat tolerance of transcriptome analyses, while many protective enzymes including glutathione S-transferase, superoxide dismutase, peroxidase, and cytochrome P450 displayed down-regulated expression. KEGG analysis mapped 4,979 and 348 differentially expressed genes and proteins, to 299 and 253 pathways, respectively. The mitogen-activated protein kinases (MAPK) signaling pathway may provide new insights for the investigation of the molecular mechanisms of heat tolerance. Correlation enriched pathways indicated that there were four pathways associated with heat acclimation involving in energy metabolism and immunity. In addition, the expression patterns of ten randomly selected genes including HSP were consistent with the transcriptome results obtained through quantitative real-time PCR. Comparisons between transcriptome and proteome results indicated the upregulation of HSPs and genes participated in ATP production, immunity and energy metabolism process. A majority of antioxidant-related genes and detoxication-related genes were down-regulated suggesting a fitness cost of heat acclimation. Our results demonstrated that heat tolerance during a long-time acclimation of N. barkeri is a fairly complicated process of physiological regulations. These findings also contribute to a better understanding of the mechanisms of thermal responses of phytoseiid mites which could provide useful information for biological control through natural enemies.

Transcriptome data of control and Ascosphaera apis infected Apis mellifera ligustica larval guts

Ascosphaera apis is an obligate fungal pathogen of honeybee larvae that leads to chalkbrood, which causes heavy losses for the apiculture in China and many other countries. In this article, guts of 4-, 5-, 6-day-old Apis mellifera ligustica larvae challenged by A. apis (AmT1, AmT2, AmT3) and normal 4-day-old larval guts (AmCK) were sequenced using next-generation sequencing technology. On average, 29,196,197, 28,690,943, 29,779,715 and 30,496,725 raw reads were yielded from these four groups; an average of 29,540,895 clean reads were obtained after quality control. In addition, the mapping ratio of clean reads in treatment and control groups to the Apis mellifera genome were over 97.16%. For more insight please see “Uncovering the immune responses of Apis mellifera ligustica larval gut to Ascosphaera apis infection utilizing transcriptome sequencing” [1]. The raw data were submitted to the National Centre for Biotechnology Information (NCBI) Sequence Read Archive (SRA) database under accession numbers: SRR4084091, SRR4084092, SRR4084095, SRR4084096, SRR4084097, SRR4084098, SRR4084099, SRR4084100, SRR4084101, SRR4084102, SRR4084093, SRR4084094.

Systematic identification of circular RNAs and corresponding regulatory networks unveil their potential roles in the midguts of eastern honeybee workers

Currently, knowledge of circular RNAs (circRNAs) in insects including honeybee is extremely limited. Here, differential expression profiles and regulatory networks of circRNAs in the midguts of Apis cerana cerana workers were comprehensively investigated using transcriptome sequencing and bioinformatics. In total, 9589 circRNAs (201-800 nt in length) were identified from 8-day-old and 11-day-old workers' midguts (Ac1 and Ac2); among them, 5916 (61.70%) A. cerana cerana circRNAs showed conservation with our previously indentified circRNAs in Apis mellifera ligucstica workers' midguts (Xiong et al., Acta Entomologica Sinica 61:1363-1375, 2018). Five circRNAs were confirmed by RT-PCR and Sanger sequencing. Interestingly, novel_circ_003723, novel_circ_002714, novel_circ_002451, and novel_circ_001980 were highly expressed in both Ac1 and Ac2. In addition, the source genes of circRNAs were involved in 34 GO terms including organelle and cellular process and 141 pathways such as endocytosis and Wnt signaling pathway. Moreover, 55 DEcircRNAs including 34 upregulated and 21 downregulated circRNAs were identified in Ac2 compared with Ac1. circRNA-miRNA regulatory networks indicated that 1060 circRNAs can target 74 miRNAs; additionally, the DEcircRNA-miRNA-mRNA networks suggested that 13 downregulated circRNAs can bind to eight miRNAs and 29 miRNA-targeted mRNAs, while 16 upregulated circRNAs can link to 9 miRNAs and 29 miRNA-targeted mRNAs. These results indicated that DEcircRNAs as ceRNAs may play a comprehensive role in the growth, development, and metabolism of the worker's midgut via regulating source genes and interacting with miRNAs. Notably, eight DEcircRNAs targeting miR-6001-y were likely to be key participants in the midgut development. Our findings not only offer a valuable resource for further studies on A. cerana cerana circRNA and novel insights into understanding the molecular mechanisms underlying the midgut development of eastern honeybee but also provide putative circRNA candidates for functional research in the near future and novel biomarkers for identification of eastern honeybee species including A. cerana cerana and honeybee diseases such as chalkbrood and microsporidiosis.

Transcriptomic investigation of immune responses of the Apis cerana cerana larval gut infected by Ascosphaera apis

Chalkbrood is the most common fungal disease in honeybees. The objective of this study was to reveal immune responses in the Apis cerana cerana larval gut following Ascosphaera apis invasion. Combining a previously assembled transcriptome of A. c. cerana larval gut and the high-throughput sequencing data obtained in this study, 6152 differentially expressed genes (DEGs) were clustered into eight profiles. Trend analysis showed three significant up-regulated profiles (p ≤ 0.05) and three down-regulated profiles. Gene Ontology (GO) term analysis suggested that DEGs within significant up-regulated and down-regulated clusters were enriched in 46 and 38 functional groups, respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis indicated a majority of DEGs were involved in ribosome structure or function, carbon metabolism, biosynthesis of amino acids, and oxidative phosphorylation. In addition, 142 and 14 DEGs were annotated in the cellular immune- and humoral immune-related pathways, respectively. Further investigation indicated that DEGs up-regulated in cellular immune and humoral immune pathways outnumbered those that were down-regulated. Moreover, immune responses of A. c. cerana and Apis mellifera ligustica larvae were compared and studied to decipher resistance of eastern honeybee larvae to A. apis. These results demonstrated that a large number of genes involved in immunity-related pathways were activated by A. apis. Our findings provided valuable information for elucidating the molecular mechanisms underlying immune responses of A. c. cerana larvae to A. apis infection and pathogen-host interactions during chalkbrood infection.

The Wisdom of Honeybee Defenses Against Environmental Stresses

As one of the predominant pollinator, honeybees provide important ecosystem service to crops and wild plants, and generate great economic benefit for humans. Unfortunately, there is clear evidence of recent catastrophic honeybee colony failure in some areas, resulting in markedly negative environmental and economic effects. It has been demonstrated that various environmental stresses, including both abiotic and biotic stresses, functioning singly or synergistically, are the potential drivers of colony collapse. Honeybees can use many defense mechanisms to decrease the damage from environmental stress to some extent. Here, we synthesize and summarize recent advances regarding the effects of environmental stress on honeybees and the wisdom of honeybees to respond to external environmental stress. Furthermore, we provide possible future research directions about the response of honeybees to various form of stressors.