Three Halloween genes from the Varroa mite, Varroa destructor (Anderson & Trueman) and their expression during reproduction

The moulting arthropod: a complete genetic toolkit review

Exoskeletons are a defining character of all arthropods that provide physical support for their segmented bodies and appendages as well as protection from the environment and predation. This ubiquitous yet evolutionarily variable feature has been instrumental in facilitating the adoption of a variety of lifestyles and the exploitation of ecological niches across all environments. Throughout the radiation that produced the more than one million described modern species, adaptability afforded by segmentation and exoskeletons has led to a diversity that is unrivalled amongst animals. However, because of the limited extensibility of exoskeleton chitin and cuticle components, they must be periodically shed and replaced with new larger ones, notably to accommodate the growing individuals encased within. Therefore, arthropods grow discontinuously by undergoing periodic moulting events, which follow a series of steps from the preparatory pre-moult phase to ecdysis itself and post-moult maturation of new exoskeletons. Each event represents a particularly vulnerable period in an arthropod's life cycle, so processes must be tightly regulated and meticulously executed to ensure successful transitions for normal growth and development. Decades of research in representative arthropods provide a foundation of understanding of the mechanisms involved. Building on this, studies continue to develop and test hypotheses on the presence and function of molecular components, including neuropeptides, hormones, and receptors, as well as the so-called early, late, and fate genes, across arthropod diversity. Here, we review the literature to develop a comprehensive overview of the status of accumulated knowledge of the genetic toolkit governing arthropod moulting. From biosynthesis and regulation of ecdysteroid and sesquiterpenoid hormones, to factors involved in hormonal stimulation responses and exoskeleton remodelling, we identify commonalities and differences, as well as highlighting major knowledge gaps, across arthropod groups. We examine the available evidence supporting current models of how components operate together to prepare for, execute, and recover from ecdysis, comparing reports from Chelicerata, Myriapoda, Crustacea, and Hexapoda. Evidence is generally highly taxonomically imbalanced, with most reports based on insect study systems. Biases are also evident in research on different moulting phases and processes, with the early triggers and late effectors generally being the least well explored. Our synthesis contrasts knowledge based on reported observations with reasonably plausible assumptions given current taxonomic sampling, and exposes weak assumptions or major gaps that need addressing. Encouragingly, advances in genomics are driving a diversification of tractable study systems by facilitating the cataloguing of putative genetic toolkits in previously under-explored taxa. Analysis of genome and transcriptome data supported by experimental investigations have validated the presence of an "ultra-conserved" core of arthropod genes involved in moulting processes. The molecular machinery has likely evolved with elaborations on this conserved pathway backbone, but more taxonomic exploration is needed to characterise lineage-specific changes and novelties. Furthermore, linking these to transformative innovations in moulting processes across Arthropoda remains hampered by knowledge gaps and hypotheses based on untested assumptions. Promisingly however, emerging from the synthesis is a framework that highlights research avenues from the underlying genetics to the dynamic molecular biology through to the complex physiology of moulting.

Ecdysteroid Biosynthesis Halloween Gene Spook Plays an Important Role in the Oviposition Process of Spider Mite, Tetranychus urticae

In insects, the ecdysteroid hormone regulates development and reproduction. However, its function in the reproduction process of spider mites is still unclear. In this study, we investigated the effect of the Halloween gene Spook on the oviposition of the reproduction process in a spider mite, Tetranychus urticae. The expression patterns of the ecdysteroid biosynthesis and signaling pathway genes, as analyzed by RT-qPCR, showed that the expression pattern of the Halloween genes was similar to the oviposition pattern of the female mite and the expression patterns of the vitellogenesis-related genes TuVg and TuVgR, suggesting that the Halloween genes are involved in the oviposition of spider mites. To investigate the function of the ecdysteroid hormone on the oviposition of the reproduction process, we carried out an RNAi assay against the Halloween gene Spook by injection in female mites. Effective silencing of TuSpo led to a significant reduction of oviposition. In summary, these results provide an initial study on the effect of Halloween genes on the reproduction in T. urticae and may be a foundation for a new strategy to control spider mites.

Host brood traits, independent of adult behaviours, reduce Varroa destructor mite reproduction in resistant honeybee populations

The ectoparasitic mite Varroa destructor is an invasive species of Western honey bees (Apis mellifera) and the largest pathogenic threat to their health world-wide. Its successful invasion and expansion is related to its ability to exploit the worker brood for reproduction, which results in an exponential population growth rate in the new host. With invasion of the mite, wild honeybee populations have been nearly eradicated from Europe and North America, and the survival of managed honeybee populations relies on mite population control treatments. However, there are a few documented honeybee populations surviving extended periods without control treatments due to adapted host traits that directly impact Varroa mite fitness. The aim of this study was to investigate if Varroa mite reproductive success was affected by traits of adult bee behaviours or by traits of the worker brood, in three mite-resistant honey bee populations from Sweden, France and Norway. The mite's reproductive success was measured and compared in broods that were either exposed to, or excluded from, adult bee access. Mite-resistant bee populations were also compared with a local mite-susceptible population, as a control group. Our results show that mite reproductive success rates and mite fecundity in the three mite-resistant populations were significantly different from the control population, with the French and Swedish populations having significantly lower reproductive rates than the Norwegian population. When comparing mite reproduction in exposed or excluded brood treatments, no differences were observed, regardless of population. This result clearly demonstrates that Varroa mite reproductive success can be suppressed by traits of the brood, independent of adult worker bees.

Temporal Expression Profiles Reveal Potential Targets during Postembryonic Development of Forensically Important Sarcophaga peregrina (Diptera: Sarcophagidae)

Sarcophaga peregrina (Robineau-Desvoidy, 1830) is a species of medical and forensic importance. In order to investigate the molecular mechanism during postembryonic development and identify specific genes that may serve as potential targets, transcriptome analysis was used to investigate its gene expression dynamics from the larval to pupal stages, based on our previous de novo-assembled genome of S. peregrina. Totals of 2457, 3656, 3764, and 2554 differentially expressed genes were identified. The specific genes encoding the structural constituent of cuticle were significantly differentially expressed, suggesting that degradation and synthesis of cuticle-related proteins might actively occur during metamorphosis. Molting (20-hydroxyecdysone, 20E) and juvenile (JH) hormone pathways were significantly enriched, and gene expression levels changed in a dynamic pattern during the developmental stages. In addition, the genes in the oxidative phosphorylation pathway were significantly expressed at a high level during the larval stage, and down-regulated from the wandering to pupal stages. Weighted gene co-expression correlation network analysis (WGCNA) further demonstrated the potential regulation mechanism of tyrosine metabolism in the process of puparium tanning. Moreover, 10 consistently up-regulated genes were further validated by qRT-PCR. The utility of the models was then examined in a blind study, indicating the ability to predict larval development. The developmental, stage-specific gene profiles suggest novel molecular markers for age prediction of forensically important flies.

Standard Methods for Dissection of Varroa destructor Females

Varroa destructor (Anderson and Trueman) is known as a major pest of Apis mellifera L, especially in the Northern Hemisphere where its effects can be deleterious. As an obligate parasite, this mite relies entirely on its host to reproduce and complete its cycle. Studies focusing on isolated organs are needed to better comprehend this organism. To conduct such targeted molecular or physiological studies, the dissection of V. destructor mites is crucial as it allows the extraction of specific organs. Here, we propose a technical article showing detailed steps of females V. destructor dissection, illustrated with pictures and videos. These illustrated guidelines will represent a helpful tool to go further in V. destructor research.

De novo RNA-Seq and Annotation of Sesquiterpenoid and Ecdysteroid Biosynthesis Genes and MicroRNAs in a Spider Mite Eotetranychus kankitus

Eotetranychus kankitus is an important mite pest in citrus, but molecular data on the developmental processes of E. kankitus are lacking. The different development stages mix of E. kankitus was used to sequence for transcriptome and small RNAs to identify genes and predict miRNAs associated with sesquiterpenoid and ecdysteroid biosynthesis and signaling pathways. More than 36 million clean reads were assembled and 67,927 unigenes were generated. Of the unigenes, 19,300 were successfully annotated through annotation databases NR, SwissProt, COG, GO, KEGG, PFAM, and KOG. The transcripts were involved in sesquiterpenoid biosynthesis (11 genes) and ecdysteroid biosynthesis and signaling pathway (13 genes). Another, small RNA library was obtained and 31 conserved miRNAs were identified. Five most abundant miRNAs were Ek-miR-5735, Ek-miR-1, Ek-miR-263a, Ek-miR-184, and Ek-miR-8. The target genes related to sesquiterpenoid and ecdysteroid showed that 10 of the conserved miRNAs could potentially target the sesquiterpenoid and ecdysteroid pathway according to four-prediction software, sRNAT, miRanda, RNAhybrid, and Risearch2. Thus, the results of this study will provide bioinformatics information for further molecular studies of E. kankitus which may facilitate improved pest control strategies.

Characterization of ecdysteroid biosynthesis in the pond wolf spider, Pardosa pseudoannulata

Ecdysteroids, as the key growth hormones, regulate moulting, metamorphosis and reproduction in arthropods. Ecdysteroid biosynthesis is catalysed by a series of cytochrome P450 monooxygenases (CYP450s) encoded by Halloween genes, including spook (spo), phantom (phm), disembodied (dib), shadow (sad) and shade (shd). The ecdysteroid biosynthesis in insects is clear with 20-hydroxyecdysone (20E) as the main ecdysteroid. However, the information on the major ecdysteroids in arachnids is limited. In this study, Halloween genes spo, dib, sad and shd, but not phm, were identified in the pond wolf spider, Pardosa pseudoannulata. Phylogenetic analysis grouped arachnid and insect Halloween gene products into two CYP450 clades, the CYP2 clan (spo and phm) and the mitochondrial clan (dib, sad, and shd). In P. pseudoannulata, the temporal expression profile of the four Halloween genes in concurrence with spiderling moulting with steady increase in the course of the 2nd instar followed by a rapid dropdown once moulting was completed. Spatially, the four Halloween genes were highly expressed in spiderling abdomen and in the ovaries of female adults. In parallel, ponasterone A (PA), but not 20E, was detected by LC-MS/MS analysis in P. pseudoannulata, and it was demonstrated as a functional ecdysteroid in the spider by accelerating of moulting with PA addition. The present study revealed the different ecdysteroid biosynthesis pathways in spiders and insects.

Juvenile hormone pathway in honey bee larvae: A source of possible signal molecules for the reproductive behavior of Varroa destructor

The parasitic mite Varroa destructor devastates honey bee (Apis mellifera) colonies around the world. Entering a brood cell shortly before capping, the Varroa mother feeds on the honey bee larvae. The hormones 20-hydroxyecdysone (20E) and juvenile hormone (JH), acquired from the host, have been considered to play a key role in initiating Varroa's reproductive cycle. This study focuses on differential expression of the genes involved in the biosynthesis of JH and ecdysone at six time points during the first 30 hr after cell capping in both drone and worker larvae of A. mellifera. This time frame, covering the conclusion of the honey bee brood cell invasion and the start of Varroa's ovogenesis, is critical to the successful initiation of a reproductive cycle. Our findings support a later activation of the ecdysteroid cascade in honey bee drones compared to worker larvae, which could account for the increased egg production of Varroa in A. mellifera drone cells. The JH pathway was generally downregulated confirming its activity is antagonistic to the ecdysteroid pathway during the larva development. Nevertheless, the genes involved in JH synthesis revealed an increased expression in drones. The upregulation of jhamt gene involved in methyl farnesoate (MF) synthesis came into attention since the MF is not only a precursor of JH but it is also an insect pheromone in its own right as well as JH-like hormone in Acari. This could indicate a possible kairomone effect of MF for attracting the mites into the drone brood cells, along with its potential involvement in ovogenesis after the cell capping, stimulating Varroa's initiation of egg laying.

Functional analysis of ecdysteroid biosynthetic enzymes of the rice planthopper, Nilaparvata lugens

Ecdysteroids, insect steroid hormones, play key roles in regulating insect development and reproduction. Hemipteran insects require ecdysteroids for egg production; however, ecdysteroid synthesis (ecdysteroidogenesis) details have not been elucidated. We identified all known genes encoding ecdysteroidogenic enzymes in Nilaparvata lugens and clarified their necessity during nymphal and ovarian development. We confirmed that N. lugens utilized 20-hydroxyecdysone as an active hormone. Assays using heterologous expression of enzymes in Drosophila S2 cells showed conserved functions of enzymes Neverland, CYP306A2, CYP314A1 and CYP315A1, but not CYP302A1. RNA interference and rescue analysis using 20-hydroxyecdysone demonstrated that most of the genes were necessary for nymphal development. The identified N. lugens enzymes showed conserved functions and pathways for ecdysteroidogenesis. Knockdown of ecdysteroidogenic enzyme genes in newly molted females caused failure of egg production: less vitellogenic and mature eggs in ovaries, fewer laid eggs and embryonic development deficiency of laid eggs. Considering the high expressions of ecdysteroidogenic enzyme genes in adults and ovaries, ecdysteroidogenesis in ovaries was critical for N. lugens ovarian development. Our study presents initial evidence that hemipteran insects require ecdysteroidogenesis for ovarian development.

Molecular characterization and functional analysis of the Halloween genes and CYP18A1 in Bemisia tabaci MED

The ecdysteroid hormone 20-hydroxyecdysone (20E), a critical hormone in arthropods, plays an essential role in insect growth, molting and reproduction. A previous study showed that 20E is actually regulated by six P450 genes (five P450 genes belonging to the Halloween family and a CYP18A1 gene) in model insects. However, the role of the six P450 genes in Bemisia tabaci Q (also call Mediterranean, MED), an important pest of field crops, remains unclear. Here, six P450 genes were cloned by RT-PCR, and the phylogenetic tree indicated a close orthologous relationship of these P450 genes between MED and other insects. Spatiotemporal expression profiling revealed that five P450 genes (CYP18A1, CYP306A1, CYP307A2, CYP314A1 and CYP315A1) were expressed at significantly higher levels in the head than in the abdomen and thorax. Four P450 genes (CYP302A1, CYP307A2, CYP314A1 and CYP315A1) were expressed at the highest levels in males, and CYP18A1 was expressed at the highest levels in the 4th nymph stage. The molting process was delayed by approximately 1-3 days after knockdown of these genes at the 4th nymph stage, and the mean proportion of shriveled or dead insects reached 8.3% (CYP18A1), 20.8% (CYP302A1), 7.0% (CYP307A2), 31.8% (CYP306A1), 28.6% (CYP314A1) and 24.1% (CYP315A1). In addition, 20E rescued the negative effect of ds-CYP306A1, ds-CYP314A1 and ds-CYP315A1 on the eclosion rate. We concluded that these Halloween genes and CYP18A1 likely participate in the development of MED, and in particular, CYP306A1 could be used as a putative insecticide target for controlling this piercing-sucking insect.

Natural selection, selective breeding, and the evolution of resistance of honeybees (Apis mellifera) against Varroa

We examine evidence for natural selection resulting in Apis mellifera becoming tolerant or resistant to Varroa mites in different bee populations. We discuss traits implicated in Varroa resistance and how they can be measured. We show that some of the measurements used are ambiguous, as they measure a combination of traits. In addition to behavioural traits, such as removal of infested pupae, grooming to remove mites from bees or larval odours, small colony size, frequent swarming, and smaller brood cell size may also help to reduce reproductive rates of Varroa. Finally, bees may be tolerant of high Varroa infections when they are resistant or tolerant to viruses implicated in colony collapse. We provide evidence that honeybees are an extremely outbreeding species. Mating structure is important for how natural selection operates. Evidence for successful natural selection of resistance traits against Varroa comes from South Africa and from Africanized honeybees in South America. Initially, Varroa was present in high densities and killed about 30% of the colonies, but soon after its spread, numbers per hive decreased and colonies survived without treatment. This shows that natural selection can result in resistance in large panmictic populations when a large proportion of the population survives the initial Varroa invasion. Natural selection in Europe and North America has not resulted in large-scale resistance. Upon arrival of Varroa, the frequency of traits to counter mites and associated viruses in European honey bees was low. This forced beekeepers to protect bees by chemical treatment, hampering natural selection. In a Swedish experiment on natural selection in an isolated mating population, only 7% of the colonies survived, resulting in strong inbreeding. Other experiments with untreated, surviving colonies failed because outbreeding counteracted the effects of selection. If loss of genetic variation is prevented, colony level selection in closed mating populations can proceed more easily, as natural selection is not counteracted by the dispersal of resistance genes. In large panmictic populations, selective breeding can be used to increase the level of resistance to a threshold level at which natural selection can be expected to take over.

Divergent evolutionary trajectories following speciation in two ectoparasitic honey bee mites

Multispecies host-parasite evolution is common, but how parasites evolve after speciating remains poorly understood. Shared evolutionary history and physiology may propel species along similar evolutionary trajectories whereas pursuing different strategies can reduce competition. We test these scenarios in the economically important association between honey bees and ectoparasitic mites by sequencing the genomes of the sister mite species Varroa destructor and Varroa jacobsoni. These genomes were closely related, with 99.7% sequence identity. Among the 9,628 orthologous genes, 4.8% showed signs of positive selection in at least one species. Divergent selective trajectories were discovered in conserved chemosensory gene families (IGR, SNMP), and Halloween genes (CYP) involved in moulting and reproduction. However, there was little overlap in these gene sets and associated GO terms, indicating different selective regimes operating on each of the parasites. Based on our findings, we suggest that species-specific strategies may be needed to combat evolving parasite communities.

Identification of Halloween Genes and RNA Interference-Mediated Functional Characterization of a Halloween Gene shadow in Plutella xylostella

Ecdysteroids play an essential role in controlling insect development and reproduction. Their pathway is regulated by a group of enzymes called Halloween gene proteins. The relationship between the Halloween genes and ecdysteroid synthesis has yet to be clearly understood in diamondback moth, Plutella xylostella (L.), a worldwide Lepidoptera pest attacking cruciferous crops and wild plants. In this study, complete sequences for six Halloween genes, neverland (nvd), shroud (sro), spook (spo), phantom (phm), disembodied (dib), shadow (sad), and shade (shd), were identified. Phylogenetic analysis revealed a strong conservation in insects, including Halloween genes of P. xylostella that was clustered with all other Lepidoptera species. Three Halloween genes, dib, sad, and shd were highly expressed in the adult stage, while nvd and spo were highly expressed in the egg and pupal stages, respectively. Five Halloween genes were highly expressed specifically in the prothorax, which is the major site of ecdysone production. However, shd was expressed predominantly in the fat body to convert ecdysone into 20-hydroxyecdysone. RNAi-based knockdown of sad, which is involved in the last step of ecdysone biosynthesis, significantly reduced the 20E titer and resulted in a longer developmental duration and lower pupation of fourth-instar larvae, as well as caused shorter ovarioles and fewer fully developed eggs of P. xylostella. Furthermore, after the knockdown of sad, the expression levels of Vg and VgR genes were significantly decreased by 77.1 and 53.0%. Meanwhile, the number of eggs laid after 3 days was significantly reduced in sad knockdown females. These results suggest that Halloween genes may play a critical role in the biosynthesis of ecdysteroids and be involved in the development and reproduction of P. xylostella. Our work provides a solid basis for understanding the functional importance of these genes, which will help to screening potential genes for pest management of P. xylostella.

A gene for resistance to the Varroa mite (Acari) in honey bee (Apis mellifera) pupae

Social insect colonies possess a range of defences which protect them against highly virulent parasites and colony collapse. The host-parasite interaction between honey bees (Apis mellifera) and the mite Varroa destructor is unusual, as honey bee colonies are relatively poorly defended against this parasite. The interaction has existed since the mid-20th Century, when Varroa switched host to parasitize A. mellifera. The combination of a virulent parasite and relatively naïve host means that, without acaricides, honey bee colonies typically die within 3 years of Varroa infestation. A consequence of acaricide use has been a reduced selective pressure for the evolution of Varroa resistance in honey bee colonies. However, in the past 20 years, several natural-selection-based breeding programmes have resulted in the evolution of Varroa-resistant populations. In these populations, the inhibition of Varroa's reproduction is a common trait. Using a high-density genome-wide association analysis in a Varroa-resistant honey bee population, we identify an ecdysone-induced gene significantly linked to resistance. Ecdysone both initiates metamorphosis in insects and reproduction in Varroa. Previously, using a less dense genetic map and a quantitative trait loci analysis, we have identified Ecdysone-related genes at resistance loci in an independently evolved resistant population. Varroa cannot biosynthesize ecdysone but can acquire it from its diet. Using qPCR, we are able to link the expression of ecdysone-linked resistance genes to Varroa's meals and reproduction. If Varroa co-opts pupal compounds to initiate and time its own reproduction, mutations in the host's ecdysone pathway may represent a key selection tool for honey bee resistance and breeding.

RNA-seq analysis of gene expression changes during pupariation in Bactrocera dorsalis (Hendel) (Diptera: Tephritidae)

Background

The oriental fruit fly, Bactrocera dorsalis (Hendel) has been considered to be one of the most important agricultural pest around the world. As a holometabolous insect, larvae must go through a metamorphosis process with dramatic morphological and structural changes to complete their development. To better understand the molecular mechanisms of these changes, RNA-seq of B. dorsalis from wandering stage (WS), late wandering stage (LWS) and white puparium stage (WPS) were performed.

Results

In total, 11,721 transcripts were obtained, out of which 1914 genes (578 up-regulated and 1336 down-regulated) and 2047 genes (655 up-regulated and 1392 down-regulated) were found to be differentially expressed between WS and LWS, as well as between WS and WPS, respectively. Of these DEGs, 1862 and 1996 genes were successfully annotated in various databases. The analysis of RNA-seq data together with qRT-PCR validation indicated that during this transition, the genes in the oxidative phosphorylation pathway, and genes encoding P450s, serine protease inhibitor, and cuticular proteins were down-regulated, while the serine protease genes were up-regulated. Moreover, we found some 20-hydroxyecdysone (20E) biosynthesis and signaling pathway genes had a higher expression in the WS, while the genes responsible for juvenile hormone (JH) synthesis, degradation, signaling and transporter pathways were down-regulated, suggesting these genes might be involved in the process of larval pupariation in B. dorsalis. For the chitinolytic enzymes, the genes encoding chitinases (chitinase 2, chitinase 5, chitinase 8, and chitinase 10) and chitin deacetylase might play the crucial role in the degradation of insect chitin with their expressions significantly increased during the transition. Here, we also found that chitin synthase 1A might be involved in the chitin synthesis of cuticles during the metamorphosis in B. dorsalis.

Conclusions

Significant changes at transcriptional level were identified during the larval pupariation of B. dorsalis. Importantly, we also obtained a vast quantity of RNA-seq data and identified metamorphosis associated genes, which would all help us to better understand the molecular mechanism of metamorphosis process in B. dorsalis.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5077-z) contains supplementary material, which is available to authorized users.

Halloween genes in panarthropods and the evolution of the early moulting pathway in Ecdysozoa

Moulting is a characteristic feature of Ecdysozoa-the clade of moulting animals that includes the hyperdiverse arthropods and less speciose groups, such as onychophorans, tardigrades and nematodes. Moulting has been best analysed in arthropods, specifically in insects and crustaceans, in which a complex neuroendocrine system acts at the genomic level and initiates the transcription of genes responsible for moulting. The key moulting hormones, ecdysone and 20-hydroxyecdysone, are subsequently synthesized from cholesterol ingested with food. Their biosynthesis is regulated by the Rieske-domain protein Neverland and cytochrome P450 enzymes encoded by the so-called 'Halloween' genes. Ecdysone is then released into the haemolymph and modified into 20-hydroxyecdysone, which binds to the nuclear receptor EcR/USP and initiates transcription of the Early genes. As little is known about the moulting pathway of other ecdysozoans, we examined the occurrence of genes involved in ecdysteroid biosynthesis and the early moulting cascade across ecdysozoan subgroups. Genomic and transcriptomic searches revealed no Halloween genes in cycloneuralians, whereas only shadow (CYP315A1) is present in onychophorans and tardigrades, suggesting that the Halloween genes evolved stepwise in panarthropods. These findings imply that the genes which were responsible for the ecdysteroid biosynthesis in the last common ancestor of Ecdysozoa are currently unknown.

Transcriptome profiling of the honeybee parasite Varroa destructor provides new biological insights into the mite adult life cycle

Background

The parasite Varroa destructor represents a significant threat to honeybee colonies. Indeed, development of Varroa infestation within colonies, if left untreated, often leads to the death of the colony. Although its impact on bees has been extensively studied, less is known about its biology and the functional processes governing its adult life cycle and adaptation to its host. We therefore developed a full life cycle transcriptomic catalogue in adult Varroa females and included pairwise comparisons with males, artificially-reared and non-reproducing females (10 life cycle stages and conditions in total).

Results

Extensive remodeling of the Varroa transcriptome was observed, with an upregulation of energetic and chitin metabolic processes during the initial and final phases of the life cycle (e.g. phoretic and post-oviposition stages), whereas during reproductive stages in brood cells genes showing functions related to transcriptional regulation were overexpressed. Several neurotransmitter and neuropeptide receptors involved in behavioural regulation, as well as active compounds of salivary glands, were also expressed at a higher level outside the reproductive stages. No difference was detected between artificially-reared phoretic females and their counterparts in colonies, or between females who failed to reproduce and females who successfully reproduced, indicating that phoretic individuals can be reared outside host colonies without impacting their physiology and that mechanisms underlying reproductive failure occur before oogenesis.

Conclusions

We discuss how these new findings reveal the remarkable adaptation of Varroa to its host biology and notably to the switch from living on adults to reproducing in sealed brood cells. By spanning the entire adult life cycle, our work captures the dynamic changes in the parasite gene expression and serves as a unique resource for deciphering Varroa biology and identifying new targets for mite control.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4668-z) contains supplementary material, which is available to authorized users.

A feeding protocol for delivery of agents to assess development in Varroa mites

A novel feeding protocol for delivery of bio-active agents to Varroa mites was developed by providing mites with honey bee larva hemolymph supplemented with cultured insect cells and selected materials delivered on a fibrous cotton substrate. Mites were starved, fed on treated hemolymph to deliver selected agents and then returned to bee larvae. Transcript levels of two reference genes, actin and glyceraldehyde 3-phosphate dehydrogenase (GAPDH), as well as for nine selected genes involved in reproductive processes showed that the starvation and feeding protocol periods did not pose a high level of stress to the mites as transcript levels remained comparable between phoretic mites and those completing the protocol. The feeding protocol was used to deliver molecules such as hormone analogs or plasmids. Mites fed with Tebufenozide, an ecdysone analog, had higher transcript levels of shade than untreated or solvent treated mites. In order to extend this feeding protocol, cultured insect cells were incorporated to a final ratio of 1 part cells and 2 parts hemolymph. Although supplementation with Bombyx mori Bm5 cells increased the amount of hemolymph consumed per mite, there was a significant decrease in the percentage of mites that fed and survived. On the other hand, Drosophila melanogaster S2 cells reduced significantly the percentage of mites that fed and survived as well as the amount of hemolymph consumed. The feeding protocol provides a dynamic platform with which to challenge the Varroa mite to establish efficacy of control agents for this devastating honey bee pest.

Identification and expression patterns of Halloween genes encoding cytochrome P450s involved in ecdysteroid biosynthesis in the cotton bollworm Helicoverpa armigera

20-Hydroxyecdysone (20E) is a key hormone which regulates growth, development and reproduction in insects. Although cytochrome P450 enzymes (P450s) participating in the ecdysteroid biosynthesis of 20E have been characterized in a few model insects, no work has been published on the molecular entity of their orthologs in the cotton bollworm Helicoverpa armigera, a major pest insect in agriculture worldwide. In this study, four cytochrome P450 homologs, namely HarmCYP302A1, HarmCYP306A1, HarmCYP314A1 and HarmCYP315A1 from H. armigera, were identified and evolutional conservation of these Halloween genes were revealed among lepidopteran. Expression analyses showed that HarmCYP302A1 and HarmCYP315A1 were predominantly expressed in larval prothoracic glands, whereas this predominance was not always observed for HarmCYP306A1 and CYP314A1. The expression patterns of Halloween genes indicate that the fat bodies may play an important role in the conversion of ecdysone into 20E in larval-larval molt and in larval-pupal metamorphosis, and raise the possibility that HarmCYP315A1 plays a role in tissue-specific regulation in the steroid biosynthesis in H. armigera. These findings represent the first identification and expression characterization of four steriodogenic P450 genes and provide the groundwork for future functional and evolutionary study of steroid biosynthesis in this agriculturally important pest.

RNA interference-mediated knockdown of the Halloween gene Spookiest (CYP307B1) impedes adult eclosion in the western tarnished plant bug, Lygus hesperus

Ecdysteroids play a critical role in coordinating insect growth, development and reproduction. A suite of cytochrome P450 monooxygenases coded by what are collectively termed Halloween genes mediate ecdysteroid biosynthesis. In this study, we describe cloning and RNA interference (RNAi)-mediated knockdown of the CYP307B1 Halloween gene (Spookiest) in the western tarnished plant bug, Lygus hesperus. Transcripts for Ly. hesperus Spookiest (LhSpot) were amplified from all life stages and correlated well with timing of the pre-moult ecdysteroid pulse. In adults, LhSpot was amplified from heads of both genders as well as female reproductive tissues. Heterologous expression of a LhSpot fluorescent chimera in cultured insect cells co-localized with a fluorescent marker of the endoplasmic reticulum/secretory pathway. RNAi-mediated knockdown of LhSpot in fifth instars reduced expression of ecdysone-responsive genes E74 and E75, and prevented adult development. This developmental defect was rescued following application of exogenous 20-hydroxyecdysone but not exogenous 7-dehydrocholesterol. The unequivocal RNAi effects on Ly. hesperus development and the phenotypic rescue by 20-hydroxyecdysone are causal proof of the involvement of LhSpot in ecdysteroid biosynthesis and related developmental processes, and may provide an avenue for development of new control measures against Ly. hesperus.

A Toolbox for Quantitative Gene Expression in Varroa destructor: RNA Degradation in Field Samples and Systematic Analysis of Reference Gene Stability

Varroa destructor is the major pest of Apis mellifera and contributes to the global honey bee health crisis threatening food security. Developing new control strategies to combat Varroa will require the application of molecular biology, including gene expression studies by quantitative real-time reverse transcription-polymerase chain reaction (qRT-PCR). Both high quality RNA samples and suitable stable internal reference genes are required for accurate gene expression studies. In this study, ten candidate genes (succinate dehydrogenase (SDHA), NADH dehydrogenase (NADH), large ribsosmal subunit, TATA-binding protein, glyceraldehyde-3-phosphate dehydrogenase, 18S rRNA (18S), heat-shock protein 90 (HSP90), cyclophilin, α-tubulin, actin), were evaluated for their suitability as normalization genes using the geNorm, Normfinder, BestKeeper, and comparative ΔCq algorithims. Our study proposes the use of no more than two of the four most stable reference genes (NADH, 18S, SDHA and HSP90) in Varroa gene expression studies. These four genes remain stable in phoretic and reproductive stage Varroa and are unaffected by Deformed wing virus load. When used for determining changes in vitellogenin gene expression, the signal-to-noise ratio (SNR) for the relatively unstable genes actin and α-tubulin was much lower than for the stable gene combinations (NADH + HSP90 +18S; NADH + HSP90; or NADH). Using both electropherograms and RT-qPCR for short and long amplicons as quality controls, we demonstrate that high quality RNA can be recovered from Varroa up to 10 days later stored at ambient temperature if collected into RNAlater and provided the body is pierced. This protocol allows the exchange of Varroa samples between international collaborators and field sample collectors without requiring frozen collection or shipping. Our results make important contributions to gene expression studies in Varroa by proposing a validated sampling protocol to obtain high quality Varroa RNA and the validation of suitable reference genes for expression studies in this globally important pest.

How Did Arthropod Sesquiterpenoids and Ecdysteroids Arise? Comparison of Hormonal Pathway Genes in Noninsect Arthropod Genomes

The phylum Arthropoda contains the largest number of described living animal species, with insects and crustaceans dominating the terrestrial and aquatic environments, respectively. Their successful radiations have long been linked to their rigid exoskeleton in conjunction with their specialized endocrine systems. In order to understand how hormones can contribute to the evolution of these animals, here, we have categorized the sesquiterpenoid and ecdysteroid pathway genes in the noninsect arthropod genomes, which are known to play important roles in the regulation of molting and metamorphosis in insects. In our analyses, the majority of gene homologs involved in the biosynthetic, degradative, and signaling pathways of sesquiterpenoids and ecdysteroids can be identified, implying these two hormonal systems were present in the last common ancestor of arthropods. Moreover, we found that the “Broad-Complex” was specifically gained in the Pancrustacea, and the innovation of juvenile hormone (JH) in the insect linage correlates with the gain of the JH epoxidase (CYP15A1/C1) and the key residue changes in the binding domain of JH receptor (“Methoprene-tolerant”). Furthermore, the gain of “Phantom” differentiates chelicerates from the other arthropods in using ponasterone A rather than 20-hydroxyecdysone as molting hormone. This study establishes a comprehensive framework for interpreting the evolution of these vital hormonal pathways in these most successful animals, the arthropods, for the first time.

Ovarian Ecdysteroidogenesis in Both Immature and Mature Stages of an Acari, Ornithodoros moubata

Ecdysteroidogenesis is essential for arthropod development and reproduction. Although the importance of ecdysteroids has been demonstrated, there is little information on the sites and enzymes for synthesis of ecdysteroids from Chelicerates. Ecdysteroid functions have been well studied in the soft tick Ornithodoros moubata, making this species an excellent candidate for elucidating ecdysteroidogenesis in Chelicerates. Results showed that O. moubata has at least two ecdysteroidogenic enzymes, Spook (OmSpo) and Shade (OmShd). RNAi showed both enzymes were required for ecdysteroidogenesis. Enzymatic assays demonstrated OmShd has the conserved functions of ecdysone 20-hydroxylase. OmSpo showed specific expression in the ovaries of final nymphal and adult stages, indicating O. moubata utilizes the ovary as an ecdysteroidogenic tissue instead of specific tissues as seen in other arthropods. On the other hand, OmShd expression was observed in various tissues including the midgut, indicating functional ecdysteroids can be produced in these tissues. In nymphal stages, expression of both OmSpo and OmShd peaked before molting corresponding with high ecdysteroid titers in the hemolymph. In fed adult females, OmSpo expression peaked at 8–10 days after engorgement, while OmShd expression peaked immediately after engorgement. Mated females showed more frequent surges of OmShd than virgin females. These results indicate that the regulation of synthesis of ecdysteroids differs in nymphs and adult females, and mating modifies adult female ecdysteroidogenesis. This is the first report to focus on synthesis of ecdysteroids in ticks and provides essential knowledge for understanding the evolution of ecdysteroidogenesis in arthropods.