A Virulent Strain of Deformed Wing Virus (DWV) of Honeybees (Apis mellifera) Prevails after Varroa destructor-Mediated, or In Vitro, Transmission

Abstract

The globally distributed ectoparasite Varroa destructor is a vector for viral pathogens of the Western honeybee (Apis mellifera), in particular the Iflavirus Deformed Wing Virus (DWV). In the absence of Varroa low levels DWV occur, generally causing asymptomatic infections. Conversely, Varroa-infested colonies show markedly elevated virus levels, increased overwintering colony losses, with impairment of pupal development and symptomatic workers. To determine whether changes in the virus population were due Varroa amplifying and introducing virulent virus strains and/or suppressing the host immune responses, we exposed Varroa-naïve larvae to oral and Varroa-transmitted DWV. We monitored virus levels and diversity in developing pupae and associated Varroa, the resulting RNAi response and transcriptome changes in the host. Exposed pupae were stratified by Varroa association (presence/absence) and virus levels (low/high) into three groups. Varroa-free pupae all exhibited low levels of a highly diverse DWV population, with those exposed per os (group NV) exhibiting changes in the population composition. Varroa-associated pupae exhibited either low levels of a diverse DWV population (group VL) or high levels of a near-clonal virulent variant of DWV (group VH). These groups and unexposed controls (C) could be also discriminated by principal component analysis of the transcriptome changes observed, which included several genes involved in development and the immune response. All Varroa tested contained a diverse replicating DWV population implying the virulent variant present in group VH, and predominating in RNA-seq analysis of temporally and geographically separate Varroa-infested colonies, was selected upon transmission from Varroa, a conclusion supported by direct injection of pupae in vitro with mixed virus populations. Identification of a virulent variant of DWV, the role of Varroa in its transmission and the resulting host transcriptome changes furthers our understanding of this important viral pathogen of honeybees.

A mutualistic symbiosis between a parasitic mite and a pathogenic virus undermines honey bee immunity and health

“…Separate independent work further corroborated this evidence, showing that viral infection in honey bees interfered with the expression of genes that participate in the Toll pathway (11,12). This finding supports the hypothesis that in honey bees, and more generally in insects, inducible antiviral barriers besides RNAi-mediated mechanisms (11)(12)(13)(14)(15)(16)(17)(18) may have an important role. Indeed, in collapsing colonies, these latter barriers under NF-κB or JAK-STAT control appear to be targeted, whereas the RNAi machinery seems to be unaffected (19).…”

“…That earlier study provided evidence supporting a major role of DWV in the immune suppression process, characterized by a negative impact on a member of the NF-κB protein family (10). Separate independent work further corroborated this evidence, showing that viral infection in honey bees interfered with the expression of genes that participate in the Toll pathway (11,12). This finding supports the hypothesis that in honey bees, and more generally in insects, inducible antiviral barriers besides RNAi-mediated mechanisms (11)(12)(13)(14)(15)(16)(17)(18) may have an important role.…”

“…The active role of V. destructor in the dispersal and enhanced replication of the virus, triggered by parasite feeding (7,10,12,22), indicates that the mite-virus association has clear benefits for the latter, whereas an adaptive value for the mite, if any, remains unknown. The feeding behavior of V. destructor is complex, characterized by prolonged use of a feeding hole created by the mother mite on abdominal sternites of the bee pupa, through which both the mother and its offspring repeatedly feed on the bee's hemolymph (23).…”

Transcriptional signatures of parasitization and markers of colony decline in Varroa-infested honey bees (Apis mellifera)

“…As in Experiment 1, several DEGs play a role in immune function (Evans et al, 2006;Ryabov et al, 2014). This include up-regulation of genes encoding antimicrobial peptides in infested bees, and changes in expression of Argonaute 3 (GB49909; Brennecke et al, 2007) and a RISC loading complex (GB47214; Brutscher et al, 2015), which are involved in the RNAi pathway.…”

A Genetic Network for Systemic RNA Silencing in Plants

“…For correlation analyses for sRNA libraries, the number of miRNA hits 740 corresponding to the previously identified set of 50 Nicotiana miRNAs was determined. All 741 analyzed sRNA libraries for local and systemic leaf samples contained similar proportions of 742 host-encoded miRNA reads (Nakasugi et al, 2013;Ryabov et al, 2014) The green fluorescence intensity was measured using ImageJ software (Supplemental Fig. S4).…”