Outcomes of honeybee pupae inoculated with deformed wing virus genotypes A and B

Abstract

Deformed wing virus and Varroa destructor virus-1 have a high percentage of nucleotide identity and might be considered as closely related viruses: DWV genotype A (DWV-A) and DWV genotype B (DWV-B) respectively. They have been implicated in overwinter colony losses in association with Varroa destructor infestations that vectored both DWV variants. In this study, we performed experimental inoculations of honeybee pupae with viral suspensions prepared from honeybee heads naturally infected by either DWV-A or DWV-B. Two outcomes were observed: the inoculated pupae exhibited either higher rates of bees with deformed wings or higher mortality rates than control bees. For both DWV variants, the viral loads quantified in the head of inoculated bees were significantly greater than those in control bees (p < 0.01). These outcomes were not correlated to the virus genotype (DWV-A or DWV-B) detected in the inocula by RT-quantitative PCR targeting the VP3 coding sequence (RT-qPCR). However, the highest mortality rates found in our study were correlated with an increase in sacbrood virus (SBV) load. Despite the fact that only either DWV-A or DWV-B was expected to be inoculated, we observed increased mortality in honeybees that were infected with both DWV and SBV. Deformed wing virus / Varroa destructor virus-1 / virulence / wing deformity / pupae mortality

The honeybee (Apis mellifera) developmental state shapes the genetic composition of the deformed wing virus-A quasispecies during serial transmission

“…Most of this evidence comes from studies on the relative virulence of DWV-A and DWV-B, the two major strains of DWV 14,44 . Both strains are capable of producing symptoms and disease upon injection into pupae, either by varroa or experimental inoculation 25,34,47 , but DWV-B is considerably more virulent than DWV-A at both individual bee 22 and colony level 21 and is more closely associated with varroa 22,48 and winter mortality 21,49 . Recombinants between the two strains are possibly even more virulent than either parent 20,31,32, and have made it possible to map this virulence to the RNA-dependent RNA polymerase region of DWV-B 22 .…”

“…Recombinants between the two strains are possibly even more virulent than either parent 20,31,32, and have made it possible to map this virulence to the RNA-dependent RNA polymerase region of DWV-B 22 . However, virulence also comes at a cost, both individual 22,50-53 and social [54][55][56] , which may explain why the natural incidence of DWV-A/DWV-B recombinant viruses is low 57 , despite having many selective advantages over their parents in laboratory studies 20,22 , and why natural recombinants often contain the "avirulent" DWV-A polymerase region 20,31,32,47,58,59 . For example, both DWV-A and DWV-B are transmitted by varroa, but only DWV-B also replicates in varroa brain tissues 41,48,60 .…”

“…This is a natural foundation for the appearance of multiple strains adapted to different contexts. Since there is little difference between DWV-A and DWV-B in pathogenicity 47,49 , titres 20,33,47,49 or adult mortality 22 , the relative persistence of DWV-A, DWV-B and/or their recombinants in bee colonies may well be driven largely by pupal mortality and the effectiveness of social hygiene 50 , where the strains differ most.…”

“…Alternatively, the clustering may reflect systematic variation in the molecular condition of the pupae used for experimentation. These were also harvested serially and could have been conditioned by their immediate environment, such as nutrition 65,66 or the presence of other bee viruses 47,67,68 , which can affect susceptibility to DWV infection 64,66 , its pathology 47 , tolerance of its effects 65 or interfere with DWV replication 26,67 or transmission 68 . These alternatives will be investigated further in future experiments and more detailed analyses of the samples.…”

Accumulation and Competition Amongst Deformed Wing Virus Genotypes in Naïve Australian Honeybees Provides Insight Into the Increasing Global Prevalence of Genotype B

“…Three DWV genotypes have been described: DWV-A, DWV-B, and DWV-C (Mordecai et al, 2016b); only DWV-A (formally DWV) and DWV-B [formally V. destructor virus 1 or VDV-1 (Ongus et al, 2004)] are currently recognized by the International Committee on Taxonomy of Viruses. For clarity, we have employed the types A and B nomenclature widely adopted in recent publications (Martin et al, 2012;McMahon et al, 2016;Mordecai et al, 2016a,b;Brettell and Martin, 2017;Kevill et al, 2017;Gisder et al, 2018;Brettell et al, 2019;Dubois et al, 2019;Kevill et al, 2019;Remnant et al, 2019;Tehel et al, 2019).…”