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

Abstract

Honeybee colony deaths are often attributed to the ectoparasitic mite Varroa destructor and deformed wing virus (DWV), vectored by the mite. In the presence of V. destructor both main genotypes (DWV-A and DWV-B) have been correlated with colony loss. Studies show that DWV-B is the most prevalent genotype in the United Kingdom and Europe. More recently DWV-B has increased in prevalence in the United States. The increasing prevalence of DWV-B at the expense of DWV-A suggests that competition exists between the genotypes. Competition may be due to disparities in virulence between genotypes, differences in fitness, such as rate of replication, or a combination of factors. In this study we investigated if DWV genotypes differ in their rate of accumulation in Australian honeybees naïve to both V. destructor and DWV, and if viral load was associated with mortality in honeybee pupae. We singly and co-infected pupae with DWV-A, DWV-B, and a recombinant strain isolated from a V. destructor tolerant bee population. We monitored viral accumulation throughout pupation, up to 192 h postinjection. We found significant differences in accumulation, where DWV-A accumulated to significantly lower loads than DWV-B and the DWV-recombinant. We also found evidence of competition, where DWV-B loads were significantly reduced in the presence of DWV-A, but still accumulated to the highest loads overall. In contrast to previous studies, we found significant differences in virulence between pupae injected with DWV-A and DWV-B. The average mortality associated with DWV-B (0.4% ± 0.33 SE) and DWV-recombinant (2.2% ± 0.83 SE) injection were significantly less than observed for DWV-A (11% ± 1.2 SE). Our results suggest that a higher proportion of DWV-B infected pupae will emerge into adults, compared to DWV-A. Overall, our data suggest that low mortality in pupae and the ability of DWV-B to accumulate to higher loads relative to DWV-A even during co-infection may favor vector transmission by V. destructor, and may thus be contributing factors to the increasing prevalence of DWV-B globally.

RNAseq of Deformed Wing Virus and Other Honey Bee-Associated Viruses in Eight Insect Taxa with or without Varroa Infestation

“…Interestingly, contrary to other studies [9,11,33], our samples were frequently co-infected with both master variants (Figures 2 and 3), potentially supporting the findings of Ryabov et al [35] that the different master variants are adapted to co-exist. While a recent experimental co-infection study showed that DWV-B replicated to higher levels than DWV-A in co-infections [36], and the vast majority of our samples contained more DWV-A reads than DWV-B, suggesting a lack of any strong competitive exclusion in this population. This was a similar finding as in the experimental infection study by Tehel et al [37] in which DWV-A replicated to slightly higher levels than DWV-B when co-infected.…”

Advances and perspectives in selecting resistance traits against the parasitic mite Varroa destructor in honey bees

“…In addition, the type of mite-transferred viruses infecting the honey bee populations may influence infestation levels, allowing for or compromising survival. Differences in the virulence of the various deformed wing virus types [ 122 ], for instance, may have a direct effect on the number of mites that can be tolerated by the colony [ 123 , 124 ]. This survival threshold can also be influenced by the haplotype of the mite [ 125 ].…”

“…This condition can hardly be reproduced in common beekeeping conditions. In Wales, the involvement of viruses in colony survival is hypothesized via the superinfection exclusion of the virulent variant DWV-A by the less virulent variant DWV-B [ 122 , 205 , 206 ]. However, other results suggest a higher virulence of DWV-B when compared to DWV-A [ 207 , 208 ].…”

Deformed wing virus prevalence and load in honeybees in South Africa

“…Africanised bees are also resistant to varroa mites, and despite the mite’s presence since the 1970s and 1980s, Brazil remains unique in that DWV-A remains the dominant genotype throughout the country, with DWV-B dominating in only one of 26 colonies studied [ 16 ]. This is unexpected, since under both controlled [ 20 , 21 ] and natural field conditions [ 9 , 10 ], DWV-B outcompetes and replaces DWV-A. This competition explains why there is no significant difference (χ 2 = 3.3, p = 0.07) between the number of DWV-A- or DWV-B-dominant colonies between the UK [ 10 ] and South Africa, which have both been infested with varroa mites for over 30 years.…”