Intra-Colonial Viral Infections in Western Honey Bees (Apis Mellifera)

Abstract

RNA viruses play a significant role in the current high losses of pollinators. Although many studies have focused on the epidemiology of western honey bee (Apis mellifera) viruses at the colony level, the dynamics of virus infection within colonies remains poorly explored. In this study, the two main variants of the ubiquitous honey bee virus DWV as well as three major honey bee viruses (SBV, ABPV and BQCV) were analyzed from Varroa-destructor-parasitized pupae. More precisely, RT-qPCR was used to quantify and compare virus genome copies across honey bee pupae at the individual and subfamily levels (i.e., patrilines, sharing the same mother queen but with different drones as fathers). Additionally, virus genome copies were compared in cells parasitized by reproducing and non-reproducing mite foundresses to assess the role of this vector. Only DWV was detected in the samples, and the two variants of this virus significantly differed when comparing the sampling period, colonies and patrilines. Moreover, DWV-A and DWV-B exhibited different infection patterns, reflecting contrasting dynamics. Altogether, these results provide new insight into honey bee diseases and stress the need for more studies about the mechanisms of intra-colonial disease variation in social insects.

Keywords: evolutionary biology; host–pathogen interactions; pollinators; population genetics; viruses.

Figure 1

Overall frequency of DWV-A and DWV-B. The distribution of infections with DWV variants (Log10) is shown as the proportion of pupae with between 10⁷ and 10¹⁵ DWV genome copies/nymph. (a) Histogram for DWV-A; (b) histogram for DWV-B.

Figure 2

Relations between DWV variants. The relationships between the DWV variant (DWV-A and DWV-B) levels in the samples are represented by plotting the infection with both variants for each pupa.

Figure 3

DWV variant genome copies across colonies and sampling periods. The graphs represent the levels of DWV-A and DWV-B across colonies and sampling periods. (a): Sampling period I (end of August to mid-September); (b): Sampling period II (October). The asterisks represent statistical significance after Bonferroni corrections (Period I: Kruskal–Wallis and Dunn tests; Period II: Wilcoxon–Mann–Whitney tests). Only groups with a sample size ≥ 14 were included (sample sizes: Period I: colony B: 33; Colony D: 14; Colony E: 22. Period II: Colony D: 18; Colony E: 18). Asterisks reflect significance levels.

Figure 4

DWV variant genome copies across patrilines. Boxplots representing the level of infection across pupae grouped by patrilines within each colony and sampling periods (Col B: period I; Col D: period II; Col E period I and II). The asterisks indicate levels of significance of the non-parametric tests conducted (Wilcoxon–Mann–Whitney or Dunn tests corrected with Bonferroni). (a): DWV-A genome copies; (b): DWV-B genome copies. Error bars represent 95% confidence intervals.