First come, first served: superinfection exclusion in Deformed wing virus is dependent upon sequence identity and not the order of virus acquisition

Deformed wing virus (DWV) is the most important globally distributed pathogen of honey bees and, when vectored by the ectoparasite Varroa destructor, is associated with high levels of colony losses. Divergent DWV types may differ in their pathogenicity and are reported to exhibit superinfection exclusion upon sequential infections, an inevitability in a Varroa-infested colony. We used a reverse genetic approach to investigate competition and interactions between genetically distinct or related virus strains, analysing viral load over time, tissue distribution with reporter gene-expressing viruses and recombination between virus variants. Transient competition occurred irrespective of the order of virus acquisition, indicating no directionality or dominance. Over longer periods, the ability to compete with a pre-existing infection correlated with the genetic divergence of the inoculae. Genetic recombination was observed throughout the DWV genome with recombinants accounting for ~2% of the population as determined by deep sequencing. We propose that superinfection exclusion, if it occurs at all, is a consequence of a cross-reactive RNAi response to the viruses involved, explaining the lack of dominance of one virus type over another. A better understanding of the consequences of dual- and superinfection will inform development of cross-protective honey bee vaccines and landscape-scale DWV transmission and evolution.

变形翅病毒（Deformed wing virus, DWV）是全球分布最为广泛的重要蜜蜂病原，当其由体外寄生螨狄斯瓦螨（Varroa destructor）介导传播时，会与蜂群大规模损失紧密相关。不同分型的变形翅病毒致病性存在差异，据报道其在序贯感染（sequential infections）条件下会表现出超感染排斥（superinfection exclusion）现象，而在狄斯瓦螨侵染的蜂群中，这类序贯感染实属常态。本研究采用反向遗传学技术（reverse genetic approach），对遗传特性迥异或亲缘相关的病毒毒株间的竞争与互作展开探究，具体分析了病毒载量随时间的动态变化、表达报告基因的病毒的组织分布特征，以及不同病毒变体间的重组事件。无论病毒侵染的先后顺序如何，均会出现瞬时竞争现象，这表明不存在感染方向性或毒株主导性。长期来看，毒株与预先定殖的感染病毒竞争的能力，与接种毒株的遗传分化程度密切相关。研究在变形翅病毒的全基因组范围内均检测到了基因重组现象；通过深度测序（deep sequencing）分析可知，重组病毒约占整个病毒种群的2%。本研究提出，若确实存在超感染排斥现象，其本质是宿主对相关病毒产生交叉反应性RNA干扰（RNA interference, RNAi）应答的结果，这也解释了为何没有某一种病毒分型能够在竞争中占据绝对主导地位。更深入地阐明双重感染与超感染所产生的影响，将有助于开发交叉保护性蜜蜂疫苗，并为景观尺度下变形翅病毒的传播与进化研究提供理论支撑。