Isolation of a natural DNA virus of Drosophila melanogaster, and characterisation of host resistance and immune responses

Drosophila melanogaster has played a key role in our understanding of invertebrate immunity. However, both functional and evolutionary studies of host-virus interaction in Drosophila have been limited by a dearth of native virus isolates. In particular, despite a long history of virus research, DNA viruses of D. melanogaster have only recently been described, and none have been available for experimental study. Here we report the isolation and comprehensive characterisation of Kallithea virus, a large double-stranded DNA virus, and the first DNA virus to have been reported from wild populations of D. melanogaster. We find that Kallithea virus infection is costly for adult flies, reaching high titres in both sexes and disproportionately reducing survival in males, and movement and late fecundity in females. Using the Drosophila Genetic Reference Panel, we quantify host genetic variance for virus-induced mortality and viral titre and identify candidate host genes that may underlie this variation, including Cdc42-interacting protein 4. Using full transcriptome sequencing of infected males and females, we examine the transcriptional response of flies to Kallithea virus infection and describe differential regulation of virus-responsive genes. This work establishes Kallithea virus as a new tractable model to study the natural interaction between D. melanogaster and DNA viruses, and we hope it will serve as a basis for future studies of immune responses to DNA viruses in insects.

黑腹果蝇（Drosophila melanogaster）在无脊椎动物免疫机制的相关研究中始终发挥着关键作用。然而，针对果蝇宿主-病毒互作的功能与进化研究，长期受制于天然病毒分离株的匮乏。具体而言，尽管病毒研究已有悠久历史，但黑腹果蝇的DNA病毒直至近年才被报道，且此前尚无可用作实验研究的毒株。本研究成功分离并完成了卡利提亚病毒（Kallithea virus）的全面表征——该病毒为大型双链DNA病毒，亦是首个从野生黑腹果蝇种群中报道的DNA病毒。研究发现，卡利提亚病毒感染对果蝇成虫存在显著有害影响：病毒可在雌雄果蝇体内均达到高滴度，且会不成比例地降低雄性存活率，同时损害雌性的运动能力与后期繁殖力。本研究借助果蝇遗传参考面板（Drosophila Genetic Reference Panel），量化了宿主遗传背景对病毒诱导死亡率与病毒滴度的影响，并筛选出可能介导该遗传变异的候选宿主基因，其中包括Cdc42相互作用蛋白4（Cdc42-interacting protein 4）。通过对感染病毒的雌雄果蝇开展全转录组测序，本研究分析了果蝇对卡利提亚病毒感染的转录应答，并揭示了病毒应答基因的差异表达调控规律。本研究确立了卡利提亚病毒作为研究黑腹果蝇与DNA病毒天然互作的新型可操作模型，我们期望该模型能够为后续昆虫DNA病毒免疫应答相关研究提供坚实基础。