Draft genome of the honey bee ectoparasitic mite, Tropilaelaps mercedesae, is shaped by the parasitic life history

Background The number of managed honey bee colonies has considerably decreased in many developed countries in recent years and the ectoparasitic mites are considered as major threats to honey bee colonies and health. However, their general biology remains poorly understood. Results We sequenced the genome and transcriptomes of Tropilaelaps mercedesae, the prevalent ectoparasitic mite infesting honey bees in Asia. The de novo assembled genome sequence (353 Mb) represents 53% of the estimated genome size because of the compression of repetitive sequences; nevertheless, we predicted 15,190 protein-coding genes which were well supported by the mite transcriptomes and proteomic dataes. Although amino acid substitutions have been accelerated within the conserved core genes in of two mites, T. mercedesae and Metaseiulus occidentalis, T. mercedesae has undergone the least gene family expansion and contraction between the seven arthropods we tested. The number of sensory system genes has been dramatically reduced; meanwhile, T. mercedesae may have evolved a specialized cuticle and water homeostasis mechanisms, as well as epigenetic control of gene expression for ploidy compensation between males and females., and water homeostasis. T. mercedesae contains all gene sets required to detoxify xenobiotics, enabling it to be miticide resistant. T. mercedesae is closely associated with a symbiotic bacteriuma (Rickettsiella grylli-like) and DWVdeformed wing virus (DWV), the most prevalent honey bee virus. The presence of DWV in both adult male and female mites was also confirmed by the proteomic analysis. Conclusions T. mercedesae has a very specialized life history and habitat as the ectoparasitic mite strictly dependsing on the honey bee inside the a stable colony. Thus, comparison of the genome and transcriptome sequences with those of a tick and free-living mites and tick has revealed the specific features of the genome shaped by interaction with the honey bee and colony environment. T. mercedesae, as well as Varroa destructor, genome and transcriptome sequences not only provide insights into the mite biology, but may also help to develop measures to control the most serious pests of the honey bee.

**背景**：近年来，诸多发达国家的人工饲养蜜蜂蜂群数量显著下降，体外寄生螨（ectoparasitic mite）被视为威胁蜜蜂蜂群健康与存续的主要危险因素，但这类螨类的基础生物学特性仍未得到充分解析。

**结果**：本研究对亚洲地区侵染蜜蜂的优势体外寄生螨——梅氏热厉螨（Tropilaelaps mercedesae）的基因组与转录组开展测序。受重复序列压缩影响，从头组装的基因组序列（de novo assembled genome，353 Mb）仅覆盖预估基因组大小的53%；不过研究共预测得到15190个蛋白质编码基因（protein-coding gene），相关结果得到了该螨转录组与蛋白质组数据的充分支持。尽管梅氏热厉螨与西方盲走螨（Metaseiulus occidentalis）的保守核心基因中氨基酸替换速率有所加快，但在本次测试的7种节肢动物中，梅氏热厉螨的基因家族扩张与收缩程度最低。其感官系统基因（sensory system gene）数量大幅缩减；同时，梅氏热厉螨可能演化出特化的表皮（cuticle）与水分稳态（water homeostasis）调控机制，以及用于实现雌雄个体间倍性补偿（ploidy compensation）的基因表达表观遗传调控（epigenetic control）通路。此外，梅氏热厉螨携带所有外源物解毒（detoxify xenobiotics）所需的基因集，使其具备杀螨剂抗性（miticide resistance）。该螨与类格里氏立克次体（Rickettsiella grylli-like）这一共生细菌（symbiotic bacterium）以及最普遍的蜜蜂病毒——变形翅病毒（Deformed Wing Virus, DWV）存在紧密关联；蛋白质组分析亦证实，雌雄成虫螨体内均存在变形翅病毒。

**结论**：梅氏热厉螨作为严格依赖稳定蜂群内蜜蜂的体外寄生螨，其生活史与栖息环境具有高度特化性。因此，将该螨的基因组与转录组序列与蜱虫以及自由生活螨类的序列进行比较，可揭示其在与蜜蜂及蜂群环境互作过程中演化形成的基因组特征。梅氏热厉螨与瓦螨（Varroa destructor）的基因组及转录组序列解析，不仅有助于深入理解螨类生物学特性，也可为防控蜜蜂这一最严重的虫害提供新思路。