Infection with a Virulent Strain of <i>Wolbachia</i> Disrupts Genome Wide-Patterns of Cytosine Methylation in the Mosquito <i>Aedes aegypti</i>

Background Cytosine methylation is one of several reversible epigenetic modifications of DNA that allow a greater flexibility in the relationship between genotype and phenotype. Methylation in the simplest models dampens gene expression by modifying regions of DNA critical for transcription factor binding. The capacity to methylate DNA is variable in the insects due to diverse histories of gene loss and duplication of DNA methylases. Mosquitoes like Drosophila melanogaster possess only a single methylase, DNMT2. Description Here we characterise the methylome of the mosquito Aedes aegypti and examine its relationship to transcription and test the effects of infection with a virulent strain of the endosymbiont Wolbachia on the stability of methylation patterns. Conclusion We see that methylation in the A. aegypti genome is associated with reduced transcription and is most common in the promoters of genes relating to regulation of transcription and metabolism. Similar gene classes are also methylated in aphids and honeybees, suggesting either conservation or convergence of methylation patterns. In addition to this evidence of evolutionary stability, we also show that infection with the virulent wMelPop Wolbachia strain induces additional methylation and demethylation events in the genome. While most of these changes seem random with respect to gene function and have no detected effect on transcription, there does appear to be enrichment of genes associated with membrane function. Given that Wolbachia lives within a membrane-bound vacuole of host origin and retains a large number of genes for transporting host amino acids, inorganic ions and ATP despite a severely reduced genome, these changes might represent an evolved strategy for manipulating the host environments for its own gain. Testing for a direct link between these methylation changes and expression, however, will require study across a broader range of developmental stages and tissues with methods that detect splice variants.背景 胞嘧啶甲基化（Cytosine methylation）是DNA的多种可逆表观遗传修饰之一，可赋予基因型与表型之间的关系更强的灵活性。在经典模型中，甲基化通过修饰转录因子结合所需的关键DNA区域，抑制基因表达。由于DNA甲基转移酶（DNA methylase）的基因丢失与复制事件存在差异，昆虫的DNA甲基化能力各不相同。与黑腹果蝇（Drosophila melanogaster）类似，蚊子仅携带一种甲基转移酶DNMT2。 研究内容 本研究对埃及伊蚊（Aedes aegypti）的甲基化组（methylome）进行表征，探究其与转录过程的关联，并测试致病性内共生菌沃尔巴克氏体（Wolbachia）强毒株感染对甲基化模式稳定性的影响。 结论 研究发现，埃及伊蚊基因组中的甲基化与转录水平降低相关，且最常出现在与转录调控及代谢相关的基因启动子区域。蚜虫与蜜蜂体内的甲基化基因也属于此类功能类别，这提示甲基化模式存在保守性或趋同性。除上述进化保守性证据外，本研究还证实，致病性wMelPop沃尔巴克氏体菌株感染可诱导基因组产生额外的甲基化与去甲基化事件。尽管此类变化大多与基因功能无明显关联，且未检测到对转录的影响，但膜功能相关基因似乎存在富集现象。鉴于沃尔巴克氏体寄生于宿主的膜包被液泡中，且尽管基因组大幅缩减，仍保留了大量用于转运宿主氨基酸、无机离子与ATP的基因，此类甲基化变化或代表其为获取自身利益而调控宿主环境的进化策略。然而，要验证这些甲基化变化与基因表达之间的直接关联，还需在更广泛的发育阶段与组织中，采用能够检测可变剪接变体（splice variants）的方法开展研究。