Compared to free-living bacteria, endosymbionts of sap-feeding insects have tiny and rapidly evolving genomes. Increased genetic drift, high mutation rates, and relaxed selection associated with host control of key cellular functions all likely contribute to genome decay. Phylogenetic comparisons have revealed massive variation in endosymbiont evolutionary rate, but such methods make it difficult to partition the effects of mutation vs. selection. For example, the ancestor of auchenorrhynchan insects contained two obligate endosymbionts, Sulcia and a betaproteobacterium (BetaSymb; called Nasuia in leafhoppers) that exhibit divergent rates of sequence evolution and different propensities for loss and replacement in the ensuing ~300 Ma. Here, we use the auchenorrhynchan leafhopper Macrosteles sp. nr. severini, which retains both of the ancestral endosymbionts, to test the hypothesis that differences in evolutionary rate are driven by differential mutagenesis. We used a high-fidelity technique known as duplex sequencing to measure and compare low-frequency variants in each endosymbiont. Our direct detection of de novo mutations reveals that the rapidly evolving endosymbiont (Nasuia) has a much higher frequency of single-nucleotide variants than the more stable endosymbiont (Sulcia) and a mutation spectrum that is even more AT-biased than implied by the 83.1% AT content of its genome. We show that indels are common in both endosymbionts but differ substantially in length and distribution around repetitive regions. Our results suggest that differences in long-term rates of sequence evolution in Sulcia vs. BetaSymb, and perhaps the contrasting degrees of stability of their relationships with the host, are driven by differences in mutagenesis.

中文翻译：

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突变压力驱动两个食液昆虫细菌内共生体中的差异基因组保守性

与自由生活的细菌相比，以树液为食的昆虫的共生菌具有微小且迅速发展的基因组。遗传漂移增加，突变率高以及与关键细胞功能的宿主控制有关的放松选择都可能导致基因组衰退。系统发育比较显示内共生体进化速度的巨大差异，但这种方法使得很难区分突变与选择的影响。例如，奇异虫的祖先包含两个专性的内共生菌，Sulcia和一个β变形杆菌（BetaSymb；称为Nasuia在随后的约300 Ma中表现出不同的序列进化速率和不同的丢失和替换倾向。在这里，我们使用auchenorrhynchan叶蝉Macrosteles sp。nr。保留了两个祖先共生共生体的塞韦里尼（Severini）检验了进化速率差异是由差异诱变驱动的假说。我们使用一种称为双链测序的高保真技术来测量和比较每个内共生体中的低频变异。我们直接检测的从头突变揭示了快速发展的共生细菌（Nasuia）的单核苷酸的高得多的频率比更稳定的共生细菌变种（Sulcia）和比其基因组83.1％AT含量所暗示的AT偏倚更大的突变谱。我们显示插入缺失在两个内共生体中都是常见的，但是长度和在重复区域周围的分布却存在很大差异。我们的结果表明，Sulcia与BetaSymb中长期序列进化的差异，以及它们与宿主关系的稳定性的对比程度，可能是由诱变的差异所驱动。