Gall wasps (Hymenoptera: Cynipidae) induce complex galls on oaks, roses and other plants, but the mechanism of gall induction is still unknown. Here we take a comparative genomic approach to revealing the genetic basis of gall induction. We focus on Synergus itoensis, a species that induces galls inside oak acorns. Previous studies suggested that this species evolved the ability to initiate gall formation recently, as it is deeply nested within the genus Synergus, whose members are mostly inquilines that develop inside the galls of other species. We compared the genome of S. itoensis to that of three related Synergus inquilines to identify genomic changes associated with the origin of gall induction. We used a novel Bayesian selection analysis, which accounts for branch-specific and gene-specific selection effects, to search for signatures of selection in 7,600 single-copy orthologous genes shared by the four Synergus species. We found that the terminal branch leading to S. itoensis had more genes with a significantly elevated dN/dS ratio (Positive Signature Genes, PSGs) than the other terminal branches in the tree; the S. itoensis branch also had more genes with a significantly decreased dN/dS ratio. Gene set enrichment analysis showed that the PSG set of S. itoensis, unlike those of the inquiline species, is enriched in several biological process Gene Ontology terms, the most prominent of which is “Ovarian Follicle Cell Development”. Our results indicate that the origin of gall induction is associated with distinct genomic changes, and provide a good starting point for further characterization of the genes involved.

中文翻译：

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从 Inquilines 到胆诱导剂：Synergus Gall Wasps 生活方式转变的基因组特征

瘿蜂（膜翅目：Cynipidae）在橡树、玫瑰等植物上诱导复杂瘿，但诱导瘿的机制尚不清楚。在这里，我们采用比较基因组方法来揭示胆汁诱导的遗传基础。我们专注于Synergus itoensis，一种在橡树橡子内诱导瘿的物种。先前的研究表明，该物种最近进化出了启动瘿形成的能力，因为它深深地嵌套在Synergus属中，其成员主要是在其他物种的瘿内发育的 inquilines。我们将S. itoensis的基因组与三个相关的Synergus的基因组进行了比较inquilines 以确定与胆汁诱导起源相关的基因组变化。我们使用了一种新的贝叶斯选择分析，它解释了分支特异性和基因特异性选择效应，以在四种Synergus物种共享的 7,600 个单拷贝直向同源基因中搜索选择特征。我们发现导致S. itoensis的末端分支比树中的其他末端分支具有更多具有显着升高的 dN/dS 比（Positive Signature Genes，PSG）的基因；的S. itoensis分支也有多种基因与显著降低分牛顿/ DS比率。基因集富集分析表明S. itoensis的 PSG 集与inquiline物种不同，它富含几个生物过程基因本体论术语，其中最突出的是“卵巢卵泡细胞发育”。我们的结果表明，胆汁诱导的起源与不同的基因组变化有关，并为进一步表征所涉及的基因提供了良好的起点。