MOLECULAR EVOLUTIONARY CONSEQUENCES OF ENDOSYMBIOSIS

大多数专性内共生细菌的基因组具有进化速度快、GC含量极低和基因组大小小的特点。这些共同的特征表明细菌的生活方式严重影响了分子进化的模式。然而，很少有比较研究已经解决了驱动序列和基因组进化的过程内共生。该研究的目的是利用系统发育和群体遗传方法区分形成内共生体基因组的分子和进化机制。我们建议对比γ -3亚型变形菌的进化速率和模式，包括自由生活的肠杆菌和相关的木蚁、采采蝇和蚜虫的细菌内共生体。本研究的具体目的是：(1)通过对几个蛋白质编码位点的分子进化和群体遗传分析，量化突变偏差、漂移和选择对细胞内细菌蛋白质进化的影响；(2)通过测试这些物种的适应性密码子使用和量化突变偏差，探索塑造内共生生物同义变异的力量。(3)通过检测木蚁中未表征的内共生体的基因组大小和修复基因缺失来研究内共生体的基因组减少模式，并探索这些小细菌基因组缺失的突变倾向。本研究的长期目标是了解细菌内共生的分子进化后果。我们将通过检查在专性、细胞内关联的背景下发生的基因组变化来解决这个问题。该项目的结果将使我们了解细胞内细菌的进化模式，包括病原体和共生菌，并将阐明在细菌进化中改变突变和选择之间平衡的力量。这项工作也将有助于开发更现实的具有偏碱基组成的DNA序列的进化模型，包括细胞器基因组，以及脊椎动物和其他分类群的偏基因区域。

Genomes of most obligately endosymbiotic bacteria are characterized by fast evolutionary rates, extremely low GC contents, and small genome sizes. These shared features suggest that bacterial lifestyle severely affects patterns of molecular evolution. However, few comparative studies have addressed the processes that drive sequence and genome evolution in endosymbionts. The goal of the proposed study is to distinguish molecular and evolutionary mechanisms that shape endosymbiont genomes using phylogenetic and population genetic approaches. We propose to contrast rates and patterns of evolution across species of gamma-3 subdivision Proteobacteria, including free- living species of enterobacteria and related bacterial endosymbionts of carpenter ants, tsetse flies, and aphids. The specific aims of this study are (1) to quantify the impact of mutational biases, drift, and selection on protein evolution of intracellular bacteria, through molecular evolutionary and population genetic analyses of several protein-coding loci, (2) to explore forces that shape synonymous variation in endosymbionts, by testing for adaptive codon usage and quantifying mutational biases in these species, (3) to examine patterns of genome reduction in endosymbionts by testing for reduced genome size and repair gene loss in the uncharacterized endosymbionts of carpenter ants, and exploring mutational biases toward deletions in these small bacterial genomes. The long-range objective of this study is to understand the molecular evolutionary consequences of endosymbiosis in bacteria. We will approach this issue by examining genomic changes that occur in the context of obligate, intracellular associations. Results of this project will inform our understanding of the modes of evolution of intracellular bacteria, including both pathogens and mutualists, and will shed light on forces that shift the balance between mutation and selection in bacterial evolution. This work will also contribute to the development of more realistic evolutionary models for DNA sequences with biased base compositions, including genomes of organelles, and biased gene regions of vertebrates and other taxa.