Accelerated evolution and Muller's rachet in endosymbiotic bacteria
Accelerated evolution and Muller's rachet in endosymbiotic bacteria
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DOI：
10.1073/pnas.93.7.2873
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发表时间：
1996-04-02
影响因子：
11.1
通讯作者：
Moran, NA
中科院分区：
文献类型：
--
作者：
Moran, NA
许多细菌只生活在动物细胞内，并通过细胞质遗传感染宿主。这些内共生谱系表现出独特的群体结构，群体规模小，有效地没有重组，因此，内共生体预计会积累轻度有害的突变，如果这些突变构成了新突变的很大一部分，内共生体将表现出(i)更快的序列进化和（ii）反映突变偏见的碱基组成可能发生变化。对五个独立衍生的内共生分支的16S rDNA的分析表明，在每种情况下，内共生生物的进化速度比自由生活的近亲更快。对于蚜虫（Bucknera属），编码基因的进化速度加快，与肠道中相同基因的同义替换率相比，同义替换率异常低。在有害突变固定增加的假设下，这种非同义替换率的增加是预期的。支持取代对多肽功能有害的假设。这些观察结果最好解释为穆勒棘轮在小型无性种群中的结果，结合突变偏倚，根据这一解释，早先为Buchnera报道的两个观察结果，修复基因的明显缺失和伴侣蛋白的过度产生，可能反映了代偿性进化。另一个假设，涉及基因组碱基组成的选择，与加速集中在非同义位点的观察结果相矛盾。
Many bacteria live only within animal cells and infect hosts through cytoplasmic inheritance. These endosymbiotic lineages show distinctive population structure, with small population size and effectively no recombination, As a result, endosymbionts are expected to accumulate mildly deleterious mutations, If these constitute a substantial proportion of new mutations, endosymbionts will show (i) faster sequence evolution and (ii) a possible shift in base composition reflecting mutational bias, Analyses of 16S rDNA of five independently derived endosymbiont clades show, in every case, faster evolution in endosymbionts than in free-living relatives. For aphid endosymbionts (genus Bucknera), coding genes exhibit accelerated evolution and unusually low ratios of synonymous to nonsynonymous substitutions compared to ratios for the same genes for enterics, This concentration of the rate increase in nonsynonymous substitutions is expected under the hypothesis of increased fixation of deleterious mutations, Polypeptides for all Buchnera genes analyzed have accumulated amino acids with codon families rich in A+T, supporting the hypothesis that substitutions are deleterious in terms of polypeptide function. These observations are best explained as the result of Muller's ratchet within small asexual populations, combined with mutational bias, In light of this explanation, two observations reported earlier for Buchnera, the apparent loss of a repair gene and the overproduction of a chaperonin, may reflect compensatory evolution, An alternative hypothesis, involving selection on genomic base composition, is contradicted by the observation that the speedup is concentrated at nonsynonymous sites.