Gómez-Valero Laura, Silva Francisco J, Christophe Simon Jean, Latorre Amparo
Institut Cavanilles de Biodiversitat i Biologia Evolutiva and Departament de Genètica, Universitat de València, Apartat 22085, 46071 Valencia, Spain.
Gene. 2007 Mar 1;389(1):87-95. doi: 10.1016/j.gene.2006.10.001. Epub 2006 Oct 14.
Genome reduction, a typical feature of symbiotic bacteria, was analyzed in the last stages of evolution of Buchnera aphidicola, the primary aphid endosymbiont, in two neutrally evolving regions: the pseudogene cmk and an intergenic region. These two regions were examined in endosymbionts from several lineages of their aphid host Rhopalosiphum padi, and different species of the same genus, whose divergence times ranged from 0.62 to 19.51 million years. Estimates of nucleotide substitution rates were between 4.3 and 6.7x10(-9) substitution/site/year, with G or C nucleotides being substituted around four times more frequently than A or T. Two different types of indel events were detected, of which many were small (1-10 nt) but one was large (about 200 nucleotides). With respect to the large one and considering the proportion and size of the deletions and insertions, the reduction rate was 1.3x10(-8) lost nucleotides/site/year. We propose a stepwise scenario for the last stages of evolution in B. aphidicola: together with a very slow and gradual degradation, considerable indels would punctually emerge. The only restriction to large deletion fixation is that the lost fragment does not contain essential genes.
基因组缩减是共生细菌的一个典型特征，在主要蚜虫内共生菌——蚜虫内共生菌（Buchnera aphidicola）进化的最后阶段，对两个中性进化区域：假基因cmk和一个基因间区域进行了分析。在其蚜虫宿主禾谷缢管蚜（Rhopalosiphum padi）的几个谱系以及同一属的不同物种的内共生菌中检查了这两个区域，这些物种的分化时间从62万年到1951万年不等。核苷酸替换率估计在4.3至6.7×10⁻⁹替换/位点/年之间，其中G或C核苷酸的替换频率比A或T高出约四倍。检测到两种不同类型的插入缺失事件，其中许多是小的（1 - 10个核苷酸），但有一个是大的（约200个核苷酸）。关于大的插入缺失事件，并考虑到缺失和插入的比例和大小，缩减率为1.3×10⁻⁸丢失核苷酸/位点/年。我们提出了蚜虫内共生菌进化最后阶段的一个逐步设想：伴随着非常缓慢和逐渐的降解，相当数量的插入缺失会不时出现。对大缺失固定的唯一限制是丢失的片段不包含必需基因。
