Department of Genetics, Forestry and Agricultural Biotechnology Institute, University of Pretoria, Faculty of Natural and Agricultural Sciences, Pretoria, South Africa.
PLoS One. 2013;8(3):e59355. doi: 10.1371/journal.pone.0059355. Epub 2013 Mar 12.
Gene conversion is the mechanism proposed to be responsible for the homogenization of multigene families such as the nuclear ribosomal gene clusters. This concerted evolutionary process prevents individual genes in gene clusters from accumulating mutations. The mechanism responsible for concerted evolution is not well understood but recombination during meiosis has been hypothesized to play a significant role in this homogenization. In this study we tested the hypothesis of unequal crossing over playing a significant role in gene conversion events within the ribosomal RNA cistron during meiosis, mitosis or both life stages in the fungal tree pathogen Ceratocystis manginecans.
Ceratocystis manginecans, a haploid ascomycete, reproduces homothallically and was found to have two distinct sequences within the internally transcribed spacer (ITS) region of the ribosomal RNA cistron. The different ITS types were scored using PCR-RFLP assays and chi-square analyses to determine the level of significance of the changes in the ratios of the ITS types.
The relative ratios of the two ITS sequence types changed when the fungal isolates were cultured vegetatively or allowed to produced sexual structures and spores. These active changes were shown to occur more frequently during meiosis than mitosis.
The evidence presented provides concrete support for homogenization in the rRNA gene clusters found in this fungus and that the most reasonable explanation for this process is unequal crossing over.
基因转换是一种被认为负责多基因家族（如核核糖体基因簇）同质化的机制。这种协同进化过程防止了基因簇中的个别基因积累突变。负责协同进化的机制尚不清楚，但有人假设减数分裂期间的重组在这种同质化中发挥了重要作用。在这项研究中，我们测试了在真菌树病原体 Ceratocystis manginecans 的减数分裂、有丝分裂或这两个生命阶段中，不等交换在核糖体 RNA 基因座内的基因转换事件中发挥重要作用的假设。
Ceratocystis manginecans 是一种单倍体子囊菌，通过同宗交配繁殖，并且在核糖体 RNA 基因座的内部转录间隔区 (ITS) 区域内发现有两个不同的序列。使用 PCR-RFLP 分析和卡方分析对不同的 ITS 类型进行评分，以确定 ITS 类型比率变化的显著性水平。
当真菌分离物进行营养培养或允许产生有性结构和孢子时，两种 ITS 序列类型的相对比例发生了变化。这些活跃的变化在减数分裂中比有丝分裂更频繁地发生。
所提供的证据为该真菌中发现的 rRNA 基因簇的同质化提供了具体支持，并且这个过程最合理的解释是不等交换。
