Wei Xiao-Xin, Wang Xiao-Quan, Hong De-Yuan
Laboratory of Systematic and Evolutionary Botany, Institute of Botany, Chinese Academy of Sciences, 20 Nanxincun, Xiangshan, Beijing 100093, China.
J Mol Evol. 2003 Dec;57(6):623-35. doi: 10.1007/s00239-003-2512-8.
Nuclear ribosomal DNA (nrDNA) of gymnosperms, especially Pinaceae, is characterized by slow concerted evolution and exhibits substantial ITS-region length variation (975-3663 bp), in sharp contrast to the narrow range (565-700 bp) in angiosperms. Here we examined intra- and intergenomic heterogeneity of the nrDNA internal transcribed spacer (ITS) region in four varieties of Larix potaninii, a species from the mountainous areas of western China. Two clones with more than a 100-bp deletion in ITS1 were detected in L. potaninii var. chinensis and L. potaninii var. australis, respectively. The deletion resulted in the loss of most part, including the motif sequence, of subrepeat 1 (SR1). Sequence divergence and phylogenetic analyses showed that some clones would be pseudogenes given their low GC content, high substitution rates, unique positions in the phylogenetic trees, or significant length variation. These clones might represent orphons or paralogues at minor loci resulting from large-scale gene or chromosome reorganization. Some recombinants characterized by chimeric structure and discordant phylogenetic positions in partitioned sequence analyses indicate that unequal crossing-over plays an important role in the process of nrDNA concerted evolution. In addition, some varieties of L. potaninii might have experienced an nrDNA founder effect parallel to their geographical isolation.
裸子植物，尤其是松科植物的核糖体DNA（nrDNA），其特点是协同进化缓慢，并且内部转录间隔区（ITS）长度存在显著变异（975 - 3663 bp），这与被子植物狭窄的范围（565 - 700 bp）形成鲜明对比。在此，我们研究了中国西部山区物种波氏落叶松四个变种nrDNA内部转录间隔区（ITS）的基因组内和基因组间异质性。在太白红杉和西南落叶松中分别检测到两个在ITS1中有超过100 bp缺失的克隆。该缺失导致亚重复序列1（SR1）的大部分，包括基序序列丢失。序列分歧和系统发育分析表明，一些克隆由于其低GC含量、高替换率、在系统发育树中的独特位置或显著的长度变异，可能是假基因。这些克隆可能代表了由于大规模基因或染色体重组导致的次要位点上的孤基因或旁系同源基因。一些在分区序列分析中具有嵌合结构和不一致系统发育位置特征的重组体表明，不等交换在nrDNA协同进化过程中起重要作用。此外，波氏落叶松的一些变种可能经历了与其地理隔离平行的nrDNA奠基者效应。
