Comparison of the evolutionary dynamics of symbiotic and housekeeping loci: a case for the genetic coherence of rhizobial lineages.
Comparison of the evolutionary dynamics of symbiotic and housekeeping loci: a case for the genetic coherence of rhizobial lineages.
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DOI：
10.1093/oxfordjournals.molbev.a026041
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发表时间：
1999
影响因子：
10.7
通讯作者：
J. Wernegreen;M. Riley
中科院分区：
文献类型：
--
作者：
J. Wernegreen;M. Riley
在原核生物中，跨染色体谱系的横向基因转移可能是由质粒、噬菌体、转座元件和其他辅助DNA元件介导的。然而，这种转移的重要性以及可能限制基因交换的进化力量在很大程度上仍未在本土环境中得到探索。本研究通过系统发育同源性检验，探讨了豆科固氮共生菌--根瘤菌不同染色体谱系间共生(Sym)基因座水平转移的范围。对从几个寄主植物属的根瘤中分离的根瘤菌进行了3个座位的测序：共生结瘤基因(nob和nodc)、染色体看家基因谷氨酰胺合成酶II(GSII)和部分16S rRNA基因。分子系统发育分析表明，每个基因座通常将菌株分为相同的主要类群，分别对应于根瘤菌属、中华根瘤菌属和中生根瘤菌属。这种广泛的系统发育一致性表明，主要染色体亚群之间缺乏横向转移，这与之前对农业群体的研究形成了鲜明对比，该研究表明，在不同的染色体谱系之间，SYM基因座广泛转移。这三个根瘤菌属与豆科主要类群的总体对应关系表明，寄主植物组合可能在根瘤菌的节位和染色体位置的分化中起重要作用，并可能限制菌株之间的横向转移。第二个主要结果是根瘤菌亚组内NOD和GSII系统发育的显著不一致，这强烈地表明NOD基因在共生组之间的水平转移。这一联合证据支持根瘤菌属“生殖隔离”和独立分化的观点。这一证据表明，基因亚群之间的横向基因转移并不存在。不同根瘤菌属间寄主关联的特异性差异表明，在原生环境中，不同谱系之间共生的进化动态有很大差异。
In prokaryotes, lateral gene transfer across chromosomal lineages may be mediated by plasmids, phages, transposable elements, and other accessory DNA elements. However, the importance of such transfer and the evolutionary forces that may restrict gene exchange remain largely unexplored in native settings. In this study, tests of phylogenetic congruence are employed to explore the range of horizontal transfer of symbiotic (sym) loci among distinct chromosomal lineages of native rhizobia, the nitrogen-fixing symbiont of legumes. Rhizobial strains isolated from nodules of several host plant genera were sequenced at three loci: symbiotic nodulation genes (nodB and nodC), the chromosomal housekeeping locus glutamine synthetase II (GSII), and a portion of the 16S rRNA gene. Molecular phylogenetic analysis shows that each locus generally subdivides strains into the same major groups, which correspond to the genera Rhizobium, Sinorhizobium, and Mesorhizobium. This broad phylogenetic congruence indicates a lack of lateral transfer across major chromosomal subdivisions, and it contrasts with previous studies of agricultural populations showing broad transfer of sym loci across divergent chromosomal lineages. A general correspondence of the three rhizobial genera with major legume groups suggests that host plant associations may be important in the differentiation of rhizobial nod and chromosomal loci and may restrict lateral transfer among strains. The second major result is a significant incongruence of nod and GSII phylogenies within rhizobial subdivisions, which strongly suggests horizontal transfer of nod genes among congenerics. This combined evidence for lateral gene transfer within, but not between, genetic subdivisions supports the view that rhizobial genera are "reproductively isolated" and diverge independently. Differences across rhizobial genera in the specificity of host associations imply that the evolutionary dynamics of the symbiosis vary considerably across lineages in native settings.