为了揭示多拷贝基因的进化方式, 对濒危植物木根麦冬(Ophiopogon xylorrhizus Wang et Dai) 3个居群、6个个体的294个5S rR NA 基因拷贝及其姐妹种林生麦冬(O. sylvicola Wang et Tang)的45个拷贝进行了DNA测序和序列分析,并以这个迄今发表的最大的单个物种的5S rRNA基因数据,以PAUP程序重建了分子系统发育树.结果表明: 1)所得序列呈高度多样性,长度变化在307～548碱基之间,仅13对(3.8%)相同,序列分化指数较高:木根麦冬是0.078,林生麦冬是0.032,两物种间是0.149; 2)100%的统计值支持两物种的5S rRNA 基因分别来自于祖先种的一个拷贝,即"建立者拷贝",这个拷贝在物种形成之后进行了一系列连续的扩增,形成一个直系的基因家族,而祖先种的其他拷贝在物种形成后被丢失; 3)不同拷贝是独立进化的,序列间的一致化过程很弱,这在串联重复的rR N A基因中是罕见的; 4)木根麦冬居群间曾存在频繁的基因交流,使5S rRNA 基因的许多拷贝扩散于不同居群,维持着种内较高的遗传多样性.可以认为是某些近期发生的变化,阻止了居群间的基因交流,导致该物种广泛的自交,发生自交衰退,并最后导致濒危.%To reveal how a large gene family evolved in the highly organized gen ome, the present study sequenced 294 copies of 5S rRNA genes from six individual s of three populations of Ophiopogon xylorrhizus Wang et Dai, and 45 copies from its closest species, O. sylvicola Wang et Tang. Based on the sequences the gene -phylogeny was inferred through PAUP. The sequences were highly diversified. Of t he 339 randomly cloned copies, only 13 pairs (3.8%) were identical, with length v aried between 307-548 bp (average 438.6 bp). The sequence differentiation index ( SDI) in O. xylorrhizus was 0.078, in O. sylvicola was 0.032, and between these two species was 0.149. In the most parsimonious tree reconstructed, all of the 294 c opies of O. xylorrhizus were grouped into one clade, and those of O. sylvicola were gr ouped into another clade with 100% bootstrap support. The result demonstrated th at the copies of 5S rRNA genes originated from just one progenitor copy, "found e r copy", in both of the species respectively, followed by a series of proliferat ion, and little, if any, homogenization ever occurred in the copies. As the comm on ancestor of the two species must have borne multiple copies of the genes, all the other copies were lost during or after the speciation. Additionally, the ph ylogeny clearly showed that the founder copy was proliferated continuously throu gh time, not just once. O. xylorrhizus is an extremely endangered species in whi ch high genetic diversity, selfing and depression on seedlings were observed pre viously. In our study the sequences from different populations or different indi viduals were mixed in the branches of the O. xylorrhizus clade, although the y ev olved independently. It is assumed that gene flow of the species happened much m ore frequently in the past, making the proliferated copies dispersed to differen t populations, and the species maintained an amount of genetic diversity. Theref ore, the selfing and seedling depression would be the result of a recent event. We suggest that some changes in outer mechanisms, such as the pollinators or see d-dispersers, might cause the species selfed, depressed and, finally, endangered .
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