Kordis Dusan, Lovsin Nika, Gubensek Franc
Department of Biochemistry and Molecular Biology, Jozef Stefan Institute, Ljubljana, Slovenia.
Syst Biol. 2006 Dec;55(6):886-901. doi: 10.1080/10635150601052637.
L1 retrotransposons constitute the largest single component of mammalian genomes. In contrast to the single remaining lineage of L1 retrotransposons in mammalian genomes, some teleost fishes contain a highly diverse L1 retrotransposon repertoire. Major evolutionary changes in L1 retrotransposon repertoires have therefore taken place in the land vertebrates (Tetrapoda). The lack of sequence data for L1 retrotransposons in the basal living Tetrapoda lineages prompted an investigation of their distribution and evolution in the genomes of the key tetrapod lineages, amphibians and reptiles, and in lungfishes. In this study, we combined genome database searches with PCR analysis to demonstrate that L1 retrotransposons are present in the genomes of lungfishes, amphibians, and lepidosaurs. Phylogenomic analysis shows that the genomes of Deuterostomia possess three highly divergent groups of L1 retrotransposons, with distinct distribution patterns. The analysis of L1 diversity shows the presence of a very large number of diverse L1 families, each with very low copy numbers, at the time of the origin of tetrapods. During the evolution of synapsids, all but one L1 lineage have been lost. This study establishes that the loss of L1 diversity and explosion in copy numbers occurred in the synapsid ancestors of mammals, and was most probably caused by severe population bottlenecks.
L1逆转座子是哺乳动物基因组中最大的单一组成部分。与哺乳动物基因组中仅存的一个L1逆转座子谱系不同，一些硬骨鱼含有高度多样化的L1逆转座子库。因此，L1逆转座子库的主要进化变化发生在陆地脊椎动物（四足动物）中。基础现存四足动物谱系中L1逆转座子缺乏序列数据，促使人们对其在关键四足动物谱系（两栖动物和爬行动物）以及肺鱼基因组中的分布和进化进行研究。在本研究中，我们将基因组数据库搜索与PCR分析相结合，以证明L1逆转座子存在于肺鱼、两栖动物和鳞龙类动物的基因组中。系统发育基因组分析表明，后口动物基因组拥有三组高度分化的L1逆转座子，具有不同的分布模式。对L1多样性的分析表明，在四足动物起源时存在大量不同的L1家族，每个家族的拷贝数都非常低。在合弓纲动物的进化过程中，除了一个L1谱系外，其他所有谱系都已消失。本研究证实，L1多样性的丧失和拷贝数的激增发生在哺乳动物的合弓纲祖先中，很可能是由严重的种群瓶颈造成的。
