The sea lamprey (Petromyzon marinus) undergoes substantial genomic alterations during embryogenesis in which specific sequences are deleted from the genome of somatic cells yet retained in cells of the germ line. One element that undergoes diminution in P. marinus is Germ1, which consists of a somatically rare (SR) region and a fragment of 28S rDNA. Although the SR-region has been used as a marker for genomic alterations in lampreys, the evolutionary significance of its diminution is unknown. We examined the Germ1 element in five additional species of lamprey to better understand its evolutionary significance. Each representative species contained sequences similar enough to the Germ1 element of P. marinus to be detected via PCR and Southern hybridizations, although the SR-regions of Lampetra aepyptera and Lethenteron appendix are quite divergent from the homologous sequences of Petromyzon and three species of Ichthyomyzon. Lamprey Germ1 sequences have a number of features characteristic of the R2 retrotransposon, a mobile element that specifically targets 28S rDNA. Phylogenetic analyses of the SR-regions revealed patterns generally consistent with relationships among the species included in our study, although the 28S-fragments of each species/genus were most closely related to its own functional rDNA, suggesting that the two components of Germ1 were assembled independently in each lineage. Southern hybridizations showed evidence of genomic alterations involving Germ1 in each species. Our results suggest that Germ1 is a R2 retroelement that occurs in the genome of P. marinus and other petromyzontid lampreys, and that its diminution is incidental to the reduction in rDNA copies during embryogenesis.

中文翻译：

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Germ1的特性和进化，Lampreys中的元素Germin1不断减少（Cyclostomata：Petromyzontidae）。

七叶树（Petromyzon marinus）在胚胎发生过程中经历了重大的基因组改变，其中特定序列从体细胞基因组中删除，但仍保留在种系的细胞中。Germ1是一种在体育假单胞菌中减少的元素，它由体细胞稀有（SR）区和28S rDNA片段组成。尽管SR-区域已被用作七lamp鳗中基因组改变的标记，但其减小的进化意义尚不清楚。我们检查了另外5种七。鳗中的Germ1元素，以更好地了解其进化意义。每个具有代表性的物种所包含的序列都与水母假单胞菌Germ1元件相似，可通过PCR和Southern杂交进行检测，尽管Lampetra aepyptera和Lethenteron阑尾的SR区与Petromyzon和3种Ichthyomyzon的同源序列差异很大。Lamprey Germ1序列具有R2反转录转座子的许多特征，R2反转录转座子是一种特异性靶向28S rDNA的移动元件。对SR区的系统发育分析揭示了与我们研究中的物种之间的关系基本一致的模式，尽管每个物种/属的28S片段与其自身功能性rDNA的关系最为密切，这表明Germ1的两个组件已组装在一起在每个血统中都是独立的 Southern杂交显示了每个物种中涉及Germ1的基因组改变的证据。我们的结果表明，Germ1是在P基因组中发生的R2逆转录因子。