Department of Biology, University of Southern Indiana, 8600 University Blvd., Evansville, IN, 47712, USA.
Department of Chemistry and Biochemistry, University of Oklahoma, 660 Parrington Oval, Norman, OK, 73019, USA.
J Mol Evol. 2019 Dec;87(9-10):298-308. doi: 10.1007/s00239-019-09909-0. Epub 2019 Sep 5.
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.
海七鳃鳗（Petromyzon marinus）在胚胎发生过程中经历了大量的基因组改变，在此过程中，体细胞中的特定序列被从基因组中删除，但在生殖细胞中保留。在海七鳃鳗中经历缩减的一个元素是 Germ1，它由一个体细胞罕见（SR）区域和一段 28S rDNA 组成。尽管 SR 区域已被用作七鳃鳗基因组改变的标记，但它缩减的进化意义尚不清楚。我们检查了另外五种七鳃鳗物种中的 Germ1 元素，以更好地了解其进化意义。每个代表物种都包含足够相似的序列，可以通过 PCR 和 Southern 杂交检测到，尽管 Lampetra aepyptera 和 Lethenteron appendix 的 SR 区域与 Petromyzon 和三种 Ichthyomyzon 物种的同源序列有很大的不同。七鳃鳗 Germ1 序列具有许多特征，这些特征是 R2 反转录转座子的特征，R2 反转录转座子是一种专门针对 28S rDNA 的可移动元件。SR 区域的系统发育分析揭示了与我们研究中包括的物种之间关系大致一致的模式，尽管每个物种/属的 28S 片段与自身的功能性 rDNA 最密切相关，这表明 Germ1 的两个组成部分在每个谱系中是独立组装的。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 是一种存在于海七鳃鳗和其他七鳃鳗科鱼类基因组中的 R2 反转录转座子，其减少是胚胎发生过程中 rDNA 拷贝减少的附带事件。
