The reissner lamprey Lethenteron reissneri, belonging to the class Cyclostomata, serves as a bridge between invertebrates and jawed vertebrates, and is considered the sister group of jawed vertebrates. However, despite this evolutionary significance, the genetic mechanisms underlying the adaptive evolution of the lamprey lineage remain unclear. Here, we assembled a 1.06 Gb chromosome‐level draft genome of L. reissneri, with 72 chromosomes (ranging in length from 4.5 Mb to 25.9 Mb) and a scaffold N50 length of 13.23 Mb. Genome quality comparisons revealed that the reissner lamprey genome has higher completeness and contiguity than the previously published sea lamprey and Japanese lamprey genomes. Moreover, reissner lamprey, sea lamprey, and Japanese lamprey species share similar transposable element profiles and Hox gene cluster compositions, suggesting that a burst of transposable element activity and whole genome duplication occurred before their divergence. Additionally, the Lip gene copy numbers, which have been studied for their functions in the host defence system, were found to be expanded uniquely in lamprey lineages, suggesting key roles for these genes in lamprey evolution and adaptation. We also identified two neural‐related genes, Nrn1 and Unc13a, with copy number expansions in jawed vertebrates, which may be functionally relevant to the origin of lamprey brains. Hence, this study not only provides the first chromosome‐level reference genome for Cyclostomata, but also highlights features of the unique biology and adaptive evolution of the lamprey lineage.

中文翻译：

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Lethenteron reissneri 的染色体水平基因组组装提供了对七鳃鳗进化的见解

赖斯纳七鳃鳗Lethenteron reissneri属于环口纲纲，是无脊椎动物和有颌脊椎动物之间的桥梁，被认为是有颌脊椎动物的姐妹群。然而，尽管具有这种进化意义，但七鳃鳗谱系适应性进化的遗传机制仍不清楚。在这里，我们组装了一个 1.06 Gb 染色体水平的莱氏乳杆菌基因组草图，具有 72 条染色体（长度从 4.5 Mb 到 25.9 Mb）和 13.23 Mb 的支架 N50 长度。基因组质量比较显示，与之前发表的海七鳃鳗和日本七鳃鳗基因组相比，赖斯纳七鳃鳗基因组具有更高的完整性和连续性。此外，赖斯纳七鳃鳗、海七鳃鳗和日本七鳃鳗物种具有相似的转座因子谱和Hox基因簇组成，表明在它们分化之前发生了转座因子活性和全基因组复制的爆发。此外，唇已经研究了它们在宿主防御系统中的功能的基因拷贝数在七鳃鳗谱系中独特地扩展，表明这些基因在七鳃鳗进化和适应中的关键作用。我们还鉴定了两个神经相关基因Nrn1和Unc13a，它们在有颌脊椎动物中具有拷贝数扩展，这可能在功能上与七鳃鳗大脑的起源有关。因此，这项研究不仅为环口虫提供了第一个染色体水平的参考基因组，而且突出了七鳃鳗谱系独特的生物学和适应性进化的特征。