There are two distinct lungless groups in caudate amphibians (salamanders and newts) (the family Plethodontidae and the genus Onychodactylus, from the family Hynobiidae). Lunglessness is considered to have evolved in response to environmental and/or ecological adaptation with respect to oxygen requirements. We performed selection analyses on lungless salamanders to elucidate the selective patterns of mitochondrial protein-coding genes associated with lunglessness. The branch model and RELAX analyses revealed the occurrence of relaxed selection (an increase of the dN/dS value) in most mitochondrial protein-coding genes of plethodontid salamander branches but not in those of Onychodactylus. Additional branch model and RELAX analyses indicated that direct-developing plethodontids showed the relaxed pattern for most mitochondrial genes, although metamorphosing plethodontids had fewer relaxed genes. Furthermore, aBSREL analysis detected positively selected codons in three plethodontid branches but not in Onychodactylus. One of these three branches corresponded to the most recent common ancestor, and the others corresponded with the most recent common ancestors of direct-developing branches within Hemidactyliinae. The positive selection of mitochondrial protein-coding genes in Plethodontidae is probably associated with the evolution of direct development.

中文翻译：

---

无肺蝾螈线粒体蛋白编码基因的自然选择模式：无肺蝾螈科中宽松的纯化选择和正选密码子位点的存在

尾状两栖动物（蝾螈和蝾螈）有两个不同的无肺类群（无肺鱼科和甲蹄属，来自Hynobiidae科）。无肺被认为是由于对氧气需求的环境和/或生态适应而进化的。我们对无肺蝾螈进行了选择分析，以阐明与无肺相关的线粒体蛋白编码基因的选择模式。分支模型和 RELAX 分析揭示了松弛选择的发生（dN/dS 的增加）值）存在于无齿蝾螈分支的大多数线粒体蛋白编码基因中，但不存在于甲指龙分支中。额外的分支模型和 RELAX 分析表明，直接发育的无齿象显示大多数线粒体基因的松弛模式，尽管变态无齿象具有较少的松弛基因。此外，aBSREL 分析在三个无齿象分支中检测到正选择的密码子，但在甲指龙中没有检测到。这三个分支之一对应于最近的共同祖先，而其他分支则对应于Hemidactyliinae内直接发育分支的最近的共同祖先。无齿象科线粒体蛋白编码基因的正向选择可能与直接发育的进化有关。