Key Points Chinese alligator has a complicated genomic organization of the TCRα/δ locus. Unidirectional transfer of V from IgH to TCR locus occurred frequently in vertebrate. A model explaining the evolutionary pattern of atypical VHδ genes was proposed. Atypical TCRδ found in sharks, amphibians, birds, and monotremes and TCRμ found in monotremes and marsupials are TCR chains that use Ig or BCR-like variable domains (VHδ/Vμ) rather than conventional TCR V domains. These unconventional TCR are consistent with a scenario in which TCR and BCR, although having diverged from each other more than 400 million years ago, continue to exchange variable gene segments in generating diversity for Ag recognition. However, the process underlying this exchange and leading to the evolution of these atypical TCR receptor genes remains elusive. In this study, we identified two TCRα/δ gene loci in the Chinese alligator (Alligator sinensis). In total, there were 144 V, 154 Jα, nine Jδ, eight Dδ, two Cα, and five Cδ gene segments in the TCRα/δ loci of the Chinese alligator, representing the most complicated TCRα/δ gene system in both genomic structure and gene content in any tetrapod examined so far. A pool of 32 VHδ genes divided into 18 subfamilies was found to be scattered over the two loci. Phylogenetic analyses revealed that these VHδ genes could be related to bird VHδ genes, VHδ/Vμ genes in platypus or opossum, or alligator VH genes. Based on these findings, a model explaining the evolutionary pattern of atypical TCRδ/TCRμ genes in tetrapods is proposed. This study sheds new light on the evolution of TCR and BCR genes, two of the most essential components of adaptive immunity.

中文翻译：

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扬子鳄TCRα/δ基因座分析揭示四足动物非典型TCRδ/TCRμ的进化模式

要点 扬子鳄的 TCRα/δ 基因座具有复杂的基因组结构。V 从 IgH 到 TCR 基因座的单向转移在脊椎动物中经常发生。提出了一个解释非典型 VHδ 基因进化模式的模型。在鲨鱼、两栖动物、鸟类和单孔类动物中发现的非典型 TCRδ 和在单孔类动物和有袋动物中发现的 TCRμ 是使用 Ig 或 BCR 样可变域 (VHδ/Vμ) 而不是常规 TCR V 域的 TCR 链。这些非常规的 TCR 与 TCR 和 BCR 尽管在 4 亿多年前彼此分歧，但继续交换可变基因片段以产生 Ag 识别的多样性的情况是一致的。然而，这种交换的基础和导致这些非典型 TCR 受体基因进化的过程仍然难以捉摸。在这项研究中，我们在扬子鳄 (Alligator sinensis) 中鉴定了两个 TCRα/δ 基因位点。扬子鳄的TCRα/δ基因座共有144个V、154个Jα、9个Jδ、8个Dδ、2个Cα和5个Cδ基因片段，代表了基因组结构和基因组中最复杂的TCRα/δ基因系统。迄今为止检查过的任何四足动物的基因含量。发现分成 18 个亚家族的 32 个 VHδ 基因库分散在两个基因座上。系统发育分析表明，这些VHδ基因可能与鸟类VHδ基因、鸭嘴兽或负鼠的VHδ/Vμ基因或鳄鱼VH基因有关。基于这些发现，提出了一个解释四足动物非典型 TCRδ/TCRμ 基因进化模式的模型。这项研究为 TCR 和 BCR 基因（适应性免疫的两个最重要组成部分）的进化提供了新的思路。