Zid Mouldi, Drouin Guy
Département de biologie et centre de recherche avancée en génomique environnementale, Université d'Ottawa, Ottawa, ON K1N 6N5, Canada.
Genome. 2014 Jun;57(6):317-25. doi: 10.1139/gen-2014-0083. Epub 2014 Aug 1.
Siglecs are cell surface proteins that belong to the immunoglobulin superfamily and which bind sialic acids. They are composed of two groups, the conserved Siglecs and the CD33-related Siglecs. Previous studies have reported the occurrence of gene conversions between human CD33-related Siglecs and suggested that these conversions are adaptive because they increase the diversity of these immunoglobulin-related genes. Here, we analyze the Siglec genes of five primate species and show that gene conversions are not observed between conserved Siglec genes but that they are frequent between primate CD33-related Siglecs. The gene conversions between CD33-related Siglec genes only occur between similar genes and equally frequently in sialic acid binding and nonbinding domains. Furthermore, dN/dS ratio tests show that most of the Ig-like V-type 1 and the Ig-like C2-type 1 domains of Siglec genes evolve either neutrally or under purifying selection and that gene conversions were not responsible for the positively selected regions detected in the Ig-like V-type1 domain of the human SIGLEC7 and SIGLEC9 genes. Our results suggest that the frequent gene conversions between CD33-related Siglec genes are simply a consequence of the high degree of sequence similarity of these genes and that they are not adaptive.
唾液酸结合免疫球蛋白样凝集素（Siglecs）是属于免疫球蛋白超家族且能结合唾液酸的细胞表面蛋白。它们由两组组成，即保守的Siglecs和与CD33相关的Siglecs。先前的研究报道了人类与CD33相关的Siglecs之间发生基因转换，并表明这些转换具有适应性，因为它们增加了这些免疫球蛋白相关基因的多样性。在此，我们分析了五种灵长类动物的Siglec基因，结果显示保守的Siglec基因之间未观察到基因转换，但在灵长类与CD33相关的Siglecs之间基因转换很频繁。与CD33相关的Siglec基因之间的基因转换仅发生在相似基因之间，且在唾液酸结合域和非结合域中发生的频率相同。此外，非同义/同义替换率（dN/dS）测试表明，Siglec基因的大多数免疫球蛋白样V型1结构域和免疫球蛋白样C2型1结构域要么中性进化，要么在纯化选择下进化，并且基因转换与在人类SIGLEC7和SIGLEC9基因的免疫球蛋白样V型1结构域中检测到的正选择区域无关。我们的结果表明，与CD33相关的Siglec基因之间频繁的基因转换仅仅是这些基因高度序列相似性的结果，并非适应性的。
