Evolution of hematopoiesis: Three members of the PU.1 transcription factor family in a cartilaginous fish, Raja eglanteria

T lymphocytes and B lymphocytes are present in jawed vertebrates, including cartilaginous fishes, but not in jawless vertebrates or invertebrates. The origins of these lineages may be understood in terms of evolutionary changes in the structure and regulation of transcription factors that control lymphocyte development, such as PU.1. The identification and characterization of three members of the PU.1 family of transcription factors in a cartilaginous fish, Raja eglanteria, are described here. Two of these genes are orthologs of mammalian PU.1 and Spi-C, respectively, whereas the third gene, Spi-D, is a different family member. In addition, a PU.1-like gene has been identified in a jawless vertebrate, Petromyzon marinus (sea lamprey). Both DNA-binding and transactivation domains are highly conserved between mammalian and skate PU.1, in marked contrast to lamprey Spi, in which similarity is evident only in the DNA-binding domain. Phylogenetic analysis of sequence data suggests that the appearance of Spi-C may predate the divergence of the jawed and jawless vertebrates and that Spi-D arose before the divergence of the cartilaginous fish from the lineage leading to the mammals. The tissue-specific expression patterns of skate PU.1 and Spi-C suggest that these genes share regulatory as well as structural properties with their mammalian orthologs.

T淋巴细胞（T lymphocyte）与B淋巴细胞（B lymphocyte）仅存在于有颌脊椎动物（jawed vertebrates）中，包括软骨鱼类，而无颌脊椎动物（jawless vertebrates）与无脊椎动物（invertebrates）体内则不存在此类细胞。此类细胞谱系的起源可通过调控淋巴细胞发育的转录因子（transcription factor）的结构与调控模式的演化改变得以阐释，例如PU.1。本文报道了在软骨鱼类猬白鳐（Raja eglanteria）体内鉴定并表征的PU.1家族转录因子的三个成员。其中两个基因分别为哺乳动物PU.1与Spi-C的直向同源基因（ortholog），第三个基因Spi-D则属于该家族的另一独立分支。此外，研究人员在无颌脊椎动物海七鳃鳗（Petromyzon marinus）体内已发现一个类PU.1基因。与哺乳动物PU.1相比，猬白鳐PU.1的DNA结合结构域（DNA-binding domain）与反式激活结构域（transactivation domain）均呈现高度保守性；与之形成鲜明对比的是七鳃鳗Spi蛋白，其序列相似性仅局限于DNA结合结构域。对序列数据开展的系统发育分析（phylogenetic analysis）显示，Spi-C的出现可能早于有颌与无颌脊椎动物的分化事件，而Spi-D则起源于软骨鱼类从哺乳类祖先谱系分化之前。猬白鳐PU.1与Spi-C的组织特异性表达模式（tissue-specific expression pattern）表明，这些基因与其哺乳动物直向同源基因不仅具备结构保守性，还共享调控特性。