Zoological Institute, University of Basel, Vesalgasse 1, 4051 Basel, Switzerland; Swiss Tropical and Public Health Institute, Socinstrasse 57, PO Box 4002, Basel.
Evolution. 2013 Oct;67(10):2999-3011. doi: 10.1111/evo.12179. Epub 2013 Jul 4.
Exon duplication and alternative splicing evolved multiple times in metazoa and are of overall importance in shaping genomes and allowing organisms to produce many fold more proteins than there are genes in the genome. No other example is as striking as the one of the Down syndrome cell adhesion molecule (Dscam) of insects and crustaceans (pancrustaceans) involved in the nervous system differentiation and in the immune system. To elucidate the evolutionary history of this extraordinary gene, we investigated Dscam homologs in two basal arthropods, the myriapod Strigamia maritima and the chelicerate Ixodes scapularis. In both, Dscam diversified extensively by whole gene duplications resulting in multigene expansions. Within some of the S. maritima genes, exons coding for one of the immunoglobulin domains (Ig7) duplicated and are mutually exclusively alternatively spliced. Our results suggest that Dscam diversification was selected independently in chelicerates, myriapods, and pancrustaceans and that the usage of Dscam diversity by immune cells evolved for the first time in basal arthropods. We propose an evolutionary scenario for the appearance of the highly variable Dscam gene of pancrustaceans, adding to the understanding of how alternative splicing, exon, and gene duplication contribute to create molecular diversity associated with potentially new cellular functions.
外显子重复和选择性剪接在后生动物中多次进化，对于塑造基因组和使生物体产生比基因组中基因多得多的蛋白质具有重要意义。没有其他例子像参与神经系统分化和免疫系统的昆虫和甲壳类动物（泛甲壳类动物）中的唐氏综合征细胞黏附分子（Dscam）那样引人注目。为了阐明这个非凡基因的进化历史，我们研究了两种基础节肢动物，多足类 Strigamia maritima 和螯肢类 Ixodes scapularis 中的 Dscam 同源物。在这两种动物中，Dscam 通过全基因重复多样化，导致多基因扩增。在某些 S. maritima 基因中，编码免疫球蛋白结构域之一（Ig7）的外显子重复并相互排斥地选择性剪接。我们的结果表明，Dscam 多样化在螯肢动物、多足类和泛甲壳类动物中是独立选择的，免疫细胞对 Dscam 多样性的利用首先在基础节肢动物中进化。我们提出了一个关于泛甲壳类动物高度可变的 Dscam 基因出现的进化情景，这增加了我们对选择性剪接、外显子和基因重复如何有助于产生与潜在新细胞功能相关的分子多样性的理解。
