Brites Daniela, McTaggart Seanna, Morris Krystalynne, Anderson Jobriah, Thomas Kelley, Colson Isabelle, Fabbro Thomas, Little Tom J, Ebert Dieter, Du Pasquier Louis
Zoologisches Institut, Evolutionsbiologie, Universität Basel, Vesalgasse 1, Switzerland.
Mol Biol Evol. 2008 Jul;25(7):1429-39. doi: 10.1093/molbev/msn087. Epub 2008 Apr 9.
In insects, the homologue of the Down syndrome cell adhesion molecule (Dscam) is a unique case of a single-locus gene whose expression has extensive somatic diversification in both the nervous and immune systems. How this situation evolved is best understood through comparative studies. We describe structural, expression, and evolutionary aspects of a Dscam homolog in 2 species of the crustacean Daphnia. The Dscam of Daphnia generates up to 13,000 different transcripts by the alternative splicing of variable exons. This extends the taxonomic range of a highly diversified Dscam beyond the insects. Additionally, we have identified 4 alternative forms of the cytoplasmic tail that generate isoforms with or without inhibitory or activating immunoreceptor tyrosine-based motifs (ITIM and ITAM respectively), something not previously reported in insect's Dscam. In Daphnia, we detected exon usage variability in both the brain and hemocytes (the effector cells of immunity), suggesting that Dscam plays a role in the nervous and immune systems of crustaceans, as it does in insects. Phylogenetic analysis shows a high degree of amino acid conservation between Daphnia and insects except in the alternative exons, which diverge greatly between these taxa. Our analysis shows that the variable exons diverged before the split of the 2 Daphnia species and is in agreement with the nearest-neighbor model for the evolution of the alternative exons. The genealogy of the Dscam gene family from vertebrates and invertebrates confirmed that the highly diversified form of the gene evolved from a nondiversified form before the split of insects and crustaceans.
在昆虫中，唐氏综合征细胞粘附分子（Dscam）的同源物是单基因座基因的一个独特例子，其表达在神经系统和免疫系统中都有广泛的体细胞多样化。通过比较研究能最好地理解这种情况是如何进化的。我们描述了两种甲壳类水蚤中Dscam同源物的结构、表达和进化方面。水蚤的Dscam通过可变外显子的可变剪接产生多达13000种不同的转录本。这将高度多样化的Dscam的分类范围扩展到了昆虫之外。此外，我们还鉴定出了细胞质尾巴的4种可变形式，它们产生具有或不具有抑制性或激活性基于免疫受体酪氨酸的基序（分别为ITIM和ITAM）的异构体，这在昆虫的Dscam中以前未曾报道过。在水蚤中，我们在大脑和血细胞（免疫效应细胞）中都检测到了外显子使用的变异性，这表明Dscam在甲壳类动物的神经系统和免疫系统中发挥作用，就像在昆虫中一样。系统发育分析表明，除了可变外显子外，水蚤和昆虫之间的氨基酸保守程度很高，可变外显子在这些分类群之间差异很大。我们的分析表明，可变外显子在两种水蚤物种分化之前就已经分化，这与可变外显子进化的最近邻模型一致。脊椎动物和无脊椎动物的Dscam基因家族的谱系证实，该基因的高度多样化形式是在昆虫和甲壳类动物分化之前从非多样化形式进化而来的。
