Langeland J A, Tomsa J M, Jackman W R, Kimmel C B
Biology Department, Kalamazoo College, 1200 Academy St. Kalamazoo, MI 49006, USA.
Dev Genes Evol. 1998 Dec;208(10):569-77. doi: 10.1007/s004270050216.
Homologs of the Drosophila snail gene have been characterized in several vertebrates. In addition to being expressed in mesoderm during gastrulation, vertebrate snail genes are also expressed in presumptive neural crest and/or its derivatives. Given that neural crest is unique to vertebrates and is considered to be of fundamental importance in their evolution, we have cloned and characterized the expression of a snail gene from amphioxus, a cephalochordate widely accepted as the sister group of the vertebrates. We show that, at the amino acid sequence level, the amphioxus snail gene is a clear phylogenetic outgroup to all the characterized vertebrate snail genes. During embryogenesis snail expression initially becomes restricted to the paraxial or presomitic mesoderm of amphioxus. Later, snail is expressed at high levels in the lateral neural plate, where it persists during neurulation. Our results indicate that an ancestral function of snail genes in the lineage leading to vertebrates is to define the paraxial mesoderm. Furthermore, our results indicate that a cell population homologous to the vertebrate neural crest may be present in amphioxus, thus providing an important link in the evolution of this key vertebrate tissue.
果蝇蜗牛基因的同源物已在几种脊椎动物中得到表征。除了在原肠胚形成期间在中胚层中表达外，脊椎动物的蜗牛基因还在假定的神经嵴和/或其衍生物中表达。鉴于神经嵴是脊椎动物所特有的，并且被认为在其进化中至关重要，我们克隆并表征了文昌鱼（一种被广泛认为是脊椎动物姐妹群的头索动物）中蜗牛基因的表达。我们表明，在氨基酸序列水平上，文昌鱼蜗牛基因是所有已表征的脊椎动物蜗牛基因的一个明确的系统发育外类群。在胚胎发生过程中，蜗牛基因的表达最初局限于文昌鱼的轴旁或体节前中胚层。后来，蜗牛基因在外侧神经板中高水平表达，并在神经胚形成过程中持续存在。我们的结果表明，在导致脊椎动物的谱系中，蜗牛基因的一个祖先功能是界定轴旁中胚层。此外，我们的结果表明，文昌鱼中可能存在与脊椎动物神经嵴同源的细胞群体，从而为这种关键的脊椎动物组织的进化提供了重要联系。
