Shimeld S M
School of Animal and Microbial Sciences, University of Reading, United Kingdom.
Dev Biol. 1997 Mar 1;183(1):74-85. doi: 10.1006/dbio.1996.8481.
The fork head/HNF-3 genes form a subclass of a family of transcription factors united by the possession of a conserved DNA binding domain known as the fork head domain. Most vertebrate HNF-3 class genes show several conserved sites of expression during development, including the dorsal lip/Hensen's node, notochord and floor plate, all structures known to organise adjacent tissues. In this paper I report the characterisation of HNF-3 class genes from the cephalochordate amphioxus. I show that amphioxus has two HNF-3 class genes, named AmHNF-3-1 and AmHNF-3-2; molecular phylogenetic analysis reveals that these derive from an independent duplication in the cephalochordate lineage. The expression of both genes in early development appears identical and shows striking similarities to that of vertebrates. In neurulae, transcripts of both genes were detected in the presumed organiser, endoderm, and notochord, supporting morphological and embryological evidence that these are homologous between vertebrates and amphioxus. This expression pattern overlaps considerably with that of amphioxus brachyury, suggesting that the functional relationship between these genes in vertebrates is conserved with amphioxus. Expression of both genes was maintained in the endoderm and notochord up to the 6 somite stage. After the 6 somite stage no expression of AmHNF-3-2 was detected and expression of AmHNF-3-1 began to decrease in the notochord, such that by the 10 somite stage transcripts were only detected in the terminal regions. At this stage, however, a column of AmHNF-3-1-expressing cells was detected at the ventral midline of the neural tube, a position occupied by the floor plate in vertebrates. This is the first evidence that amphioxus has a floor plate which is specified by a mechanism conserved with vertebrates. Taken together these data support two conclusions: Firstly, that the role of the dorsal lip/Hensen's node, notochord, and floor plate as organisers of the vertebrate body plan evolved prior to the separation of the vertebrate and cephalochordate lineages, at least 520 million years ago. Secondly, that the role of HNF-3 genes in these structures predates the origin of the multiple HNF-3 genes found in vertebrates.
叉头/HNF-3基因构成了一类转录因子家族的一个亚类，这类转录因子因拥有一个被称为叉头结构域的保守DNA结合结构域而统一起来。大多数脊椎动物的HNF-3类基因在发育过程中显示出几个保守的表达位点，包括背唇/亨氏结、脊索和底板，所有这些结构都已知可组织相邻组织。在本文中，我报告了头索动物文昌鱼的HNF-3类基因的特征。我发现文昌鱼有两个HNF-3类基因，命名为AmHNF-3-1和AmHNF-3-2；分子系统发育分析表明，这些基因源自头索动物谱系中的一次独立复制。这两个基因在早期发育中的表达看起来相同，并且与脊椎动物的表达有显著相似之处。在神经胚中，两个基因的转录本在假定的组织者、内胚层和脊索中都被检测到，这支持了形态学和胚胎学证据，即这些在脊椎动物和文昌鱼之间是同源的。这种表达模式与文昌鱼短尾相关蛋白的表达模式有相当大的重叠，表明这些基因在脊椎动物中的功能关系在文昌鱼中是保守的。这两个基因的表达在内胚层和脊索中一直维持到6体节阶段。在6体节阶段之后，未检测到AmHNF-3-2的表达，并且AmHNF-3-1在脊索中的表达开始下降，以至于到10体节阶段，转录本仅在末端区域被检测到。然而，在这个阶段，在神经管的腹中线检测到一列表达AmHNF-3-1的细胞，这个位置在脊椎动物中被底板占据。这是文昌鱼有一个由与脊椎动物保守的机制所确定的底板的第一个证据。综合这些数据支持两个结论：第一，背唇/亨氏结、脊索和底板作为脊椎动物身体蓝图组织者的作用在脊椎动物和头索动物谱系分离之前就已经进化出来了，至少在5.2亿年前。第二，HNF-3基因在这些结构中的作用早于在脊椎动物中发现的多个HNF-3基因的起源。
