Neidert A H, Panopoulou G, Langeland J A
Department of Biology, Kalamazoo College, MI 49006-3291, USA.
Evol Dev. 2000 Nov-Dec;2(6):303-10. doi: 10.1046/j.1525-142x.2000.00073.x.
The organizer is a central feature of vertebrate embryogenesis and is functionally subdivided into the head organizer that gives rise primarily to the prechordal plate and induces forebrain structures, and the trunk/tail organizer that gives rise primarily to the notochord and induces more posterior structures. Goosecoid(gsc) encodes a homeodomain-containing transcription factor that is expressed in the vertebrate head organizer and prechordal plate, and can induce a secondary axis when expressed ectopically. To investigate the evolution of the vertebrate head organizer and prechordal plate, we have cloned and characterized a gsc homolog from the cephalochordate amphioxus. Amphioxus, it is important to note, lacks a prechordal plate in that the notochord extends to the extreme anterior end of the animal, and lacks elaborate differentiation of its forebrain. Gsc expression in amphioxus is initially localized during gastrulation to the mesendodermal layer of the dorsal lip of the blastopore. However, gsc expression in amphioxus is not maintained in anterior axial mesoderm, as is the case with the vertebrate prechordal plate. Rather, gsc is expressed in the dorsal axial mesoderm of the blastopore lip throughout gastrulation, appearing transiently in the presumptive notochord that underlies all regions of the amphioxus brain. The similarities in gsc expression in amphioxus and vertebrates suggest that a primitive version of the head organizer evolved prior to the origin of the vertebrates. The differences in gsc expression can be interpreted either as the loss of the prechordal plate domain in the cephalochordate lineage, or the gain of a distinct gsc-expressing prechordal plate that plays a role in forebrain induction in the vertebrate lineage.
组织者是脊椎动物胚胎发育的一个核心特征，在功能上可细分为头部组织者和躯干/尾部组织者。头部组织者主要产生前索板并诱导前脑结构的形成，而躯干/尾部组织者主要产生脊索并诱导更靠后的结构。鹅膏蕈氨酸（gsc）编码一种含同源异型结构域的转录因子，该因子在脊椎动物的头部组织者和前索板中表达，异位表达时可诱导形成第二轴。为了研究脊椎动物头部组织者和前索板的进化，我们克隆并鉴定了来自头索动物文昌鱼的gsc同源物。需要注意的是，文昌鱼没有前索板，因为脊索延伸到动物的最前端，且其前脑缺乏精细的分化。文昌鱼中的gsc表达最初在原肠胚形成期间定位于胚孔背唇的中内胚层。然而，文昌鱼中的gsc表达并不像脊椎动物前索板那样在前轴中胚层中持续存在。相反，在整个原肠胚形成过程中gsc都在胚孔唇的背轴中胚层中表达，短暂出现在构成文昌鱼脑所有区域基础的假定脊索中。文昌鱼和脊椎动物中gsc表达的相似性表明，头部组织者的原始版本在脊椎动物起源之前就已进化。gsc表达的差异既可以解释为头索动物谱系中前索板结构域的丧失，也可以解释为脊椎动物谱系中获得了一个在诱导前脑过程中起作用的、表达gsc的独特前索板。
