Holland P W, Koschorz B, Holland L Z, Herrmann B G
School of Animal and Microbial Sciences, University of Reading, Whiteknights, UK.
Development. 1995 Dec;121(12):4283-91. doi: 10.1242/dev.121.12.4283.
Homologues of the murine Brachyury (T) gene have been cloned from several vertebrates, and are implicated in mesoderm formation and in differentiation of the notochord. In contrast, the roles of the ascidian Brachyury gene may be restricted to presumptive notochord. To understand the evolution of Brachyury genes and their developmental roles, we have searched for homologues in amphioxus, representing the third chordate subphylum and the probable closest relative of the vertebrates. We report the isolation of two amphioxus cDNA clones with clear homology to Brachyury genes, and demonstrate that these derive from separate loci resultant from a recent gene duplication. This finding represents an exception to the emerging consensus of an archetypal prevertebrate genome in amphioxus. The spatial and temporal distribution of Brachyury transcripts during amphioxus development is remarkably similar to vertebrate Brachyury, in presumptive mesoderm, posterior mesoderm and the notochord. Gene expression extends throughout the anteroposterior axis of the notochord, despite the most rostral regions being a more recent specialization; it also persists into larval stages, despite differentiation into contractile tissue. We propose that roles of Brachyury in notochord differentiation are more ancient than roles in mesoderm formation, and that the latter are shared by cephalochordates and all vertebrates.
鼠类短尾（T）基因的同源物已从几种脊椎动物中克隆出来，它们与中胚层形成以及脊索的分化有关。相比之下，海鞘短尾基因的作用可能仅限于推定的脊索。为了了解短尾基因的进化及其发育作用，我们在文昌鱼中寻找同源物，文昌鱼代表了脊索动物的第三个亚门，也是脊椎动物可能最亲近的亲属。我们报告了两个与短尾基因具有明显同源性的文昌鱼cDNA克隆的分离，并证明这些克隆来自最近基因复制产生的不同位点。这一发现代表了文昌鱼原型脊椎动物前体基因组新出现的共识的一个例外。在文昌鱼发育过程中，短尾转录本的时空分布与脊椎动物的短尾在推定的中胚层、后中胚层和脊索中非常相似。尽管最前端区域是最近才特化的，但基因表达在脊索的前后轴上都有延伸；尽管已分化为收缩组织，但它在幼虫阶段也持续存在。我们提出，短尾在脊索分化中的作用比在中胚层形成中的作用更为古老，而后者是头索动物和所有脊椎动物共有的。
