Egg Cytoplasmic Factors that Regulate Developmental Pattern

最近基于18S rDNA序列比较的分子系统发育研究表明，蝌蚪幼虫的形成是理解脊索动物起源和进化的关键发育事件。在海鞘的胚胎发生过程中，幼虫尾部形成约40个单细胞和横纹肌细胞以及40个脊索细胞。因此，研究参与这些细胞分化的分子机制对于了解个体发育以及动物的系统发育都是很重要的。我们已经分离到小鼠T(Brachyury)基因的海鞘同源基因。海鞘T基因仅在脊索细胞中表达，其表达开始于其发育命运的限制时间。因此，T基因是理解脊索进化的线索。我们目前正在研究T基因在发育成无尾幼虫的腹水中的表达。在肌肉肌动蛋白基因、生肌调节基因和细胞质因子(或肌肉决定因素)的水平上，已经研究了海囊胚胎中肌肉细胞的分化。我们还研究了无尾幼虫的海鞘中的肌肉肌动蛋白基因，发现无尾幼虫的海鞘中含有几个肌肉肌动蛋白的假基因，这些假基因在我们通常采用的研究条件下是不存在于有尾巴的海鞘中的。

A recent molecular-phylogenetic study based on the comparison of 18S rDNA sequences has suggested that formation of tadpole larvae is a key developmental event to understand the origin and evolution of chordates. During embryogenesis of ascidians, about forty unicellular and striated muscle cells as well as forty notochord cells are formed in the tail of the larva. Therefore, it is important to study molecular mechanisms involved in differentiation of these cells to understand not only ontogeny by also phylogeny of animals.We have isolated an ascidian homolog of the mouse T (Brachyury) gene. The ascidian T gene is expressed exclusively in notochord cells and the expression begins with the time of restriction of their developmental fates. Therefore, the T gene is cue to understand the chordate evolution. We are now studying the T gene expression in ascidians which develop tailless larvae.Muscle cell differentiation in ascidian embryos has been studied at the levels of muscle actin genes, myogenic regulatory gene, and cytoplasmic factors (or muscle determinants). We have also studied muscle actin genes in ascidians with tailless larvae, and it was found that the ascidian with tailless larvae contains several pseudogenes of muscle actin, which were not found in ascidians with tailed larvae under usual research conditions we adopted.