Origin and Development of the Chordate Traits

脊索小体的形成是通过一系列复杂的形态发生运动完成的，包括脊索细胞的会聚伸展。在尾索海鞘中，短臂(Ci-Bra)在脊索的形成中起着关键作用，目前已鉴定出30多个Ci-Bra下游的脊索基因。在本研究中，我们研究了9个Ci-Bra下游脊索基因的功能抑制效应，这些基因包括Ci-PTP(蛋白酪氨酸磷酸酶)、Ci-acl(ATP：柠檬酸裂解酶)、Ci-Pickle、Ci-netrin、Ci-trop(原肌球蛋白样蛋白)、Ci-Noto3、Ci-ASAK(三磷酸腺苷硫酰酶/APS激酶)、Ci-ERM(ezrin/redisin/meiosin)和Ci-pellino。当前两个基因(Ci-PTP和Ci-acl)的功能被特定的吗啉抑制时，脊索细胞不能收敛，而Ci-棘细胞的功能抑制导致着床失败，因此这三种类型的胚胎中的细胞保持在胚胎的中背部。Ci-netrin、Ci-trop、Ci-Noto3和Ci-ASAK四个基因的功能抑制导致了部分嵌合缺陷，脊索不是由一排硬币状的细胞组成。此外，当最后两个基因(Ci-ERM和Ci-Pellino)的功能被抑制时，即使汇聚/延伸正常发生，胚胎仍表现出脊索细胞伸长失败。这些结果表明，许多Ci-Bra-下游脊索基因参与了胚胎的汇聚/延伸。结果还表明，导致脊索特异性特征或脊索形成的进化事件是胚胎细胞汇聚/延伸所需的基因的招募。

Formation of the chordate body is accomplished by a complex set of morphogenetic movements including convergent extension of notochord cells. In the urochordate ascidian Ciona intestinalis, Brachyury (Ci-Bra) plays a key role in the formation of the notochord, and more than thirty Ci-Bra-downstream notochord genes have been identified. In the present study, we examined the effects of functional suppression of nine Ci-Bra-downstream notochord genes, which include Ci-PTP (protein tyrosine phosphatase), Ci-ACL (ATP: citratelyase), Ci-prickle, Ci-netrin, Ci-trop (tropomyosin-like), Ci-Noto3, Ci-ASAK (ATP sulfurylase/APS kinase), Ci-ERM (ezrin/redixin/meiosin) and Ci-pellino. When function of first two genes (Ci-PTP and Ci-ACL) was suppressed with specific morpholinos, the notochord cells failed convergent, while functional suppression of Ci-prickle resulted in a failure of intercalation, and therefore the cells in these three types of embryo remained in mid-dorsal region of the embryo. Functional suppression of the next four genes (Ci-netrin, Ci-trop, Ci-Noto3 and Ci-ASAK) resulted in the partial defect of intercalation, and the notochord did not consist of a single row of coin-shaped cells. In addition, when the function of the last two genes (Ci-ERM and Ci-pellino) was suppressed, the embryo showed failure of notochord cell elongation, even though convergent/extension took place normally. These results indicate that many Ci-Bra-downstream notochord genes are involved in convergent/extension of the embryo. The results also suggest that an evolutionary event leading to the formation of a chordate-specific character or notochord was the recruitment of genes required for convergent/extension of embryonic cells.