Nishino A, Satou Y, Morisawa M, Satoh N
Department of Zoology, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
Dev Genes Evol. 2001 May;211(5):219-31. doi: 10.1007/s004270100141.
Appendicularia (Larvacea) is a subgroup of Urochordata (Tunicata) comprised of holoplanktonic organisms that retain their tailed architecture throughout their life history, while other tunicates, including ascidians and doliolids, resorb the tail after metamorphosis. In order to investigate the characteristics of the appendicularian unresorbed notochord, we isolated a partial genomic clone and a full-length cDNA sequence homologous to the mouse Brachyury (T) gene from the appendicularian Oikopleura longicauda. Brachyury is known to be predominantly expressed in the notochord cells and plays an important role in their differentiation in other chordates. While phylogenetic analysis robustly supports the orthology of the isolated Brachyury gene, the exon-intron organization found in the genomic clone was distinct from that well-conserved among other T-box genes. In addition to a detailed observation of notochord development in living specimens, whole-mount double in situ hybridization was carried out using a Brachyury probe along with a muscle actin probe. The Brachyury transcripts were found in the notochord of the tailbud embryos and persisted into later stages. The present study highlights characteristics of notochord development in the appendicularian. Furthermore, these results provide basic knowledge for comprehensive understanding of the cellular- and molecular-based mechanisms needed to build the characteristic cytoarchitecture of notochord that varies among tunicate species.
尾海鞘纲（幼形纲）是尾索动物亚门（被囊动物亚门）的一个亚群，由终生保留尾部结构的全浮游生物组成，而其他被囊动物，包括海鞘和住囊虫，在变态后会吸收尾部。为了研究尾海鞘纲未吸收的脊索的特征，我们从长尾住囊虫中分离出一个与小鼠短尾（T）基因同源的部分基因组克隆和一个全长cDNA序列。已知短尾基因主要在脊索细胞中表达，并在其他脊索动物的脊索细胞分化中起重要作用。虽然系统发育分析有力地支持了分离出的短尾基因的直系同源性，但在基因组克隆中发现的外显子-内含子组织与其他T-box基因中保守的组织不同。除了对活体标本中的脊索发育进行详细观察外，还使用短尾基因探针和肌肉肌动蛋白探针进行了全胚胎原位杂交。在尾芽胚胎的脊索中发现了短尾基因转录本，并持续到后期阶段。本研究突出了尾海鞘纲脊索发育的特征。此外，这些结果为全面理解构建不同被囊动物物种中脊索特征细胞结构所需的细胞和分子机制提供了基础知识。
