Evolutionary developmental Biological study of Acoela, Turbellaria

1.为了制作一种无壳涡虫--内凯旋螺（Convolutanaikaiensis）的正常发育表。用扫描电镜观察胚胎发育过程。受精后90 h左右，胚胎开始孵化，整个孵化过程分为16个阶段.在体腔中，表皮细胞和下面的肌肉细胞显示出复杂的结构，没有介于其间的基底结构。为了解肌肉-表皮复合体的形成是否由退化引起，用透射电镜观察了复合体形成的发育过程。该复合体形成于胚胎发育的早期，在肌肉-表皮复合体形成过程中，基本结构从未出现过.基于18 s核糖体DNA的分子系统发育分析表明，无腔类是涡虫目中最原始的类群.比较了涡虫纲7个目线粒体密码子的差异，除Acoel外的6个目中AAA和ATA分别编码天冬氨酸和异亮氨酸，而Acoel中AAA和XPA分别编码赖氨酸和蛋氨酸。这一事实表明，Acoela在其他Turbellarians被指定之前就已经分化了。

1. In order to make the normal table of development in an acoel turbellarian, Convoluta naikaiensis. the process of the embryonic development was examined by scanning electron microscopy. The embryo hatched about 90 hr after fertilization and the process was divided into 16 stages.2. In the acoel, the epidermal cells and the underlying muscle cells show a complex structure without an intervening basal structure. In order to knpw whether the muscle-epidermis complex results from degeneration, developmental process of the formation of the complex was examined by transmission electron microscopy. The complex was formed in the early stage of the embryonic development and the basal structure never appeared during the course of the muscle-epidermis complex.Molecular phylogenetic analysis on the basis of the 18s ribosomal DNA using 9 orders of the turbellarians revealed that the Acoela is the most primitive in the turbellarian.4. Mitochondrial codons were compared among 7 orders of the turbellarians, In the 6 orders except the Acoel, AAA and ATA code aspartic acid and isoleucine, respectively, but in the Acoel, AAA and XPA codes lysine and methionine, respectively. This fact suggests the Acoela diverged before other turbellarians were specified.