Department of Biological Sciences, Graduate School of Science, Osaka University, 560-0043 Toyonaka, Osaka, Japan;
Department of Biological Sciences, Graduate School of Science, Osaka University, 560-0043 Toyonaka, Osaka, Japan.
Proc Natl Acad Sci U S A. 2020 Feb 25;117(8):4188-4198. doi: 10.1073/pnas.1916858117. Epub 2020 Feb 6.
Larvaceans are chordates with a tadpole-like morphology. In contrast to most chordates of which early embryonic morphology is bilaterally symmetric and the left-right (L-R) axis is specified by the Nodal pathway later on, invariant L-R asymmetry emerges in four-cell embryos of larvaceans. The asymmetric cell arrangements exist through development of the tailbud. The tail thus twists 90° in a counterclockwise direction relative to the trunk, and the tail nerve cord localizes on the left side. Here, we demonstrate that larvacean embryos have nonconventional L-R asymmetries: 1) L- and R-cells of the two-cell embryo had remarkably asymmetric cell fates; 2) Ca oscillation occurred through embryogenesis; 3) , an evolutionarily conserved left-determining gene, was absent in the genome; and 4) bone morphogenetic protein gene () homolog showed right-sided expression in the tailbud and larvae. We also showed that Ca oscillation is required for expression, and that BMP signaling suppresses ectopic expression of neural genes. These results indicate that there is a chordate species lacking that utilizes Ca oscillation and for embryonic L-R patterning. The right-side expression may have arisen via cooption of conventional BMP signaling in order to restrict neural gene expression on the left side.
樽海鞘是具有蝌蚪状形态的脊索动物。与大多数脊索动物不同，后者的早期胚胎形态是左右对称的，并且左右（L-R）轴后来由 Nodal 途径指定，而樽海鞘的四细胞胚胎中则出现了不变的 L-R 不对称性。这种不对称的细胞排列存在于尾芽的发育过程中。因此，尾巴相对于躯干逆时针旋转 90°，而尾巴的神经管位于左侧。在这里，我们证明了樽海鞘胚胎具有非传统的 L-R 不对称性：1） 两细胞胚胎的 L- 和 R- 细胞具有明显的不对称细胞命运；2） Ca 振荡贯穿胚胎发生；3） ，一个进化上保守的左侧决定基因，在基因组中缺失；4） 骨形态发生蛋白基因（）同源物在尾芽和幼虫中表现出右侧表达。我们还表明，Ca 振荡是 表达所必需的，并且 BMP 信号抑制神经基因的异位表达。这些结果表明，有一种脊索动物物种缺乏 ，它利用 Ca 振荡和 来进行胚胎的 L-R 模式形成。右侧的 表达可能是通过对传统 BMP 信号的共激活而产生的，以限制左侧的神经基因表达。
