Hashimoto Hisashi, Aritaki Masato, Uozumi Kaori, Uji Susumu, Kurokawa Tadahide, Suzuki Tohru
Bioscience and Biotechnology Center, Nagoya University, Japan.
Zoolog Sci. 2007 Feb;24(2):137-46. doi: 10.2108/zsj.24.137.
Although it is well known that flounder form external asymmetry by migration of one eye at metamorphosis, the control system that forms this asymmetry is unknown. To help elucidate this mechanism, we here describe the embryogenesis and expression profiles of the Nodal-pathway genes in the Japanese flounder, Paralichthys olivaceus. We also perform a comparative study of the laterality of the expression of these genes in sinistral (P. olivaceus) and dextral (Verasper variegatus) flounders. In P. olivaceus, Kupffer's vesicle forms at the 2-somite stage, after which left-sided expression of spaw starts at the 8-somite stage. Left-sided expression of pitx2 occurs in the gut field at the 15-somite to high-pec stages, in the heart field at the 21-somite stage, and in the dorsal diencephalon at the 27-somite to high-pec stages. In response to left-sided pitx2 expression, the heart, gut, and diencephalon begin asymmetric organogenesis at the pharyngula (heart) and the long-pec (gut and diencephalon) stages, whereas the eyes do not show signs of asymmetry at these stages. In both sinistral and dextral flounders, the Nodal-pathway genes are expressed at the left side of the dorsal diencephalon and left lateral-plate mesoderm. Considering these data together with our previous finding that reversal of eye laterality occurs to some extent in the P. olivaceus mutant reversed, in which embryonic pitx2 expression is randomized, we propose that although the Nodal pathway seems to function to fix eye laterality, embryonic expression of these genes does not act as a direct positional cue for eye laterality.
虽然人们都知道比目鱼在变态发育过程中通过一只眼睛的迁移形成外部不对称，但形成这种不对称的控制系统尚不清楚。为了帮助阐明这一机制，我们在此描述了日本比目鱼（Paralichthys olivaceus）中Nodal信号通路基因的胚胎发育过程和表达谱。我们还对左旋（P. olivaceus）和右旋（Verasper variegatus）比目鱼中这些基因表达的不对称性进行了比较研究。在P. olivaceus中，Kupffer囊泡在2体节期形成，之后spaw在8体节期开始在左侧表达。pitx2在左侧的表达出现在15体节至高胸鳍期的肠道区域、21体节期的心脏区域以及27体节至高胸鳍期的背侧间脑。响应左侧pitx2的表达，心脏、肠道和间脑在咽期（心脏）和长胸鳍期（肠道和间脑）开始不对称器官发生，而眼睛在这些阶段没有显示出不对称的迹象。在左旋和右旋比目鱼中，Nodal信号通路基因均在背侧间脑左侧和左侧侧板中胚层表达。结合这些数据以及我们之前的发现，即在P. olivaceus突变体reversed中，胚胎pitx2表达随机化，眼睛的左右不对称性在一定程度上发生了反转，我们提出，虽然Nodal信号通路似乎在固定眼睛的左右不对称性方面发挥作用，但这些基因的胚胎表达并不是眼睛左右不对称性的直接位置线索。
