Bruce A E, Shankland M
Program in Neuroscience, Harvard Medical School, Boston, Massachusetts, USA.
Dev Biol. 1998 Sep 1;201(1):101-12. doi: 10.1006/dbio.1998.8968.
The orthodenticle orthologue Lox22-Otx was isolated from an annelid worm, the leech Helobdella triserialis. In situ hybridization reveals that embryonic expression of Lox22-Otx RNA is primarily restricted to an unsegmented head domain, including tissues in the foregut, surface ectoderm, and the head ganglion of the central nervous system. The patterns of head expression form concentric rings about the stomadeum and mark tissue domains that exhibit discrete behaviors during later morphogenesis and differentiation. Expression was also observed in one to two bilateral pairs of neurons in each segmental ganglion or neuromere of the body trunk. The largely head-specific expression of Lox22-Otx in this annelid species supports data from two other bilaterian phyla in suggesting the existence of a genetically defined head/trunk distinction. We suggest here that this head/trunk distinction is a synapomorphy of the Bilateria as a whole, and that it reflects the body plan of an early bilaterian ancestor. In addition, we discuss the possibility that the radial organization of gene expression and cell lineages in the leech's head domain may reflect the symmetry properties of a prebilaterian ancestor that had a radially symmetric body plan.
从环节动物蛭类三带远孔蛭（Helobdella triserialis）中分离出了直向同源异形基因Lox22 - Otx。原位杂交显示，Lox22 - Otx RNA的胚胎表达主要局限于一个未分节的头部区域，包括前肠组织、表面外胚层以及中枢神经系统的头部神经节。头部表达模式围绕口凹形成同心环，并标记了在后期形态发生和分化过程中表现出离散行为的组织区域。在躯干的每个节段神经节或神经原中，也观察到一到两对双侧神经元中有表达。在这种环节动物物种中，Lox22 - Otx在很大程度上具有头部特异性的表达，这支持了来自其他两个两侧对称动物门的数据，表明存在遗传定义的头部/躯干区分。我们在此提出，这种头部/躯干区分是整个两侧对称动物的一个共衍征，并且它反映了早期两侧对称动物祖先的身体结构。此外，我们还讨论了水蛭头部区域基因表达和细胞谱系的径向组织可能反映具有径向对称身体结构的前两侧对称动物祖先的对称特性的可能性。
