D'Aniello Salvatore, D'Aniello Enrico, Locascio Annamaria, Memoli Alessandra, Corrado Marcella, Russo Monia Teresa, Aniello Francesco, Fucci Laura, Brown Euan R, Branno Margherita
Neurobiology Laboratory, Stazione Zoologica Anton Dohrn, Villa Comunale, 80121 Naples, Italy.
Differentiation. 2006 Jun;74(5):222-34. doi: 10.1111/j.1432-0436.2006.00071.x.
The tadpole larvae prosencephalon of the ascidian Ciona intestinalis contains a single large ventricle, along the inner walls of which lie two sensory organs: the otolith (a gravity-sensing organ) and the ocellus (a photo-sensing organ composed of a single cup-shaped pigment cell, about 20 photoreceptor cells, and three lens cells). Comparison has been drawn between the morphology and physiology of photoreceptor cells in the ascidian ocellus and the vertebrate eye. The development of vertebrate and invertebrate eyes requires the activity of several conserved genes and it is regulated by precise expression patterns and cell fate decisions common to several species. We have isolated a Ciona homeobox gene (Ci-Rx) that belongs to the paired-like class of homeobox genes. Rx genes have been identified from a variety of organisms and have been demonstrated to have a role in vertebrate eye formation. Ci-Rx is expressed in the anterior neural plate in the middle tailbud stage and subsequently in the larval stage in the sensory vesicle around the ocellus. Loss of Ci-Rx function leads to an ocellus-less phenotype that shows a loss of photosensitive swimming behavior, suggesting the important role played by Ci-Rx in basal chordate photoreceptor cell differentiation and ocellus formation. Furthermore, studies on Ci-Rx regulatory elements electroporated into Ciona embryos using LacZ or GFP as reporter genes indicate the presence of Ci-Rx in pigment cells, photoreceptors, and neurons surrounding the sensory vesicle. In Ci-Rx knocked-down larvae, neither basal swimming activity nor shadow responses develop. Thus, Rx has a role not only in pigment cells and photoreceptor formation but also in the correct development of the neuronal circuit that controls larval photosensitivity and swimming behavior. The results suggest that a Ci-Rx "retinal" territory exists, which consists of pigment cells, photoreceptors, and neurons involved in transducing the photoreceptor signals.
海鞘肠鳃纲动物的蝌蚪幼虫前脑包含一个单一的大心室，沿着其内壁有两个感觉器官：耳石（重力感应器官）和眼点（由单个杯状色素细胞、约20个光感受器细胞和三个晶状体细胞组成的光感应器官）。人们已经对海鞘眼点中的光感受器细胞与脊椎动物眼睛的形态和生理进行了比较。脊椎动物和无脊椎动物眼睛的发育需要几个保守基因的活动，并且它由几个物种共有的精确表达模式和细胞命运决定来调节。我们分离出了一个海鞘同源框基因（Ci-Rx），它属于同源框基因的配对样类别。Rx基因已从多种生物体中被鉴定出来，并已被证明在脊椎动物眼睛形成中起作用。Ci-Rx在中尾芽阶段的前神经板中表达，随后在幼虫阶段在眼点周围的感觉泡中表达。Ci-Rx功能的丧失导致无眼点表型，表现出光敏游泳行为的丧失，这表明Ci-Rx在基部脊索动物光感受器细胞分化和眼点形成中发挥着重要作用。此外，使用LacZ或GFP作为报告基因对导入海鞘胚胎的Ci-Rx调控元件进行的研究表明，Ci-Rx存在于色素细胞、光感受器以及感觉泡周围的神经元中。在Ci-Rx敲低的幼虫中，基础游泳活动和阴影反应均未发育。因此，Rx不仅在色素细胞和光感受器形成中起作用，而且在控制幼虫光敏性和游泳行为的神经回路的正确发育中也起作用。结果表明存在一个Ci-Rx“视网膜”区域，它由色素细胞、光感受器以及参与转导光感受器信号的神经元组成。
