Papatsenko D, Sheng G, Desplan C
Howard Hughes Medical Institute, The Rockefeller University, New York, NY 10021, USA.
Development. 1997 May;124(9):1665-73. doi: 10.1242/dev.124.9.1665.
The photoreceptor cells of the Drosophila compound eye are precisely organized in elementary units called ommatidia. The outer (R1-R6) and inner (R7, R8) photoreceptors represent two physiologically distinct systems with two different projection targets in the brain (for review see Hardie, 1985). All cells of the primary system, R1-R6, express the same rhodopsin and are functionally identical. In contrast, the R7 and R8 photoreceptors are different from each other. They occupy anatomically precise positions, with R7 on top of R8. In fact, there are several classes of R7/R8 pairs, which differ morphologically and functionally and are characterized by the expression of one of two R7-specific opsins, rh3 or rh4. Here, we describe the identification of a new opsin gene, rhodopsin 5, expressed in one subclass of R8 cells. Interestingly, this subclass represents R8 cells that are directly underneath the R7 photoreceptors expressing rh3, but are never under those expressing rh4. These results confirm the existence of two subpopulations of R7 and R8 cells, which coordinate the expression of their respective rh genes. Thus, developmental signaling pathways between R7 and R8 lead to the exclusive expression of a single rhodopsin gene per cell and to the coordinate expression of another one in the neighboring cell. Consistent with this, rh5 expression in R8 disappears when R7 cells are absent (in sevenless mutant). We propose a model for the concerted evolution of opsin genes and the elaboration of the architecture of the retina.
果蝇复眼的光感受器细胞精确地组织成称为小眼的基本单位。外层（R1 - R6）和内层（R7、R8）光感受器代表两个生理上不同的系统，在大脑中有两个不同的投射靶点（综述见Hardie，1985）。初级系统的所有细胞，即R1 - R6，表达相同的视紫红质且功能相同。相比之下，R7和R8光感受器彼此不同。它们占据精确的解剖学位置，R7在R8之上。实际上，有几类R7/R8对，它们在形态和功能上不同，其特征是表达两种R7特异性视蛋白之一，即rh3或rh4。在这里，我们描述了一个新的视蛋白基因视紫红质5的鉴定，它在R8细胞亚类中表达。有趣的是，这个亚类代表的R8细胞直接位于表达rh3的R7光感受器下方，但从不位于表达rh4的R7光感受器下方。这些结果证实了R7和R8细胞存在两个亚群，它们协调各自视紫红质基因的表达。因此，R7和R8之间的发育信号通路导致每个细胞仅表达单个视紫红质基因，并导致相邻细胞中另一个视紫红质基因的协调表达。与此一致的是，当R7细胞缺失时（在无七细胞突变体中），R8中的rh5表达消失。我们提出了一个视蛋白基因协同进化和视网膜结构精细形成的模型。
