Visual perception of the environment is mediated by specialized photoreceptor (PR) neurons of the eye. Each PR expresses photosensitive opsins, which are activated by a particular wavelength of light. In most insects, the visual system comprises a pair of compound eyes that are mainly associated with motion detection, color or polarized light perception and a triplet of ocelli that are thought to be critical during flight to detect horizon and movements. It is widely believed that evolutionary diversification of compound eye and ocelli in insects occurred from an ancestral visual organ around 500 million years ago. Concurrently, opsin genes were also duplicated to provide distinct spectral sensitivities to different PRs of compound eye and ocelli. In the fruit fly Drosophila melanogaster, Rhodopsin1 (Rh1) and Rh2 are closely related opsins that are originated from the duplication of a single ancestral gene. However, in the visual organs, Rh2 is uniquely expressed in ocelli whereas Rh1 is uniquely expressed in outer PRs of the compound eye. It is currently unknown how this differential expression of Rh1 and Rh2 in the two visual organs is controlled to provide unique spectral sensitivities to ocelli and compound eyes. Here, we show that Homothorax (Hth) is expressed in ocelli and confers proper rhodopsin expression. We find that Hth controls a binary rhodopsin switch in ocelli to promote Rh2 expression and repress Rh1 expression. Genetic and molecular analysis of rh1 and rh2 supports that Hth acts through their promoters to regulate rhodopsin expression in the ocelli. Finally, we also show that when ectopically expressed in the retina, hth is sufficient to induce Rh2 expression only at the outer PRs in a cell autonomous manner. We therefore propose that the diversification of rhodpsins in the ocelli and retinal outer PRs occurred by duplication of an ancestral gene, which is under the control of Homothorax.

中文翻译：

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同胸控制果蝇中的二元视紫红质开关

环境的视觉感知由眼睛的专门光感受器 (PR) 神经元介导。每个 PR 表达光敏视蛋白，由特定波长的光激活。在大多数昆虫中，视觉系统包括一对主要与运动检测、颜色或偏振光感知相关的复眼，以及被认为在飞行过程中检测地平线和运动至关重要的三组单眼。人们普遍认为，昆虫的复眼和单眼的进化多样化发生在大约 5 亿年前的祖先视觉器官。同时，视蛋白基因也被复制，以对复眼和单眼的不同 PR 提供不同的光谱敏感性。在果蝇 Drosophila melanogaster 中，视紫红质 1 (Rh1) 和 Rh2 是密切相关的视蛋白，起源于单个祖先基因的复制。然而，在视觉器官中，Rh2 在 ocelli 中唯一表达，而 Rh1 在复眼的外部 PR 中唯一表达。目前尚不清楚如何控制 Rh1 和 Rh2 在两个视觉器官中的这种差异表达，从而为单眼和复眼提供独特的光谱灵敏度。在这里，我们显示同胸 (Hth) 在 ocelli 中表达并赋予正确的视紫质表达。我们发现 Hth 控制 ocelli 中的二元视紫红质开关，以促进 Rh2 表达并抑制 Rh1 表达。rh1 和rh2 的遗传和分子分析支持Hth 通过它们的启动子来调节小眼中视紫红质的表达。最后，我们还表明，当在视网膜中异位表达时，hth 足以以细胞自主方式仅在外部 PR 处诱导 Rh2 表达。因此，我们提出在单眼和视网膜外 PRs 中视紫红质的多样化是通过复制祖先基因而发生的，这是在同胸的控制下。