Evolutionary conservation of the initial eye genetic pathway in planarians

Eyes of all organisms share a common function, visual perception. In addition, the different types of eyes (camera-, mirror-, and compound) are present in different phyla and share the same visual pigment, rhodopsin, and the same initial genetic pathway triggered by the master control gene Pax-6. Although the developmental mechanisms are quite diverse, all data suggest that the different eye types found in metazoans derive from a common prototype and evolved in the different phyla by parallelism, intercalating new genes independently. In this manuscript, we describe the isolation and characterization of several genes that constitute the eye gene regulatory network in the planarian Girardia tigrina (Platyhelminthes; Turbellaria; Tricladida). Two Pax-6 genes, GtPax6A and GtPax6B, do not show an obvious correspondence to the two Pax-6 of Drosophila ey and toy. Two sine oculis genes Gtsix-1 and Gtsix-3 are closely related to the Six 1-2 and Six-3 families respectively. Furthermore, we demonstrate that the opsin gene Gtops shows greater similarity to mollusc opsins. GtPax-6B is expressed in both cell types of the planarian eye spots: the photoreceptor cells and the pigmented cells. In addition, Gtsix-1 and the opsin gene Gtops are expressed in the photoreceptor cells. This expression pattern is present throughout the whole eye regeneration process and maintained in adults. Gtops double strand RNA injection does not inhibit eye regeneration but produces light insensitive eyes due to the absence of photopigment. The loss of function of Gtsix-1 by dsRNA injection produces a non-eye phenotype in head regenerating blastemas, while the injected intact adult heads show a loss of the differentiated state of the photoreceptor cells through inhibition of opsin expression and the production of a blind phenotype. Our results on the prototypic eye spots of Platyhelminthes provide farther important support for the idea of a universally conserved early eye genetic cascade in the Metazoa.