Eyes of all organisms share a common function, visual perception. In additi
on, the different types of eyes (camera-, mirror-, and compound) are presen
t in different phyla and share the same visual pigment, rhodopsin, and the
same initial genetic pathway triggered by the master control gene Pax-6. Al
though the developmental mechanisms are quite diverse, all data suggest tha
t 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 character
ization 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 correspondenc
e 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 respectiv
ely. Furthermore, we demonstrate that the opsin gene Gtops shows greater si
milarity to mollusc opsins. GtPax-6B is expressed in both cell types of the
planarian eye spots: the photoreceptor cells and the pigmented cells. In a
ddition, Gtsix-1 and the opsin gene Gtops are expressed in the photorecepto
r cells. This expression pattern is present throughout the whole eye regene
ration process and maintained in adults. Gtops double strand RNA injection
does not inhibit eye regeneration but produces light insensitive eyes due t
o the absence of photopigment. The loss of function of Gtsix-1 by dsRNA inj
ection produces a non-eye phenotype in head regenerating blastemas, while t
he injected intact adult heads show a loss of the differentiated state of t
he photoreceptor cells through inhibition of opsin expression and the produ
ction of a blind phenotype. Our results on the prototypic eye spots of Plat
yhelminthes provide farther important support for the idea of a universally
conserved early eye genetic cascade in the Metazoa.