Searching for the prototypic eye genetic network: Sine oculis is essentialfor eye regeneration in planarians

We have identified a sine oculis gene in the planarian Girardia tigrina (Pl atyhelminthes; Turbellaria; Tricladida). The planarian sine oculis gene (Gt so) encodes a protein with a sine oculis (Six) domain and a homeodomain tha t shares significant sequence similarity with so proteins assigned to the S ix-2 gene family. Gtso is expressed as a single transcript in both regenera ting and fully developed eyes. Whole-mount in situ hybridization studies sh ow exclusive expression in photoreceptor cells. Loss of function of Gtso by RNA interference during planarian regeneration inhibits eye regeneration c ompletely. Gtso is also essential for maintenance of the differentiated sta te of photoreceptor cells. These results, combined with the previously demo nstrated expression of Pax-6 in planarian eyes, suggest that the same basic gene regulatory circuit required for eye development in Drosophila and mou se is used in the prototypic eye spots of platyhelminthes and, therefore, i s truly conserved during evolution.