Isolation and expression of a Pax-6 gene in the regenerating and intact Planarian Dugesia(G)tigrina

The Pax-6 gene encodes a transcription factor containing both a paired and a homeodomain and is highly conserved among Metazoa, In both vertebrates an d invertebrates, Pax-6 is required for eye morphogenesis, development of pa rts of the central nervous system, and, in some phyla, for the development of olfactory sense organs. Ectopic expression of Pax-6 from insects, mammal s, cephalopods, and ascidians induces ectopic eyes in Drosophila, suggestin g that Pax-6 may be a universal master control gene for eye morphogenesis. Platyhelminthes are an ancient phylum, originating from the base of spirali an protostomes, that bear primitive eyes, consisting of a group of rhabdome ric photoreceptor cells enclosed in a cup of pigment cells. The analysis of Pax-6 and its expression pattern should provide insights into the ancestra l function of Pax-6 in eye morphogenesis. We have identified the Pax-6 gene of the planarian Dugesia(G)tigrina (Platyhelminthes; Turbellaria; Tricladi da), This gene shares significant sequence identity and conserved genomic o rganization with Pax-6 proteins from other phyla, Phylogenetic analysis ind icates that it clusters with the other Pax-6 genes, but in the most basal p osition. DtPax-6 is expressed as a single transcript in both regenerating a nd fully grown eyes, and electron microscopy studies show strong expression in the perykarion of both photoreceptor and pigment cells. Very low levels of expression also are detectable in other body regions. Because a bona fi de Pax-6 homolog so far has not been detected in diploblastic animals, we s peculate that Pax-6 may be typical for triploblasts and that the appearance of additional Pax genes may have coincided with increasingly complex body plans.