A homeobox gene of the orthodenticle family is involved in antero-posterior patterning of regenerating planarians.

We studied the expression of DtOtx, a homeobox gene of the freshwater planarian Dugesia tigrina closely related to the Drosophila orthodenticle (otd) and vertebrate Otx genes, which are known to control head development in both fruit flies and vertebrates. DtOtx was not significantly expressed in adult planarians but it was activated within one hour in regenerating tissues with a clearly asymmetric pattern. Animals sectioned transversally, either between the head and the pharynx, or caudal to the pharynx, give rise to a head-containing fragment regenerating a tail region and to a tail-containing fragment regenerating a head region. DtOtx was found to be activated in both regeneration blastemas but its transcripts were much more abundant in the head-regenerating tissues than in the tail-regenerating tissues. The same asymmetric distribution of DtOtx transcripts was observed in central portions of the body regenerating both head and tail structures and in animals laterally regenerating after a longitudinal cut. These data suggest a role of this gene in patterning the body axis of these primitive bilateria, at least during regeneration.