Search

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.

POU domain proteins are a large family of transcription factors that have been identified in a variety of metazoans, from freshwater sponges, planarians and nematodes to arthropods, echinoderms and vertebrates. Many of these proteins are implicated in the development and establishment of the nervous system. In this paper we describe the identification of the planarian genes GtPOU-1, GtPOU-3 and GtPOU-4, which belong to the subclasses III and IV of POU-domain genes. Their similarity with other members of the POU family is restricted to the POU and homeo domains, plus some peptide sequences scattered in the linker and flanking regions. As with other subclass III POU genes, GtPOU-1 is devoid of introns. Axial transcript distribution by RT-PCR and immunohistochemical assays, performed with a polyclonal antibody raised against the GtPOU-1 fusion protein, indicate that both the GtPOU-1 transcript and protein are continuously expressed along the antero-posterior axis. A monoclonal antibody raised against the same fusion protein indicates that a GtPOU-1-specific epitope, probably obtained by post-translational modification, is present in neural cells from both the central and peripheral nerve systems of the adult planarian's anterior third. Moreover, the GtPOU-1-specific epitope shows a dynamic expression pattern during regeneration, always marking the most anterior region of the planarian nervous system. Both the rapid and general GtPOU-1-specific epitope modification, during posterior regeneration, indicate that regeneration is a global process involving all planarian regions, including those that are far from the wound, by a combination of morphallactic and epimorphic mechanisms., (Copyright 1998 Elsevier Science Ireland Ltd. All Rights Reserved.)

Platyhelminthes are widely considered to be the sister group of coelomates (Philippe, H., Chenuil, A. and Adoutte, A. (1994)Development 1994 Supplement, 15-24) and the first organisms to show bilateral symmetry and cephalization. Within this phylum, the freshwater planarians (Turbellaria, Tricladida) have been used as model systems for studying bidirectional regeneration (Slack, J. M. W. (1980) J. Theor. Biol 82, 105-140). We have been attempting to identify potential pattern-control genes involved in the regeneration of planarian heads and tails after amputation. Since Hox cluster genes determine positional identity along the anteroposterior axis in a wide range of animals (Slack, J. M. W., Holland, P. W. H. and Graham, C. F. (1993) Nature 361,490-492), we performed an extensive search for Hox-related genes in the planarian Dugesia(G)tigrina. Sequence analyses of seven planarian Dthox genes (Dthox-A to Dthox-G) reveal high similarities with the homeodomain region of the Hox cluster genes, allowing us to assign planarian Dthox genes to anterior and medial Hox cluster paralogous groups. Whole-mount in situ hybridization studies in regenerating adults showed very early, synchronous and colocalized activation of Dthox-D, Dthox-A, Dthox-C, Dthox-E, Dthox-G and Dthox-F. After one hour of regeneration a clear expression was observed in all Dthox genes studied. In addition, all seemed to be expressed in the same regenerative tissue, although in the last stages of regeneration (9 to 15 days) a differential timing of deactivation was observed. The same Dthox genes were also expressed synchronously and were colocalized during intercalary regeneration, although their expression was delayed. Terminal regeneration showed identical Dthox gene expression in anterior and posterior blastemas, which may prevent these genes from directing the distinction between head and tail. Finally, continuous expression along the whole lateral blastema in sagittal regenerates reflected a ubiquitous Dthox response in all types of regeneration that was not related specifically with the anteroposterior polarity.