PLANARIAN HOX GENES - NOVEL PATTERNS OF EXPRESSION DURING REGENERATION

Platyhelminthes are widely considered to be the sister group of coelom ates (Philippe, H., Chenuil, A. and Adoutte, A. (1994) Development 199 4 Supplement, 15-24) and the first organisms to show bilateral symmetr y and cephalization. Within this phylum, the freshwater planarians (Tu rbellaria, Tricladida) have been used as model systems for studying bi directional regeneration (Slack, J. M. W. (1980) J. Theor Biol. 82, 10 5-140). 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 alon g 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 D ugesia(G)tigrina. Sequence analyses of seven planarian Dthox genes (Dt hox-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, s ynchronous and colocalized activation of Dthox-D, Dthox-A, Dthox-C, Dt hox-E, Dthox-G and Dthox-F. After one hour of regeneration a clear exp ression was observed in all Dthox genes studied. In addition, all seem ed to be expressed in the same regenerative tissue, although in the la st stages of regeneration (9 to 15 days) a differential timing of deac tivation was observed. The same Dthox genes were also expressed synchr onously 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 p revent these genes from directing the distinction between head and tai l. Finally, continuous expression along the whole lateral blastema in sagittal regenerates reflected a ubiquitous Dthox response in all type s of regeneration that was not related specifically with the anteropos terior polarity.