Synchronous and early activation of planarian Hox genes and the re-specification of body axes during regeneration in Dugesia (G.) tigrina (Turbellaria; Tricladida)

Seven Hox cluster-related genes (Dthox-A to -G) have been isolated from the freshwater triclad Dugesia (G.) tigrina, their sequence compared to other Hox genes and their expression in intact and regenerating organisms analyze d by whole mount in situ hybridization. Sequence comparison analyses show h igh similarities of D. tigrina Hox genes to anterior and medial groups of c oelomate Hox genes. Expression analyses show very early, synchronous, and o verlapping expression of Dthox -A, -E, -G and -F in anterior, posterior and lateral regenerative tissues. At one hour of regeneration all Dthox genes studied showed a neat, clear expression at the wound boundary. Later, as th e blastema grows, the expression area expands to more proximal regions cove ring the blastema and the distal postblastema regions. Blastemas formed by intercalary regeneration also show a synchronous expression of the same Hox genes though the onset of activation is much delayed. The finding that the same set of Hox genes is synchronously activated in anterior, posterior, i ntercalary and lateral regeneration is in sharp contrast to its well establ ished role in specifying antero-posterior pattern during embryonic developm ent. The implications of these results as regards ancestral versus co-opted roles of Hox genes in development and regeneration are discussed.