Position-specific and non-colinear expression of the planarian posterior (Abdominal-B-like) gene

Hox genes are pivotal molecules in the control of morphogenesis along the a nterior-posterior (AP) axis in various bilaterians. Planarians are key anim als for understanding the evolution of the bilaterian body plan. Furthermor e, they are also known for their strong regeneration ability and are though t to use the Hox genes in the process of reconstruction of the AP axis. In the present paper, the identification and analysis of expression of two pos terior (Abdominal-B-like) genes, DjAbd-Ba and DjAbd-Bb, is reported in the planarian Dugesia japonica. DjAbd-Ba is expressed in the entire tail region and its anterior boundary is the posterior pharyngeal region. In contrast, DjAbd-Bb is expressed in several types of cells throughout the body. Durin g regeneration, the expression of DjAbd-Ba rapidly recovers a pattern simil ar to that in the normal worm. These findings suggest the possibility that DjAbd-Ba is involved in the specification of the tail region. The anterior boundary of the expression domain of the posterior gene DjAbd-Ba is anterio r to the domains of the central genes Plox4-Dj and Plox5-Dj. These expressi on patterns of planarian Hox genes seem out of the rule of spatial colinear ity and may reflect an ancestral feature of bilaterian Hox genes.