WNT AND TGF-BETA SIGNALS SUBDIVIDE THE ABDA HOX DOMAIN DURING DROSOPHILA MESODERM PATTERNING

Hox genes have large expression domains yet control the formation of f ine pattern elements at specific locations. We have examined the mecha nism underlying subdivision of the abdominal-A (abdA) Hox domain in th e visceral mesoderm, AbdA directs formation of an embryonic midgut con striction at a precise location within the broad and uniform abdA expr ession domain. The constriction divides the abdA domain of the midgut into two chambers, the anterior one producing the Pointed (Pnt) ETS tr anscription factors and the posterior one the Odd-paired (Opa) zinc fi nger protein. Transcription of both pnt and opa is activated by abdA b ut the adjacent non-overlapping patterns are not due to mutual opa-pnt regulation. Near the anterior limit of the abdA domain, two signals, Dpp (a TGF beta) and Wg (a Wnt), are produced, in adjacent non-overlap ping patterns, under Hox control in mesoderm cells. The two signals ar e known to regulate local mesodermal cell fates and to signal to the e ndoderm, We find that, in addition, they precisely subdivide the abdA domain: Wg acts upon anterior abdA domain cells to activate pnt transc ription, while Dpp is essential in the same region to prevent abdA fro m activating opa transcription, pnt activation is required to determin e the appropriate numbers of mesodermal cells in the third midgut cham ber.