HEDGEHOG AND WINGLESS INDUCE METAMERIC PATTERN IN THE DROSOPHILA VISCERAL MESODERM

The Drosophila visceral mesoderm (VM) is a favorite system for studyin g the regulation of target genes by Hox proteins. The VM is formed by cells from only the anterior subdivision of each mesodermal parasegmen t (PS). We show here that the VM itself acquires modular anterior-post erior subdivisions similar to those found in the ectoderm. As VM proge nitors merge to form a continuous band running anterior to posterior a long the embryo, expression of connectin (con) in 11 metameric patches within the VM reveals VM subdivisions analagous to ectodermal compart ments. The VM subdivisions form in response to ectodermal production o f secreted signals encoded by the segment polarity genes hedgehog (hh) and wingless (wg) and are independent of Hox gene activity. A cascade of induction from ectoderm to mesoderm to endoderm thus subdivides th e gut tissues along the A-P axis. Induction of VM subdivisions may con verge with Hox-mediated information to refine spatial patterning in th e VM. Con patches align with ectodermal engrailed stripes, so the VM s ubdivisions correspond to PS 2-12 boundaries in the VM. The PS boundar ies demarcated by Con in the VM can be used to map expression domains of Hox genes and their targets with high resolution. The resultant map suggests a model for the origins of VM-specific Hox expression in whi ch Hox domains clonally inherited from blastoderm ancestors are modifi ed by diffusible signals acting on VM-specific enhancers. (C) 1998 Aca demic Press.