SEGMENTATION AND SPECIFICATION OF THE DROSOPHILA MESODERM

Patterning of the developing mesoderm establishes primordia of the vis ceral, somatic, and cardiac tissues at defined anteroposterior and dor soventral positions in each segment. Here we examine the mechanisms th at locate and determine these primordia. We focus on the regulation of two mesodermal genes: bagpipe (bap), which defines the anlagen of the visceral musculature of the midgut, and serpent (srp), which marks th e anlagen of the fat body. These two genes are activated in specific g roups of mesodermal cells in the anterior portions of each parasegment . Other genes mark the anlagen of the cardiac and somatic mesoderm and these are expressed mainly in cells derived from posterior portions o f each parasegment. Thus the parasegments appear to be subdivided, at least with respect to these genes, a subdivision that depends on pair- rule genes such as even-skipped (eve). We show with genetic mosaics th at eve acts autonomously within the mesoderm. We also show that hedgeh og (hh) and wingless (wg) mediate pair-rule gene functions in the meso derm, probably partly by acting within the mesoderm and partly by indu ctive signaling from the ectoderm. hh is required for the normal activ ation of bap and sip in anterior portions: of each parasegment, wherea s wg is required to suppress bap and srp expression in posterior porti ons. Hence, hh and wg play opposing roles in mesoderm segmentation.