Generation of medial and lateral dorsal body domains by the pannier gene of Drosophila

The pannier (pnr) gene encodes a GATA transcription factor and acts in seve ral developmental processes in Drosophila, including embryonic dorsal closu re, specification of cardiac cells and bristle determination. We show that pnr is expressed in the mediodorsal parts of thoracic and abdominal segment s of embryos, larvae and adult flies. Its activity confers cells with speci fic adhesion properties that make them immiscible with non-expressing cells . Thus there are two genetic domains in the dorsal region of each segment: a medial (MED) region where pnr is expressed and a lateral (LAT) region whe re it is not. The homeobox gene iroquois (iro) is expressed in the LAT regi on. These regions are not formed by separate polyclones of cells, but are d efined topographically. We show that ectopic pnr in the wing induces MED th oracic development, indicating that pnr specifies the identity of the MED r egions. Correspondingly, when pnr is removed from clones of cells in the ME D domain, they sort out and apparently adopt the LAT fate. We propose that (1) the subdivision into MED and LAT regions is a general feature of the Dr osophila body plan and (2) pnr is the principal gene responsible for this s ubdivision. We argue that pnr acts like a classical selector gene but diffe rs in that its expression is not propagated through cell divisions.