INTRINSIC AND EXTRINSIC DETERMINANTS OF MESODERMAL DIFFERENTIATION INDROSOPHILA

Early experiments on insect embryos suggested that the patterning of t he mesoderm is decisively influenced by inductive signals from the ect oderm. A series of experiments in which putative signalling molecules are ectopically expressed in expanded domains identifies two of the si gnals involved, Decapentaplegic and Wingless, and confirms that they a ct to regulate patterns of mesodermal gene expression and differentiat ion during development. Ubiquitous expression of these signalling mole cules leads to patterned responses in the mesoderm. Extrinsic signals thus interact with intrinsic restrictions on the competence of mesoder mal cells to respond and so initiate patterns of mesodermal differenti ation in the fly.