AXES, BOUNDARIES AND COORDINATES - THE ABCS OF FLY LEG DEVELOPMENT

Recent studies of gene expression in the developing fruitfly leg suppo rt a model - Meinhardt's Boundary Model - which seems to contradict th e prevailing paradigm for pattern formation in the imaginal discs of D rosophila - the Polar Coordinate Model. Reasoning from geometric first principles, this article examines the strengths and weaknesses of the se hypotheses, plus some baffling phenomena that neither model can com fortably explain. The deeper question at issue is: how does the fly's genome encode the three-dimensional anatomy of the adult? Does it dema rcate territories and boundaries (as in a geopolitical map) and then u se those boundaries and their points of intersection as a scaffolding on which to erect the anatomy (the Boundary Model)? Or does it assign cellular fates within a relatively seamless coordinate system (the Pol ar Coordinate Model)? The existence of hybrid Cartesian-polar models s hows that the alternatives may not be so clear-cut: a single organ mig ht utilize different systems that are spatially superimposed or tempor ally sequential.