Controlling Reaction-Diffusion Pattern with Gradients: Lessons from Drosophila, and Trajectories Through Parameter Space

Abstract

Since it was first reported by Hafen et al.(1984), the beautifully simple periodic pattern of pair-rule stripes in Drosophila embryos has intrigued theoretical and experimental biologists alike. A primary concern among theoreticians has been the question of whether pairrule stripes are, to any extent, Turing structures, sustained by far-from-equilibrium kinetic processes. Current opinion among experimentalists tends to the view that they are not. It is instead proposed that the precise boundaries of all seven pair-rule stripes are each independently specified by the underlying expression patterns of gap genes in a concentration-dependent fashion, in the same manner that the gap genes are themselves activated by the broader maternal gradients. This means the iterative operation of a process of gradient reading. The problems with this view have already been discussed at this meeting at some length by Axel Hunding. His message is that it is difficult if not impossible to test either view critically on the basis of currently available data. Models that depend solely on iterative gradient reading require, in their extreme form, many untested suppositions. A Turing mechanism avoids some of the apparent problems, but there is as yet little direct evidence to support the idea that a Turing mechanism is operating, or if it is, what the key components, i. e. the morphogens, might be.