Establishement of the Dorso-ventral Pattern During Embryonic Development ofDrosophila melanogaster: a Logical Analysis

This report focuses on dorso-ventral patterning in the segmented region of theDrosophila melanogasterembryo. According to the concept of positional information, this pattern results from the different response of cells to the Dorsal-protein morphogen. This protein shows a distribution gradient along the dorso-ventral axis, with the highest concentration on the ventral side. Using the generalized logical formalism developed by R. Thomas and co-workers, the different cellular responses were analysed in terms of the intracellular loops between the regulatory genes. Two positive loops were found to be involved, each constituting a switch which can be acted upon by the Dorsal morphogen to determine the different cell types that make up the embryonic dorso-ventral pattern. The novelty in this use of generalized logical formalism is the employment of a multilevel variable to represent a morphogen gradient. The proposed model accounts for the essential qualitative effects of the Dorsal gradient in the dorso-ventral determination process. Three main conclusions may be drawn. Firstly, the genetwistneeds to have two functional threshold concentrations, one for autoactivation and the other for activation of the genesnail. Secondly, the autoactivation threshold must be smaller than that which activatessnail. Thirdly, the action of the genesnailon the maintenance function of the genetwistis crucial for cells to be able to choose between the mesoderm or neuroectoderm developmental pathways. Furthermore, it is predicted that if the genesnailshows autoregulation, this will not be crucial for the determination of the embryonic D-V pattern.