Models for maternally supplied positional information and the activation of segmentation genes in Drosophila embryogenesis

Recent experiments concerned with early Drosophila development require some modification of models proposed earlier. The addition of a positional information system spreading out from the anterior pole (as indicated by the bicoid mutation) allows very good size regulation. Additional steep gradients at both poles (as suggested by the mutations of the torso group) allow simpler mechanisms for the activation of the gap (or cardinal) genes. Simulations of pattern alterations due to mutations in one of these systems agree well with the observed phenotypes. Mechanisms that can lead to bicaudal phenotypes as well as mechanisms that keep the anteroposterior and the dorsoventral axes perpendicular to each other are discussed. According to the model, the sharpness of the region-specific activation of the gap genes results from local competition of the gene products. For the correct activation of the pair-rule genes, cooperation between gap genes is required. Basic features of the pair-rule pattern can be described by two binary sequences whose phase shift contains the polarity information. Segmentation results from the reiteration of at least three, but probably four, cell states. Each of these cell states is induced at two different positions in the pair-rule pattern, such that 14 segments are formed from 7 double segments. A computer simulation is provided.