DNA-Binding Proteins in Drosophila Development

The Drosophila embryo is composed of serially repeating units, or segments, which differentiate into particular structures and patterns according to their position along the longitudinal axis of the embryo. The generation and diversification of the segments depend on two distinct but integrated processes during early embryogenesis: the subdivision of the embryo into metameric units, and the specification of their identities. Both processes occur simultaneously at the blastoderm stage of embryogenesis which is reached after about 2 h postfertilization. At this stage, the embryo is a single-layered sheet of cells which develops in a characteristic way: nuclei generated by the initial syncitial cleavage divisions migrate to the periphery (egg cortex), and undergo three more divisions before membranes grow down from the cortex to separate each nucleus, thereby forming the cellular blastoderm (for details see Campos-Ortega and Hartenstein 1985). By the completion of this process, the fate of each newly formed cell is already established with regard to its role in the segmentation process. The purpose of this review is to survey the recent progress towards understanding the molecular mechanism of how blastoderm cells are programmed for specific developmental pathways. We first summarize the different components of the genetic circuitry that establish the segment pattern along the longitudinal axis of the Drosophila embryo. We then focus on the activities of the different maternal and zygotic segmentation genes, most of which encode DNA-binding proteins that bind cis-regulatory elements of developmental target genes in a complex network of interactions.