45 The Bicoid Morphogen: Concentration-dependent Transcriptional Activation of Zygotic Target Genes during Early Drosophila Development

Abstract

OVERVIEW
The Drosophila maternal gene bicoid encodes a homeodomain protein, Bicoid, which activates various zygotic genes at distinct threshold concentrations in the embryo. Bicoid is distributed in a concentration gradient along the anterior-posterior axis of the embryo. The structures of target gene promoters appear to specify their spatially restricted responses: Genes with high-affinity binding sites for Bicoid are active at low concentrations of Bicoid (in the thoracic region), whereas those with low-affinity sites for Bicoid appear to be expressed only in more anterior positions (the head region) where high Bicoid concentrations prevail. Investigation of Bicoid will reveal how a continuous gradient of a single regulator can achieve discretely different regulatory effects within its field of distribution.

INTRODUCTION
Control of Early Development in the Drosophila Embryo
During embryonic development of Drosophila melanogaster, a hierarchy of genes acts to specify the body pattern (Nüsslein-Volhard and Wieschaus 1980; Akam 1987; Ingham 1988). The fertilized egg develops first as a polynucleated syncytium to form, after about 2.5 hours of development, a peripheral layer of blastoderm nuclei. At this stage, the earliest genes to be transcribed in the embryo (zygotic genes) already display local regions of expression that reflect the organization of the embryonic axis. Thus, information that initiates the process of pattern formation must preexist in the egg.

Genetic analysis has demonstrated that maternal gene products (transcribed during oogenesis) regulate the establishment of regional identity in the early Drosophila embryo (Nüsslein-Volhard et al. 1987). These studies reveal that four maternal gene systems are sufficient...