The organization of the antero-posterior axis.

The components specifying the spatial coordinates of the Drosophila embryo are deposited in the egg during oogenesis. Three maternal pathways control the pattern of the embryo along its antero-posterior axis. Genetic and molecular analysis has identified the key-genes in each of these pathways: (1) the bicoid gene encodes an anterior signal in the embryo that directs head and thorax formation via transcriptional activation of anteriorly expressed zygotic genes. (2) A posterior signal, the nanos gene product, antagonizes an inhibitor of abdominal development, hunchback, by translational regulation. (3) A terminal signal controls development at both poles of the embryo. It is probably induced by the somatic follicle cells and transmitted to the embryo via a membrane bound receptor encoded by the gene torso. Other maternal genes function in the localization of these signals or in signal transduction.