DETERMINATION OF THE DORSOVENTRAL POLARITY OF DROSOPHILA

Embryonic pattern formation has been studied extensively in many organ isms. In Drosophila, the powerful combination of genetics cytoplasm tr ansplantation experiments, as well as recent molecular data, have help ed to elucidate the mechanisms responsible for the establishment of em bryonic polarity. A small number of genes, most of them maternally. ex pressed, are involved in this process and participate in four independ ent systems - three for the antero-posterior axis (A/P) and one for do rsoventral axis (D/V) - which define various embryonic territories by specifically localized cues. This review concerns the definition of th e dorsoventral polarity responsible for the establishment of the germ layers of the embryo. Dorsoventral development is regulated by a singl e group of maternally expressed genes : the << dorsal group >> of gene s. It includes 11 genes, the loss of function of any of which results in a dorsalized development, whereas mutation of the 12th gene, cactus , results in a ventralized development. These genes are arranged accor ding to a functional hierarchy, and have been shown to cooperate in th e formation of a graded nuclear concentration of the dorsal gene produ ct. The dorsal product corresponds to the dorsoventral morphogen and i s homologous to the transcription factor NF-kappaB. Among the 11 genes of the dorsal group, 3 are required in the somatic line. This suggest s the existence of inductive signals originating during oogenesis from the follicle cells that surround the developing oocyte. This somatica lly expressed spatial information probably controls dorsoventral devel opment by defining the polarity of a signal transducing pathway that s pecifically activates the nuclear uptake of the dorsal product. This m odel, highlights the importance of the polarity of the egg chamber, an d suggests that it is the oocyte nucleus due to its asymmetrical local ization that determines the dorsoventral pattern formation of the embr yo.