ANTERIOR-POSTERIOR SUBDIVISION AND THE DIVERSIFICATION OF THE MESODERM IN DROSOPHILA

We have used a novel cell marker, in which the twist promoter directs the synthesis of the cell surface protein CD2 (twi-CD2) to examine the development of the mesoderm in the Drosophila embryo after gastrulati on and to locate the progenitor cell populations for different mesoder mal derivatives. We find that the early mesoderm in each segment is di vided into a more anterior region with relatively low levels of twist and twi-CD2 expression and a more posterior region where twist and twi -CD2 expression are high. This subdivision coincides with regional ass ignments of cells to form different progenitors: dorsal anterior cells invaginate to form an internal layer from which the visceral mesoderm is derived. Ventral anterior cells form progenitors of mesodermal gli al cells. Dorsal posterior cells form heart. Ventral and dorsal poster ior cells form somatic muscles. We conclude that the metamerically rep eated anterior-posterior subdivision of the mesoderm is an essential e lement in laying out the pattern of mesodermal progenitor cells and in distinguishing between an internal cell layer which will give rise to the progenitors of visceral muscles and an external layer which will generate the somatic muscles and the heart.