Fate mapping of Drosophila embryonic mitotic domain 20 reveals that the larval visual system is derived from a subdomain of a few cells.

In an attempt to study the fates of cells in the dorsal head region of Drosophila embryos at gastrulation, we used the photoactivated gene expression system to mark small numbers of cells in selected mitotic domains. We found that mitotic domain 20, which is a cluster of approximately 30 cells on the dorsal posterior surface, gives rise to various ectodermal cell types in the head, including dorsal pouch epithelium, the optic lobe, and head sensory organs, including Bolwig's organ, the larval photoreceptor organ. We found that the optic lobe and larval photoreceptors share the same origin of a few adjacent cells near the center of mitotic domain 20, suggesting that within the mitotic domain, there is a subdomain from which the larval visual system is specified. In addition to the components of the larval visual system, this central region of mitotic domain 20 also generates a part of the eye-antennal disc placode; cells that gives rise to the adult visual system. We also observed that a significant amount of cell death occurred within this domain and used cell ablation experiments to determine the ability of the embryo to compensate for cell loss.