even skippedis required to produce atrans-acting signal for larval neuroblast proliferation that can be mimicked by ecdysone

Development of a multicellular organism requires precise coordination of cell division and cell type determination. The selector homeoprotein Even skipped (Eve) plays a very specific role in determining cell identity in the Drosophila embryo, both during segmentation and in neuronal development. However, studies of gene expression in eve mutant embryos suggest that eve regulates the embryonic expression of the vast majority of genes. We present here genetic interaction and phenotypic analysis showing that eve functions in the trol pathway to regulate the onset of neuroblast division in the larval CNS. Surprisingly, Eve is not detected in the regulated neuroblasts, and culture experiments reveal that Eve is required in the body, not the CNS. Furthermore, the effect of an eve mutation can be rescued both in vivo and in culture by the hormone ecdysone. These results suggest that eve is required to produce a trans-acting factor that stimulates cell division in the larval brain.