THE ROLE OF MORPHOGENETIC CELL-DEATH DURING DROSOPHILA EMBRYONIC HEADDEVELOPMENT

This article addresses the role of programmed cell death (apoptosis) d uring embryonic head development of Drosophila. Previous studies showe d that reaper (rpr) is expressed in and required by cells undergoing a poptosis. We have analyzed the correlation between the pattern of expr ession of rpr and morphogenetic movements affecting head development. Furthermore, we have investigated the defects in head development resu lting from the absence of apoptosis in embryos deficient for rpr. Our results show that, in the head, domains of high incidence of cell deat h as marked by expression of rpr correlate with regions where most mor phogenetic movements occur; these regions are involved in formation of mouth structures, the internalization of neural progenitors, and head involution. Cellular events driving these movements are delamination, invagination, and intercalation as well as disruption and reformation of contacts among epithelial cells. The analysis of rpr-deficient emb ryos demonstrates that, despite of the widespread occurrence of apopto sis during normal head morphogenesis, many aspects of this process pro ceed in an apparently unperturbed manner even when cell death is block ed. In particular, movements that happen early during embryonic develo pment and that are evolutionarily more ancient (e.g., formation of the dorsal ridge and the pharynx) take place almost normally in rpr-defic ient embryos. Later events which are mostly associated with head invol ution (e.g., retraction of the clypeolabrum, formation of the dorsal p ouch, fusion of lateral gnathal lobes) are evolutionarily more recent and fail to occur normally in rpr deficient embryos. (C) 1998 Academic Press.