Actin organization in the early Drosophila embryo

Organization of the cortical cytoplasm during the syncytial blastoderm stages of early Drosophila embryogenesis relies on cyclic transitions between transient microfilament structures. Microtubule-organizing centres (MTOCs) appear to provide the instructive cues governing this dynamic, cell-cycle-dependent process. Using a genetic approach, we have identified key roles for two molecular pathways in mediating these events. The conserved Arp2/3 microfilament nucleation machinery, likely acting in response to the activating element SCAR, plays an essential role in establishment of a cortical F-actin array, and contributes to specific aspects of cyclic microfilament restructuring. Defective cortical microfilament organization is the primary phenotypic feature of embryos derived from mothers bearing mutations in the sponge locus. Several lines of investigation suggest that the primary defect in sponge lies in a faulty cortical microfilament response, downstream of the centrosomal signal. We have determined that sponge encodes a Drosophila homologue of the evolutionarily-conserved CDM (DOCK180) protein family.