The early Drosophila embryo is a syncytium. Cellularisation only occurs after 14 very rapid nuclear divisions, the last four of which occur at the periphery and are associated with transient ingressions of the plasma membrane between dividing nuclei. How do these transient furrows form and what is their function? On p. , Todd Blankenship and co-workers address these questions using live-imaging approaches. First, the authors characterise the dynamics of furrow formation and retraction. They then test whether the RalA GTPase, known to be important in other systems for targeted membrane addition via the exocyst complex, plays a role in furrow dynamics. Depletion of RalA leads to a lack of membrane ingression, and the authors also find that the Rab8 GTPase and the exocyst component Sec5 are important for ingression. In the absence of RalA, and hence furrows, two different types of nuclear division defects are seen: spindle anchoring is impaired, leading to chromosome mis-segregation at anaphase, and nuclei are not efficiently separated, leading to fusion of adjacent genomes. RalA-dependent transient furrow formation is therefore important to maintain genome integrity during the early syncytial divisions in Drosophila.
