The Rap1 GTPase performs two separable functions during cellularization to enable ventral furrow formation.

Morphogenetic processes involve dynamic cell-level events, such as contractility, shape changes, and adhesion between cells and extracellular structures. These cell-level changes require intercellular coordination in order to generate the appropriate tissue-level output. Although the repertoire of these cell-level changes is limited, these processes are reused serially during development to generate diverse tissue-level transformations. Dissecting any one of them benefits from perturbations controlled with high spatiotemporal resolution. Despite extensive analysis of ventral furrow formation, a model gastrulation event in the early Drosophila embryo, it remains unclear how tissue-level events are orchestrated at the molecular level. We therefore developed a biosensor and optogenetic tools for spatiotemporal modulation of Rap1, a small GTPase known to be required for ventral furrow formation. Using the biosensor, we show that Rap1 activity declines in ventral cells as the furrow forms. Optogenetic dissection of Rap1 activity reveals that acute inhibition during furrowing has little effect. Instead, Rap1 contributes to furrowing through two separable functions during the preceding process, cellularization: it is continuously required for efficient membrane ingression, and transiently required to nucleate the adherens junctions that later coordinate contractility across the ventral epithelium.