In resource-constrained wireless sensor networks (WSNs), communication protocols need to be energyefficient. In this context, information-fusion techniques have been used to reduce packet transmissions, and role assignment algorithms have been proposed to define which nodes should perform information fusion. However, most of the role assignment algorithms for WSNs are proactive in the sense that roles are assigned even when no event is being detected. In this work, we propose a reactive algorithm that assigns roles only when an event is detected, therefore saving energy during the network inactivity. In addition, to save energy during event notification, our algorithm searches for the shortest paths that maximize data aggregation.