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Broadcast communication in wireless ad hoc networks can be implemented by means of a virtual backbone. Backbone nodes forward broadcast messages for others. A classical way of constructing such a backbone is to find a connected dominating set in the underlying graph. Based on this concept, a great amount of research has been devoted to backbone formation in static networks. In contrast, distributed backbone management in the presence of node mobility has been explored to a less extent. We present a distributed algorithm for managing a dynamic broadcast infrastructure in mobile ad hoc networks. The algorithm relies on neither location information nor any kind of global knowledge of the network. What's more, the algorithm does not require time synchronization between nodes. Decisions of joining and leaving the backbone are made locally at nodes. A node joins the backbone if connectivity between some neighbors is seemingly broken. To keep the size of the backbone small, a backbone node applies randomized pruning if its neighbors are connected through other backbone nodes. In our simulation experiments we examine backbone size and connectivity with respect to node mobility. The results demonstrate the effectiveness of the algorithm.