A decomposition theorem and two algorithms for reticulation-visible networks

In studies of molecular evolution, phylogenetic trees are rooted binary trees, whereas phylogenetic networks are rooted acyclic digraphs. Edges are directed away from the root and leaves are uniquely labeled with taxa in phylogenetic networks. For the purpose of validating evolutionary models, biologists check whether or not a phylogenetic tree (resp. cluster) is contained in a phylogenetic network on the same taxa. These tree and cluster containment problems are known to be NP-complete. A phylogenetic network is reticulation-visible if every reticulation node separates the root of the network from at least a leaf. We answer an open problem by proving that the problem is solvable in quadratic time for reticulationvisible networks. The key tool used in our answer is a powerful decomposition theorem. It also allows us to design a linear-time algorithm for the cluster containment problem for networks of this type and to prove that every galled network with n leaves has 2(n − 1) reticulation nodes at most.