Lassoing and Corralling Rooted Phylogenetic Trees

The construction of a dendogram on a set of individuals is a key component of a genomewide association study. However, even with modern sequencing technologies the distances on the individuals required for the construction of such a structure may not always be reliable making it tempting to exclude them from an analysis. This, in turn, results in an input set for dendogram construction that consists of only partial distance information, which raises the following fundamental question. For what (proper) subsets of a dendogram’s leaf set can we uniquely reconstruct the dendogram from the distances that it induces on the elements of such a subset? By formalizing a dendogram in terms of an edge-weighted, rooted, phylogenetic tree on a pre-given finite set X with |X|≥3 whose edge-weighting is equidistant and subsets Y of X for which the distances between every pair of elements in Y is known in terms of sets of 2-subsets of X, we investigate this problem from the perspective of when such a tree is lassoed, that is, uniquely determined by the elements in . For this, we consider four different formalizations of the idea of “uniquely determining” giving rise to four distinct types of lassos. We present characterizations for all of them in terms of the child-edge graphs of the interior vertices of such a tree. Our characterizations imply in particular that in case the tree in question is binary, then all four types of lasso must coincide.