  An acyclic digraph each vertex of which has indegree at most $i$ and
outdegree at most $j$ is called an $(i, j)$ digraph for some positive integers
$i$ and $j$. Lee {\it et al.} (2017) studied the phylogeny graphs of $(2, 2)$
digraphs and gave sufficient conditions and necessary conditions for $(2, 2)$
digraphs having chordal phylogeny graphs. Their work was motivated by problems
related to evidence propagation in a Bayesian network for which it is useful to
know which acyclic digraphs have their moral graphs being chordal (phylogeny
graphs are called moral graphs in Bayesian network theory).
  In this paper, we extend their work. We completely characterize phylogeny
graphs of $(1, i)$ digraphs and $(i,1)$ digraphs, respectively, for a positive
integer $i$. Then, we study phylogeny graphs of a $(2,j)$ digraphs, which is
worthwhile in the context that a child has two biological parents in most
species, to show that the phylogeny graph of a $(2,j)$ digraph $D$ is chordal
if the underlying graph of $D$ is chordal for any positive integer $j$.
Especially, we show that as long as the underlying graph of a $(2,2)$ digraph
is chordal, its phylogeny graph is not only chordal but also planar.

Combinatorics