Mader's subdivision conjecture for acyclic digraphs

Let $F$ be an acyclic digraph, and let $\delta^+(D)$ denote the minimum out-degree of a digraph $D$. A subdivision of $F$ is obtained by replacing the arcs of $F$ by directed paths with the same endpoints and internally vertex-disjoint interiors. Mader's subdivision conjecture. For every acyclic digraph $F$ there exists a number $K=K(F)$ such that every digraph $D$ with

$$\delta^+(D)\ge K$$

contains a subdivision of $F$. This remains an open problem concerning degree conditions that force directed subdivisions.