DNR and incomparable Turing degrees

We construct an increasing $\omega$-sequence $(a_n)$ of Turing degrees which forms an initial segment of the Turing degrees, and such that each~$a_{n+1}$ is diagonally noncomputable relative to $a_n$. It follows that the~$\mathsf{D}\mathsf{N}\mathsf{R}$ principle of reverse mathematics does not imply the existence of Turing incomparable degrees.