Non-Asplund Banach spaces and operators

Let $W$ and $Z$ be Banach spaces such that $Z$ is separable and let $R:W⟶Z$ be a (continuous, linear) operator. We study consequences of the adjoint operator $R^{\ast }$ having non-separable range. From our main technical result we obtain applications to the theory of basic sequences and the existence of universal operators for various classes of operators between Banach spaces. We also obtain an operator-theoretic characterisation of separable Banach spaces with non-separable dual.