Free group automorphisms and knotted tori in S-4

Let phi be an automorphism of a finitely generated free group F, omega and element of F, and H the subgroup of F generated by the orbit {phi(n)(omega), \n is an element of Z}. We describe sufficient conditions ensuring that H is non-finitely generated. Using this we give a simple construction of tori T embedded in S-4 in such a way that the commutator subgroup of pi(1)(S-4 - T) is finitely generated but not finitely presented. Such tori have no minimal Seifert manifolds. An example of an embedded torus with the latter property was given recently by T. Maeda using different methods.