Hyperelliptic Jacobians and isogenies

In the second part we consider a closed subvariety Y⊂Ag of the moduli space of principally polarized varieties of dimension g≥3. We show that if a very general element of Y is dominated by the Jacobian of a curve C and dim⁡Y≥2g, then C is not hyperelliptic. In particular, if the general element in Y is simple, its Kummer variety does not contain rational curves. Finally we show that a closed subvariety Y⊂Mg of dimension 2g−1 such that the Jacobian of a very general element of Y is dominated by a hyperelliptic Jacobian is contained either in the hyperelliptic or in the trigonal locus.