Decomposition and minimality of Lagrangian submanifolds in nearly K\"ahler manifolds

We show that Lagrangian submanifolds in six-dimensional nearly K\"ahler (non K\"ahler) manifolds and in twistor spaces $Z\sp{4n+2}$ over quaternionic K\"ahler manifolds $Q\sp{4n}$ are minimal. Moreover, we will prove that any Lagrangian submanifold $L$ in a nearly K\"ahler manifold $M$ splits into a product of two Lagrangian submanifolds for which one factor is Lagrangian in the strict nearly K\"ahler part of $M$ and the second factor is Lagrangian in the K\"ahler part of $M$. Using this splitting theorem we then describe Lagrangian submanifolds in nearly K\"ahler manifolds of dimensions six, eight and ten.