A single three-dimensional chromatin compartment in amphioxus indicates a stepwise evolution of vertebrate Hox bimodal regulation by Acemel, R, Tena, J, Irastorza-Azcarate, I, Marlétaz, F, Gómez-Marín, C, de la Calle-Mustienes, E, Bertrand, S, Diaz, S, Aldea, D, Aury, J, Mangenot, S, Holland, P, Devos, D, Maeso, I, Escrivá, H, Gómez-Skarmeta, J