**6. Acknowledgments**

We thank Sandra M. Ocampo, Francesc X. Blasco and the Research Support Services from the Biology Unit of Bellvitge (University of Barcelona) for their technical assistance, and Dr. Maria Molas for invaluable assistance in reviewing the manuscript. A.V.A received a fellowship from DURSI (Generalitat de Catalunya), A.M.L. received a fellowship from F.P.I. (Ministerio de Educación y Ciencia, Spain). This study was supported by a grant from the Ministerio de Educación y Ciencia (BFU2006-02802 and BFU2009-07506).
