1 University of Hamburg, Biozentrum Klein Flottbek, Department of Developmental Biology, Ohnhorststr. 18, 22609 Hamburg, Germany. Electronic address: arp.schnittger@uni-hamburg.de.
2 RIKEN Center for Sustainable Resource Science, Yokohama, Kanagawa 230-0045, Japan. Electronic address: keiko.sugimoto@riken.jp.