We thank members of the Zarkower and Bardwell labs for many helpful discussions; Dan Camerini-Otero, John Logsdon, and Scott Hawley for insights into evolution of meiotic regulators; Hiromitsu Tanaka, Yoshitake Nishimune, and David Elliot for generously sharing antibodies; and Dies Meijer for sharing Dhh-Cre mice. We thank the University of Minnesota Mouse Genetics Laboratory for ES cell injection. Some images were obtained using the Middlebury Biology Imaging Facility, equipped in part through the National Science Foundation (Course, Curriculum, and Laboratory Improvement, 0088412). SHN is a research fellow of the Japan Society for Promotion of Science.