The Y chromosome that lost the male-determining function behaves as an X chromosome in the medaka fish, Oryzias latipes Hiroyuki Otake 1 , Yusuke Hayashi 1 , Satoshi Hamaguchi, Mitsuru Sakaizumi 2 1 These authors contributed equally to this work. Graduate School of Science and Technology, Niigata University, Ikarashi, Niigata 950-2181, Japan Genetics: Published Articles Ahead of Print, published on August 9, 2008 as 10.1534/genetics.108.090167