Male: spelled without a Y

In most mammals, us included, biological sex is determined by a lottery between two letters: X and Y, the sex chromosomes. But there are rare, mysterious exceptions: A small number of rodents have no Y chromosomes, yet are born as females or males, not hermaphrodites. Both female and male Amami spiny rats have only one X chromosome, for example. At some point the rats lost their Y chromosome and, along with it, an important gene called SRY that’s considered the “master switch” of male anatomical development in most mammals. Recently, Japanese scientists injected stem cells derived from a female rat into male embryos of laboratory mice. The cells developed into and survived as sperm precursors in adult males. This result shows that the spiny rat’s sex cells have “astounding” fluidity, said Diana Laird, at the University of California, San Francisco, who was not part of the study.