Scientists at CSIRO in Australia have genetically modified the Carp (Cyprinus carpio) to make all individuals turn male, according to this Reuters article. Apparently the gene sequence used, called "daughterless", is found naturally in the carp genome. The hope is that if these modified carp are released into bodies of water, over the next several decades they will breed with wild carp, resulting in many fewer females, and theoretically fewer carp offspring. Since carp interefere with the ecosystems they are introduced to by competing with native species for food and polluting the water, this could be a good thing. If you want to know more, you can check out this information sheet (.pdf) from CSIRO's website. There is also a great web page about Aquaculture Genetics .