No equivalent to the mammalian Sry sex-determining gene has been found in non-mammalian vertebrates. In an Advanced Online Publication in Nature, Masaru Matsuda and colleagues describe the characterization of the first fish gene required for male development (Nature 2002, DOI://10.1038/nature751).

Matsuda et al. chose the medaka fish (Oryzias latipes), a cousin of the zebrafish that is widely used a genetic model organism to study vertebrate development. The major difference between the medaka Y and X chromosomes is thought to be the sex-determining region. They used a Y congenic strain and a positional cloning approach to narrow down the sex-determining region to a 530 kb region that includes 52 predicted genes. Study of an XY female with a large deletion of the Y chromosome allowed them to narrow the search further, to a 250 kb region with 27 predicted genes. They tested each of these candidates...

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