Regulatory putsches create new ways of determining sexual development

The development of multicellular organisms is controlled by gene‐regulatory cascades. Generally, these cascades are headed by a master regulator that determines the development of a precursor cell lineage towards a given tissue or organ during embryogenesis. A notable feature is that the genes at the top of the cascade are more highly conserved than the downstream‐acting factors. An outstanding example of this phenomenon is the homeobox‐containing paired box 6 ( PAX6 ) gene in its function as the master regulator of eye development. When, for example, the human gene is transgenically expressed in a Drosophila leg, it is able to induce the development of an eye at this ectopic and xenogenic position (Halder et al , 1995). What develops is not a vertebrate‐type eye but a fly eye because the genes downstream in the eye‐development cascade are not conserved and are those that specify the development of the insect visual organ, which consists of the complex and very specific ommatidia.However, the opposite is true for the genes of the sex‐determining cascade. In this case, the genes at the top of the cascade are not conserved, whereas the downstream genes have homologues in a much broader spectrum of species (Graham et al , 2003). The SRY gene—the master male sex‐determining gene of mammals—has not been detected outside the eutherians. However, the other known members of the gene network involved in the development of the bipotential gonad anlage of the embryo towards either an ovary or testis—for example, the transcription factors Wilms tumour 1 (WT1) and SOX9 (sex‐determining region Y‐type high‐mobility group box 9), and the bone morphogenetic protein (BMP) growth factor family member anti‐Mullerian hormone (AMH; for a review, see Wilhelm et al , 2007)—are present in all vertebrates, including fish (Devlin & Nagahama, 2002). The most downstream component of this cascade, DMRT1 (doublesex and …