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Vertebrate sex determination: questioning the hierarchy

Abstract : Within vertebrates, one of the most striking features of the mammalian and avian genomes with respect to sexual development is the presence of sex chromosomes, which harbour the sex-determining genes. Within these two clades, sex chromosomes arose twice, independently in the therian mammals and avian ancestors, giving rise to XY and ZW systems, respectively. Noticeably, in the other vertebrate clades, sex determination is much more divergent even between closely related groups. For example, fish have independently evolved and re-evolved a remarkable array of sex-determining mechanisms, including WZ and XY sex chromosomes, multilocus autosomal sex determinants and environmental sex determination. At the molecular level, the common concept of sex determination is regulation of the decision and differentiation programme by a cascade or network of genes. In particular, for XY chromosomal sex determination, there is a single master regulator gene, which switches a default female expression programme towards male. Intuitively explained by the plethora of independently evolved sex-determination systems, a remarkable diversity of ‘master sex-determining genes’ exists at the top of the genetic hierarchies. A contrario, downstream components are surprisingly more evolutionarily conserved and undoubtedly tend to converge upon the regulation of common effectors generally encoding transcription factors. The SRY gene – the master male sex-determining gene of mammals – has not been detected outside the placental mammals. However, other 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), sex-determining region y-type high-mobility group box 9 (SOX9) and doublesex and Mab-3 related factors (Dmrt1), and the bone morphogenetic protein growth factor family member anti-Mullerian hormone (AMH) – are present in all vertebrates, including fish. Although the primary involvement during the sex-determination process of most of the conserved ‘usual suspects’ mentioned above remains to be further investigated outside mammals, one transcription factor stands out: Dmrt1. Indeed, the rise to fame of Dmrt1 in the field of sex determination came with the findings that, although a gene causing dosage-dependent sex-reversal in humans, in birds it is located on the avian Z, but not on the W chromosome. Likewise, a duplicate of Dmrt1 was shown to be the only functional gene in the male-specific region on the Y chromosome of the medaka fish and consequently the master regulator of male development. The evolutionary conserved commitment of Dmrt1 during the sex-determination process in organisms as phylogenetically disparate as corals, Caenorhabditis elegans, Drosophila, frogs, fish, birds and mammals reliably establishes the DM domain factors as the most widespread downstream genetic sex-determination effectors across all the metazoans. Given this obviously somehow stereotype-conserved position and function, it independently emerged as the master sex determinant in fish (Dmrt1bY), frogs (DM-W) and birds (Dmrt1). After describing the canonical mammalian sex-determination pathway and notably focusing on the latest relationships between SRY and its direct SOX9 downstream target and their regulation, the three remaining minireviews in this series place particular emphasis on the exciting questions connected to DMRT1′s rise to the top in fish, frog and birds
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Amaury Herpin, Manfred Schartl. Vertebrate sex determination: questioning the hierarchy. FEBS Journal, Wiley, 2011, 278 (7), pp.1001. ⟨10.1111/j.1742-4658.2011.08028.x⟩. ⟨hal-02651989⟩

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