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Molecular and Chromosomal Aspects of Sex Determination

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Abstract

The sexual fate of an organism is determined by one of many means. Sex chromosomes provide the genetic material for sex determination in some organisms or the combination of several genes scattered throughout the genome might influence this fate choice. Additionally environmental or social cues determine sex in some species. Once the sexual fate is determined, sex differentiation leads to the development of sex-specific morphologies typically commencing with the development of the sexually dimorphic structures of the ovary and testis from an undifferentiated gonad primordium. The genes governing gonadal sex differentiation in fish are beginning to be elucidated.

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Although the sex of most animals is determined by genetic information, sex-determining genes had been identified only in mammals, several flies, and the worm Caenorhabditis elegans until the recent discovery of DMY (DM-domain gene on the Y chromosome) in the sex-determining region on the Y chromosome of the teleost fish medaka, Oryzias latipes. Functional and expression analyses of DMY have shown it to be the master gene for male sex determination in the medaka. The only sex-determining genes found so far in vertebrates are Sry and DMY. Therefore, the medaka is expected to become a good experimental animal for investigating the precise mechanisms involved in primary sex determination in nonmammalian vertebrates. This article reviews the origin of DMY and the sexual development of gonads in the medaka. The putative functions of DMY are also discussed.
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In contrast to the rather stable regulatory regimes established over more that 100 million years in birds and mammals, sex determination in fish might frequently undergo evolutionary changes bringing the sex-determining cascade under new master sex regulators. This phenomenon, possibly associated with the emergence of new sex chromosomes from autosomes, would explain the frequent switching between sex determination systems observed in fish. In the medaka Oryzias latipes, the Y-specific master sex-determining gene dmrt1bY has been formed through duplication of the autosomal gene dmrt1 onto another autosome, thus generating a new Y chromosome. Dmrt1bY emerged about 10 million years ago and is restricted to several Oryzias species, indicating that the Y chromosome of the medaka is evolutionarily much younger than mammalian and bird sex chromosomes. Fertile males without dmrt1bY have been detected in some medaka populations, and this gene might even have been inactivated in one Oryzias species, indicating the existence of sexual regulators already able to supplant dmrt1bY. Studies on other models have confirmed that fish sex chromosomes are generally young and occurred independently in different fish lineages. The identification of new sex-determining genes in these species will shed new light on the exceptional evolutionary instability governing sex determination in fish.
Reproduction: The Diversity of Fish Reproduction: An Introduction. Social and Reproductive Behaviors: Socially Controlled Sex Change in Fishes. The Reproductive Organs and Processes: Anatomy and Histology of Fish Testis; Regulation of Spermatogenesis. Further Reading Baroiller JF
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Endocrine Regulation of Fish Reproduction. Reproduction: The Diversity of Fish Reproduction: An Introduction. Social and Reproductive Behaviors: Socially Controlled Sex Change in Fishes. The Reproductive Organs and Processes: Anatomy and Histology of Fish Testis; Regulation of Spermatogenesis. Further Reading Baroiller JF, D'Cotta H, Bezault E, Wessels S, and Hoerstgen-Schwark G (2009) Tilapia sex determination: Where temperature and genetics meet. Comparative Biochemistry and Physiology. Part A. Molecular and Integrative Physiology 153: 30-38.
A duplicated copy of DMRT1 in the sex-determining region of the Y chromosome of the medaka
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  • A Shinomiya
Matsuda, M., Nagahama, Y., Shinomiya, A., et al., 2002. DMY is a Y-specific DM-domain gene required for male development in the medaka fish. Nature 417, 559-563. Nanda, I., Kondo, M., Hornung, U., et al., 2002. A duplicated copy of DMRT1 in the sex-determining region of the Y chromosome of the medaka, Oryzias latipes. Proceedings of the National Academy of Sciences of the United States of America 99, 11778-11783.