Avian sex determination: What, when and where?

Murdoch Childrens Research Institute and University of Melbourne, Department of Paediatrics, Royal Childrens Hospital, Parkville, Australia.
Cytogenetic and Genome Research (Impact Factor: 1.56). 02/2007; 117(1-4):165-73. DOI: 10.1159/000103177
Source: PubMed


Sex is determined genetically in all birds, but the underlying mechanism remains unknown. All species have a ZZ/ZW sex chromosome system characterised by female (ZW) heterogamety, but the chromosomes themselves can be heteromorphic (in most birds) or homomorphic (in the flightless ratites). Sex in birds might be determined by the dosage of a Z-linked gene (two in males, one in females) or by a dominant ovary-determining gene carried on the W sex chromosome, or both. Sex chromosome aneuploidy has not been conclusively documented in birds to differentiate between these possibilities. By definition, the sex chromosomes of birds must carry one or more sex-determining genes. In this review of avian sex determination, we ask what, when and where? What is the nature of the avian sex determinant? When should it be expressed in the developing embryo, and where is it expressed? The last two questions arise due to evidence suggesting that sex-determining genes in birds might be operating prior to overt sexual differentiation of the gonads into testes or ovaries, and in tissues other than the urogenital system.

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    • "Embryonic testes exhibit greater medullary development by the appearance of testicular cords containing the male germ cells, supporting Sertoli cells inside and hormone-producing Leydig cells outside the cords. On the other hand, the ovary exhibits greater cortical development, and female germ cells locate in this layer [4]. Early differences between male and female embryos are thought to include a greater number and size of female germ cells at an earlier stage than in males [5], based on the localization of glycogen granules in germ cells by PAS staining [6], [7]. "
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