Smith et al 2007 - Fax +41 61 306 12 34 E-Mail...

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Unformatted text preview: Fax +41 61 306 12 34 E-Mail karger@karger.ch www.karger.com Gene Expression Cytogenet Genome Res 117:165173 (2007) DOI: 10.1159/000103177 Avian sex determination: what, when and where? C.A. Smith a K.N. Roeszler a Q.J. Hudson b A.H. Sinclair a a Murdoch Childrens Research Institute and University of Melbourne, Department of Paediatrics, Royal Childrens Hospital, Parkville (Australia); b The Jackson Laboratory, Bar Harbor , ME (USA) though SRY is absent in birds, it is generally thought that the principle is the same. That is, a sex determinant is first ex- pressed in the embryonic gonads, triggering gonadal sex differentiation, followed by hormone production to induce secondary sexual differentiation (ducts, external genitalia and brain sex). In this review, we consider the evidence for this idea in birds. Firstly, what is the nature of the avian sex determinant? Does is depend on the presence/absence of a single gene, as in mammals? Secondly, when is it expressed? The two most promising sex-determining candidates in birds, the Z-linked DMRT1 and W-linked HINTW genes, are expressed in the gonads well prior to sexual differentia- tion. Lastly, where is the sex determinant(s) expressed? There is evidence that candidate sex-determining genes may be expressed independently in other tissues in addition to the gonads, as occurs in marsupials (O et al., 1988) and probably also in eutherian mammals (Dewing et al., 2003). Chicken sex chromosomes and sex determination The sex chromosomes in most birds are heteromorphic, characterised by a large Z chromosome and smaller hetero- chromatic W chromosome (Takagi and Sasaki, 1974; Solari, Abstract. Sex is determined genetically in all birds, but the underlying mechanism remains unknown. All species have a ZZ/ZW sex chromosome system characterised by fe- male (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 chromo- Request reprints from Craig A. Smith Murdoch Childrens Research Institute and University of Melbourne Department of Paediatrics, Royal Childrens Hospital Parkville, Vic. 3052 (Australia) telephone: +61 3 8341 6353; fax: + 61 3 8341 6429 e-mail: craig.smith@mcri.edu.au 2007 S. Karger AG, Basel 14248581/07/11740165$23.50/0 Accessible online at: www.karger.com/cgr somes 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 develop- ing embryo, and where is it expressed? The last two ques- tions arise due to evidence suggesting that sex-determining...
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Smith et al 2007 - Fax +41 61 306 12 34 E-Mail...

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