Is sexual development conserved between birds and mammals?

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Mammalian sex development is considered to be a three step process: 1 isthe establishment of chromosomal sex at fertilisation, 2 is the establishment of the primary sexual characteristcics (gonads), and 3 is development of the secondary sexual characteristics. Central to this model is the concept that the mammalian fetus groes through a sexually indifferent period early in development and that the adult sexual phenotype depends on the type of gonad formed. This model is now wideley accepted and is considered to apply to all vertebrate species. However, we have evidence that indicates that the secondary sexual characteristics in birds do not depend on the nature of the gonads formed and suggests an inherent cellular sex identitity. We have also identified a novel-specific transcript that is expressed in all female cells throughout development, most significantly during the supposedly indifferent phase of development. This raises the possibility that an inherent sex identity is conferred on avian cells at fertilisation and gonadal determination and/or the secondary sexual characteristics are largely a result of this cell autonomous sex identity. We will investigate this possibility by generating and analysing male:female chimeras. We will monitor how the presence of cells of the opposite sex affectsthe phenotype of the tissue/organism, and how the cellular envirnment affects the funstion of these cells. We will generate three independant model systems to address these questions: 1 adult birds composed by genetically male and genetically female cells, 2. Embryos containing gonalds composed of genetically male and generally female cells, and 3 transgenic animals where putative sex-determining genes are used to generate genetic females with testes and genetic males with ovaries. If aspects of sexual development in birds are cell autonomous, this will necessitate a re-evaluation of the established concept of sex differentiation in all vertebrates.

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