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

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 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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Available from: Quanah Hudson, Sep 26, 2015
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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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    ABSTRACT: a functional ovary develops on the left, whereas the right gonad regresses. In males, however, testes develop on both sides. We examined the distribution of germ cells using Vasa/Cvh as a marker. Expression is asymmetric in both sexes: at stage 35 the left gonad contains significantly more germ cells than the right. A similar expression pattern is seen for expression of ERNI (Ens1), a gene expressed in chick embryonic stem cells while they self-renew, but downregulated upon differentiation. Other pluripotency-associated markers (PouV/Oct3/4, Nanog and Sox2) also show asymmetric expression (more expressing cells on the left) in both sexes, but this asymmetry is at least partly due to expression in stromal cells of the developing gonad, and the pattern is different for all the genes. Therefore germ cell and pluripotency-associated genes show both sex-dependent and independent left-right asymmetry and a complex pattern of expression.
    PLoS ONE 07/2013; 8(7):e69893. DOI:10.1371/journal.pone.0069893 · 3.23 Impact Factor
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    • "Mammals use an XX/XY sexdetermination system, in which females have two identical sex chromosomes (XX) while males have two distinct sex chromosomes (XY). The ZW sex-determination system, found in birds (Smith et al., 2007) and some insects (Arunkumar et al., 2009), instructs female development from two different sex chromosomes (ZW), while males possess two of the same kind of chromosomes (ZZ). Some reptiles do not have sex chromosomes and their sex is determined by temperature (Modi and Crews, 2005). "
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    ABSTRACT: The zebrafish is a popular model for genetic analysis and its sex differentiation has been the focus of attention for breeding purposes. Despite numerous efforts, very little is known about the mechanism of zebrafish sex determination. The lack of discernible sex chromosomes and the difficulty of distinguishing the sex of juvenile fish are two major obstacles that hamper the progress in such studies. To alleviate these problems, we have developed a scheme involving methyltestosterone treatment followed by natural mating to generate fish with predictable sex trait. Female F1 fish that gave rise to all-female offspring were generated. This predictable sex trait enables characterization of gonadal development in juvenile fish by histological examination and gene expression analysis. We found the first sign of zebrafish sex differentiation to be ovarian gonocyte proliferation and differentiation at 10 to 12 days post-fertilization (dpf). Somatic genes were expressed indifferently at 10 to 17 dpf, and then became sexually dimorphic at three weeks. This result indicates clear distinction of male and female gonads derived independently from primordial gonads. We classified the earliest stages of zebrafish sex determination into the initial preparation followed by female germ cell growth, oocyte differentiation, and somatic differentiation. Our genetic selection scheme matches the prediction that female-dominant genetic factors are required to determine zebrafish sex.
    Developmental Biology 08/2010; 344(2):849-56. DOI:10.1016/j.ydbio.2010.05.515 · 3.55 Impact Factor
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    • "There is not yet a model reptile GSD system to serve as a focus for coordinated attention in the way that the human, mouse and chicken have served this role. Research on genetic variants within single species that has led to so many remarkable insights in humans and mice has not occurred in reptiles, and the level of understanding of the reptilian system is not yet at a stage to allow experimental in vivo manipulation of the regulation of key genes, as has been achieved for mammals and birds [Smith et al., 2007; Wilhelm et al., 2007]. "
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    ABSTRACT: Vertebrates show an astonishing array of sex determining mechanisms, including male and female heterogamety, multiple sex chromosome systems, environmental sex determination, parthenogenesis and hermaphroditism. Sex determination in mammals and birds is extraordinarily conservative compared to that of reptiles, amphibians and fish. In this paper, we explore possible explanations for the diversity of sex determining modes in reptiles, and in particular, address the prevalence of reptilian temperature-dependent sex determination (TSD) and its almost haphazard distribution across the reptile phylogeny. We suggest that reptiles are predisposed to evolving TSD from genotypic sex determination (GSD) by virtue of the uniquely variable thermal environment experienced by their embryos during the critical period in which sex is determined. Explicit mechanisms for canalization of sexual phenotype in the face of high thermal variation during development provide a context for thermolability in sex determination at extremes and the raw material for natural selection to move this thermolability into the developmental mainstream when there is a selective advantage to do so. Release of cryptic variation when canalization is challenged and fails at extremes may accelerate evolutionary transitions between GSD and TSD.
    Sexual Development 03/2010; 4(1-2):7-15. DOI:10.1159/000279441 · 2.29 Impact Factor
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