Developmental consequences and molecular basis of cell-autonomous sex identity in the chicken

哺乳动物的性发育模式被认为适用于所有脊椎动物，但我们已经证明情况并非如此。在哺乳动物中，雄性Y染色体上的SRY基因指导性腺发育的路径，使睾丸而不是卵巢。性腺激素决定了男性和女性的第二性征。在鸟类中，就像在哺乳动物中一样，性染色体决定个体是雄性还是雌性，但第二性征主要不是由血液中循环的性腺产物决定的。我们从对稀有的自然发生的鸟类的分析中知道了这一点，我们已经证明了这种鸟类是由雄性细胞和雌性细胞组成的嵌合体。这些鸟显示出惊人的不对称性，赋予身体一侧的雄性特征（雄性羽毛颜色，大肌肉组织，大垂肉）和另一侧的雌性特征-尽管身体两侧暴露于相同水平的循环激素。我们已经在发育第2天的正常鸡胚胎中进行了移植研究，在性腺形成之前。随着胚胎的发育，来自一种性别的移植细胞可以整合到另一种性别的组织中，包括性腺。如果移植的供体细胞来自与宿主相同的性别，供体细胞将完全整合到性腺的关键区室中，并表达胚胎睾丸或卵巢的特征基因。我们已经证明，来自一种性别的细胞不会整合到另一种性别的性腺的关键功能区室。看来移植的假定性腺细胞可以正常分化，但无论环境如何，都能保持其性别身份。我们认为，鸟类体细胞具有“细胞自主性性别认同”（CASI）。该项目旨在i）确定CASI对不同发育阶段表型的影响，ii）建立这种现象的分子基础。我们将扩展我们目前的移植研究，以确定细胞受CASI影响的程度，确定CASI如何影响不同的细胞类型，并确定从2天胚胎移植的组织是否在动物的整个生命周期中保持其性别身份。鸟类的性染色体被命名为Z和W：雄性有两条Z染色体，雌性有一条Z和一条W染色体。鸟类有一种不寻常的机制来调节Z染色体基因的表达，这导致大约300个Z染色体基因在雄性组织中的表达水平与雌性组织不同（除了W染色体上的大约15个基因）。我们相信，性染色体基因表达的模式建立了一个“基态”，将体内的每个细胞定义为男性或女性。为了验证我们的假设，并确定参与这种基态的基因，我们将建立早期男性和女性胚胎的基因表达模式，并确定分化和性别逆转如何影响这些模式。

The mammalian model of sexual development is thought to apply to all vertebrates but we have shown that this is not the case. In mammals the SRY gene on the Y chromosome in males directs the path of gonad development toward making a testis rather than an ovary. Gonadal hormones then determine the secondary sexual characteristics of males and females. In birds, as in mammals, the sex chromosomes determine whether the individual is male or female but the secondary sex characteristics are not primarily determined by gonadal products circulating in the blood stream. We know this from our analysis of rare, naturally occurring birds known as gynandromorphs, which we have shown to be chimeras comprised of both male cells and female cells. These birds display a striking asymmetry that confers male characteristics on one side of the body (male feather colour, large musculature, large wattle) and female characteristics on the other - despite the fact that both sides of the body are exposed to the same levels of circulating hormones. We have carried out transplantation studies in normal chicken embryos at day 2 of development, before the gonads form. As the embryo develops, transplanted cells from one sex can integrate into tissues in the opposite sex, including gonad. If transplanted donor cells are from the same sex as the host, the donor cells will fully integrate into key compartments in the gonads and express genes characteristic of the embryonic testis or ovary. We have shown that cells from one sex will not integrate into the key functional compartment of the gonads of the opposite sex. It seems that transplanted presumptive gonadal cells can differentiate properly, but retain their sex identity regardless of environment. We maintain that bird somatic cells possess a 'cell-autonomous sex identity' (CASI). This project aims to i) establish the influence of CASI on phenotype at different stages of development and ii) establish the molecular basis underlying this phenomenon. i) We will extend our current transplantation studies to determine the extent to which cells are affected by CASI, to determine how CASI affects different cell types, and determine whether tissue grafted from a 2-day embryo retains its sex identity throughout the lifetime of the animal. ii) The sex chromosomes in birds are designated Z and W: males have two Z chromosomes and females have a Z and a W chromosome. Birds have an unusual mechanism for regulating expression of Z-chromosome genes which results in around 300 Z-chromosome genes that are expressed at different levels in male tissues than in female tissues (in addition to around 15 genes on the W-chromosome). We believe that there is a pattern of sex chromosome gene expression that establishes a 'ground state' that defines every cell in the body as male or female. To test our hypothesis and to identify the genes involved in this ground state, we will establish patterns of gene expression for the early male and female embryos and determine how differentiation and sex reversal affects these patterns.