Temperature-Dependent Male Sex Determination in the Red-Eared Slider Turtle

许多下蛋的爬行动物缺乏性染色体，取而代之的是依靠卵的孵化温度来确定后代的性别，这一过程被称为温度依赖性性别决定(TSD)。温度如何刺激和抑制遗传级联反应来确定性腺的类型和直接的性别分化是这一应用的重点。这位研究人员和他的同事开发了红耳滑龟(Trachemys Scripta Elegans)作为研究TSD的动物模型系统。这些研究人员已经证明，男性和女性的性腺发育是受类固醇激素影响的不同遗传级联反应的结果，特定的酶激活或抑制类固醇激素对性别决定的影响。在红耳滑块系统中，雌性(或卵巢)决定涉及雌激素，而男性(或睾丸)决定涉及非芳香化雄激素。哺乳动物和海龟有着共同的进化史，最近的发现表明，尽管触发因素(环境温度和性染色体)不同，但红耳海龟与哺乳动物系统有许多共同的性发育特征，包括性别决定途径中涉及的相同基因。实验旨在确定参与睾丸形成的基因，然后确定温度和性激素是如何完成这一壮举的。例如，将使用定量逆转录聚合酶链式反应(RT-PCR)和Northern印迹分析来测量基因表达的相对水平。基因表达的定位将通过原位杂交进行分析。在原始脊椎动物物种中，通过孵化温度、外源激素和其他因素来操控性别的能力提供了无与伦比的实验控制，从而使性别决定过程中的正常基因表达模式能够比其他有性染色体的羊栖生物(爬行动物、鸟类和哺乳动物被称为羊膜动物或高等脊椎动物)更详细地分析。这项工作之所以重要，有几个原因。首先，长期以来，人们一直认为母体或胚胎来源的类固醇激素不参与哺乳动物和鸟类的性腺形成。对TSD爬行动物的研究表明，这一结论可能为时过早。其次，由于TSD可能是性染色体进化的先驱，在温血脊椎动物中，可能存在潜在的温度和类固醇对性别决定的影响，但部分或全部被掩盖。第三，尽管大量研究证明激素反应和激素作用明显依赖于温度，但在温血动物中，温度作为类固醇激素作用的遗传水平的一个因素还没有得到充分的研究。

Many egg-laying reptiles lack sex chromosomes, depending instead upon the incubation temperature of the egg to determine the sex of their offspring, a process known as temperature-dependent sex determination (TSD). How temperature both stimulates and inhibits genetic cascades to determine the type of gonad and direct sexual differentiation is the focus of this application. This investigator and his colleagues have developed the red-eared slider turtle (Trachemys scripta elegans) as an animal model system to study TSD. These researchers have demonstrated that male and female gonadal development are the result of separate genetic cascades influenced by steroid hormones, and that specific enzymes activate or inhibit steroid hormone effects on sex determination. In the red-eared slider system, female (or ovary) determination involves estrogens whereas male (or testis) determination involves nonaromatizable androgens. Mammals and turtles share a common evolutionary history and recent findings indicate that despite a difference in the trigger (environmental temperature vs. sex chromosomes), the red-eared slider shares many characteristics of sexual development with the mammalian system, including the same genes involved along the sex determination pathway. Experiments are designed to identify the genes involved in testis formation and then to determine how temperature and sex hormones accomplish this feat. For example, relative levels of gene expression will be measured using quantitative reverse transcriptase polymerase chain reaction (RT-PCR) and Northern blot analysis. Localization of gene expression will be analyzed with in situ hybridization. The ability to manipulate sex in a primitive vertebrate species by incubation temperature, exogenous hormones, and other agents provides unparalleled experimental control, thereby enabling more detailed analysis of the normal pattern of gene expression during sex determination than is possible with other amniotes (reptiles, birds and mammals are known as the amniote or higher vertebrates) having sex chromosomes. This work is important for several reasons. First, it has long been assumed that steroid hormones of maternal or embryonic origin are not involved in gonad formation in mammals and birds. The work with TSD reptiles indicates that this conclusion may be premature. Second, since TSD may represent the evolutionary precursor to sex chromosomes, potential temperature and steroid effects in sex determination may be present, but partly or wholly masked, in warm-blooded vertebrates. Third, temperature has not been adequately investigated as a factor in steroid hormone action at the genetic level in warm-blooded animals despite numerous studies documenting how both hormone responsiveness and hormone action are markedly dependent on temperature.