Merchant-Larios H, Ruiz-Ramirez S, Moreno-Mendoza N, Marmolejo-Valencia A
Department of Cell Biology, Instituto de Investigaciones Biomédicas, México, D.F., 04510, México.
Gen Comp Endocrinol. 1997 Sep;107(3):373-85. doi: 10.1006/gcen.1997.6946.
Reptile embryos with temperature sex determination have a thermosensitive period (TSP). The finding that exogenous estradiol (E2) overcomes the effect of male-promoting temperature led to the idea that temperature may regulate estrogen concentration in the gonad during TSP. Since interspecific variations in TSP and in the effect of exogenous E2 exist, we undertook a study in the olive ridley Lepidochelys olivacea. Four parameters were correlated: the TSP (time dimension), the thermosensitive stages (rate of development), gonad development (histological aspect), and the estradiol response. Two kinds of experiments were performed: (1) Eggs were shifted once, at different stages of development, from a male-promoting temperature to a female-temperature (or vice versa) for the remainder of development. (2) Eggs at male-promoting temperature were treated once with 6 or 12 microg of estradiol (E2) at various times of incubation. Sex ratio was established around hatching in each experimental series. We found that the temporal dimension of the TSP was around 7 days (Days 20-27 of incubation) at a male-promoting or a female-promoting temperature. The rate of development of the whole embryo and gonadal growth was faster at female-promoting temperature than at male-promoting temperature. Formation of the genital ridge began at stage 21-22 and histological differentiation of the gonads occurred around stage 26-27. Although these stages coincided with the TSP, at male-promoting temperature the thermosensitive stages occurred earlier (from stages 20-21 to stages 23-24) than at female-promoting temperature (from stages 23-24 to stages 26-27). Thus, at male promoting-temperature, sex was determined in embryos with incipient or undifferentiated gonads. In contrast, E2 treatment continued to feminize the gonads of embryos at a male-promoting temperature beyond the TSP up to stage 25-26, but the E2-induced ovaries were significantly smaller than temperature-induced ovaries. It is suggested that the doses of E2 used were higher than the concentration of endogenous E2 required for normal sex determination. The lack of correlation between sex determination and gonad differentiation suggests that irreversible molecular processes underlying sex determination occur earlier at male- than at female-promoting temperature. Results suggest that the male sex may be the default state and that the female condition must be imposed upon it.
具有温度依赖型性别决定的爬行动物胚胎有一个温度敏感期（TSP）。外源性雌二醇（E2）能克服促进雄性温度的影响，这一发现引发了这样一种观点，即温度可能在温度敏感期调节性腺中的雌激素浓度。由于温度敏感期和外源性E2的作用存在种间差异，我们对榄蠵龟（Lepidochelys olivacea）进行了一项研究。四个参数相互关联：温度敏感期（时间维度）、温度敏感阶段（发育速率）、性腺发育（组织学方面）和雌二醇反应。进行了两种实验：（1）在发育的不同阶段，将卵从促进雄性的温度转移到促进雌性的温度（或反之亦然）一次，直至发育结束。（2）在孵化的不同时间，对处于促进雄性温度的卵用6或12微克雌二醇（E2）处理一次。在每个实验系列中，在孵化时确定性别比例。我们发现，在促进雄性或促进雌性的温度下，温度敏感期的时间维度约为7天（孵化的第20 - 27天）。促进雌性温度下整个胚胎的发育速率和性腺生长比促进雄性温度下更快。生殖嵴在第21 - 22阶段开始形成，性腺的组织学分化在第26 - 27阶段左右发生。尽管这些阶段与温度敏感期重合，但在促进雄性温度下，温度敏感阶段比促进雌性温度下更早出现（从第20 - 21阶段到第23 - 24阶段）（促进雌性温度下从第23 - 24阶段到第26 - 27阶段）。因此，在促进雄性的温度下，性别在性腺刚开始发育或未分化的胚胎中就已确定。相比之下，在促进雄性的温度下，E2处理在温度敏感期之后一直到第25 - 26阶段都能使胚胎的性腺雌性化，但E2诱导的卵巢明显小于温度诱导的卵巢。这表明所使用的E2剂量高于正常性别决定所需的内源性E2浓度。性别决定与性腺分化之间缺乏相关性表明，性别决定背后的不可逆分子过程在促进雄性的温度下比在促进雌性的温度下更早发生。结果表明，雄性性别可能是默认状态，而雌性状态必须施加于其上。
