Wageningen University & Research Animal Breeding and Genomics, Wageningen Livestock Research, PO Box 338, 6700 AH Wageningen, The Netherlands.
Department of Zoology, University of Otago, 340 Great King Street, PO Box 56, Dunedin 9054, New Zealand.
Gen Comp Endocrinol. 2019 Aug 1;279:129-138. doi: 10.1016/j.ygcen.2019.02.018. Epub 2019 Feb 20.
An inverse relation exists between the maturation stage at the start of the oceanic reproductive migration and the migration distance to the spawning grounds for the various eel species. The European eel Anguilla anguilla migrates up to 5-6000 km and leaves in a previtellogenic state. The shortfinned eel A. australis migrates 2-4000 km and leaves in an early vitellogenic state. In this study, we compared the early pubertal events in European silver eels with those in silver shortfinned eels to gain insights into the initiation of vitellogenesis. Immediately after being caught, yellow and silver eels of both species were measured and sampled for blood and tissues. Eye index (EI), gonadosomatic index (GSI) and hepatosomatic index (HSI) were calculated. Plasma 11-ketotestosterone (11-KT) and 17β-estradiol (E2) levels were measured by radioimmunoassay. Pituitary, liver and ovaries were dissected for quantitative real-time PCR analyses (pituitary dopamine 2b receptor d2br, gonadotropin-releasing hormone receptors 1 and 2 gnrhr1 and gnrhr2, growth hormone gh and follicle-stimulating hormone-β fshb; liver estrogen receptor 1 esr1; gonad follicle-stimulating hormone receptor fshr, androgen receptors α and β ara and arb, vitellogenin receptor vtgr and P450 aromatase cyp19). Silver eels of both species showed a drop in pituitary gh expression, progressing gonadal development (GSI of ∼1.5 in European eels and ∼3.0 in shortfinned eels) and steroid level increases. In shortfinned eels, but not European eels, expression of fshb, gnrhr1 and gnrhr2, and d2br in the pituitary was up-regulated in the silver-stage as compared to yellow-stage females, as was expression of fshr, ara and arb in the ovaries. Expression of esr1 in European eels remained low while esr1 expression was up-regulated over 100-fold in silver shortfinned eels. The mechanistic model for anguillid vitellogenesis that we present suggests a first step that involves a drop in Gh and a second step that involves Fsh increase when switching in the life history trade-off from growth to reproduction. The drop in Gh is associated with gonadal development and plasma steroid increase but precedes brain-pituitary-gonad axis (BPG) activation. The Fsh increase marks BPG activation and increased sensitivity of the liver to estrogenic stimulation, but also an increase in D2br-mediated dopaminergic signaling to the pituitary.
存在这样一种反向关系，即各种鳗鱼物种在开始进行海洋生殖洄游时所处的成熟阶段与它们向产卵场的洄游距离成反比。欧洲鳗 Anguilla anguilla 最远可洄游 5000-6000 公里，并处于预卵黄生成状态。短鳍鳗 A. australis 最远可洄游 2000-4000 公里，并处于早期卵黄生成状态。在这项研究中，我们比较了欧洲银鳗和银色短鳍鳗的早期青春期事件，以深入了解卵黄生成的启动。在被捕获后，立即测量并采集这两种物种的黄色和银色鳗鱼的血液和组织样本。计算眼指数（EI）、性腺指数（GSI）和肝体指数（HSI）。通过放射免疫分析测量血浆 11-酮睾酮（11-KT）和 17β-雌二醇（E2）水平。解剖垂体、肝脏和卵巢进行定量实时 PCR 分析（垂体多巴胺 2b 受体 d2br、促性腺激素释放激素受体 1 和 2 gnrhr1 和 gnrhr2、生长激素 gh 和促卵泡激素-β fshb；肝脏雌激素受体 1 esr1；性腺卵泡刺激素受体 fshr、雄激素受体 α 和 β ara 和 arb、卵黄蛋白原受体 vtgr 和 P450 芳香化酶 cyp19）。两种鳗鱼的银鳗均表现出垂体 gh 表达下降，性腺发育进展（欧洲鳗的 GSI 约为 1.5，短鳍鳗的 GSI 约为 3.0）和类固醇水平升高。在短鳍鳗中，但在欧洲鳗中，与黄色阶段的雌性相比，垂体中 fshb、gnrhr1 和 gnrhr2 以及 d2br 的表达在银鳗阶段上调，卵巢中 fshr、ara 和 arb 的表达也上调。欧洲鳗的 esr1 表达保持较低水平，而银色短鳍鳗的 esr1 表达上调 100 多倍。我们提出的鳗鲡类卵黄生成的机制模型表明，第一步涉及 Gh 下降，第二步涉及 Fsh 增加，这是从生长到繁殖的生命周期权衡中的一个转折点。Gh 的下降与性腺发育和血浆类固醇增加有关，但先于脑垂体性腺轴（BPG）的激活。Fsh 的增加标志着 BPG 的激活和肝脏对雌激素刺激的敏感性增加，但也标志着 D2br 介导的多巴胺信号向垂体的增加。
