Jennings D H, Hanken J
Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder, Colorado, 80309-0334, USA.
Gen Comp Endocrinol. 1998 Aug;111(2):225-32. doi: 10.1006/gcen.1998.7111.
Direct development is a widespread, alternative life history in Recent amphibians. There is no free-living, aquatic larva; adult features form in the embryo and are present at hatching. The mechanistic bases of direct development remain relatively unexplored. The current study describes the embryonic ontogeny of the thyroid gland in the direct-developing frog Eleutherodactylus coqui (Leptodactylidae) and quantifies histological changes that occur in the gland after its initial appearance. The thyroid gland of E. coqui is first apparent at Townsend-Stewart stage 10, approximately two-thirds of the way through embryogenesis. Soon after this the thyroid begins to accumulate follicular colloid. Quantitative analyses of thyroid histology reveal embryonic peaks in two measures, follicle number and follicle volume, which are followed by declines in these measures prior to hatching. These peaks in thyroid activity in E. coqui are correlated with morphological changes that are directly comparable to metamorphic changes in frogs that retain the ancestral, biphasic life history. In metamorphic taxa, a histologically identifiable thyroid gland does not form until the larval period, well after hatching. Nevertheless, measures of thyroid histology observed in E. coqui follow the pattern reported for metamorphosing amphibians. The present results support the hypothesis that the evolution of direct development in anurans is associated with precocious development and activity of the thyroid axis.
直接发育是现代两栖动物中一种广泛存在的替代生活史。不存在自由生活的水生幼体；成体特征在胚胎中形成并在孵化时就已具备。直接发育的机制基础仍相对未被探索。当前研究描述了直接发育的青蛙库奎雨蛙（细趾蟾科）甲状腺的胚胎发育过程，并对甲状腺初次出现后腺体中发生的组织学变化进行了量化。库奎雨蛙的甲状腺在汤森 - 斯图尔特第10阶段首次显现，此时胚胎发育已进行了大约三分之二。在此之后不久，甲状腺开始积累滤泡胶体。甲状腺组织学的定量分析揭示了两项指标（滤泡数量和滤泡体积）在胚胎期出现峰值，随后在孵化前这两项指标下降。库奎雨蛙甲状腺活动的这些峰值与形态变化相关，这些形态变化与保留祖先双相生活史的青蛙的变态变化直接可比。在变态类群中，直到幼体期、孵化后很久才会形成组织学上可识别的甲状腺。然而，在库奎雨蛙中观察到的甲状腺组织学指标遵循了变态两栖动物所报道的模式。目前的结果支持了无尾目动物直接发育的进化与甲状腺轴的早熟发育和活动相关的假说。
