Department of Biology, Ludwig-Maximilians-University Munich, Planegg-Martinsried, Germany.
Department of Biological Sciences, East Tennessee State University, Johnson City, Tennessee, USA.
J Morphol. 2021 Jul;282(7):1080-1122. doi: 10.1002/jmor.21380. Epub 2021 May 26.
We review morphological features of the amniote egg and embryos in a comparative phylogenetic framework, including all major clades of extant vertebrates. We discuss 40 characters that are relevant for an analysis of the evolutionary history of the vertebrate egg. Special attention is given to the morphology of the cellular yolk sac, the eggshell, and extraembryonic membranes. Many features that are typically assigned to amniotes, such as a large yolk sac, delayed egg deposition, and terrestrial reproduction have evolved independently and convergently in numerous clades of vertebrates. We use phylogenetic character mapping and ancestral character state reconstruction as tools to recognize sequence, order, and patterns of morphological evolution and deduce a hypothesis of the evolutionary history of the amniote egg. Besides amnion and chorioallantois, amniotes ancestrally possess copulatory organs (secondarily reduced in most birds), internal fertilization, and delayed deposition of eggs that contain an embryo in the primitive streak or early somite stage. Except for the amnion, chorioallantois, and amniote type of eggshell, these features evolved convergently in almost all major clades of aquatic vertebrates possibly in response to selective factors such as egg predation, hostile environmental conditions for egg development, or to adjust hatching of young to favorable season. A functionally important feature of the amnion membrane is its myogenic contractility that moves the (early) embryo and prevents adhering of the growing embryo to extraembryonic materials. This function of the amnion membrane and the liquid-filled amnion cavity may have evolved under the requirements of delayed deposition of eggs that contain developing embryos. The chorioallantois is a temporary embryonic exchange organ that supports embryonic development. A possible evolutionary scenario is that the amniote egg presents an exaptation that paved the evolutionary pathway for reproduction on land. As shown by numerous examples from anamniotes, reproduction on land has occurred multiple times among vertebrates-the amniote egg presenting one "solution" that enabled the conquest of land for reproduction.
我们在比较系统发育框架中回顾了羊膜动物卵和胚胎的形态特征，包括现存脊椎动物的所有主要类群。我们讨论了 40 个与脊椎动物卵进化历史分析相关的特征。特别关注细胞卵黄囊、蛋壳和胚胎外膜的形态。许多通常被认为是羊膜动物的特征，如大卵黄囊、延迟产卵和陆地繁殖，在脊椎动物的许多类群中独立且趋同进化。我们使用系统发育特征映射和祖先特征状态重建作为工具来识别形态进化的序列、顺序和模式，并推断出羊膜动物卵的进化历史假说。除了羊膜和绒毛膜胎盘，羊膜动物的祖先具有交配器官（在大多数鸟类中二次退化）、体内受精和延迟产卵，卵中胚胎处于原条或早期体节阶段。除了羊膜、绒毛膜胎盘和羊膜动物类型的蛋壳外，这些特征在几乎所有主要的水生脊椎动物类群中都有趋同进化，可能是为了应对卵捕食、卵发育的恶劣环境条件或调整幼体孵化到有利季节等选择因素。羊膜膜的一个功能重要的特征是其肌原性收缩性，它可以移动（早期）胚胎并防止胚胎附着在胚胎外材料上。羊膜膜的这个功能和充满液体的羊膜腔可能是在含有发育胚胎的卵的延迟沉积的要求下进化而来的。绒毛膜胎盘是一个临时的胚胎交换器官，支持胚胎发育。一个可能的进化场景是，羊膜动物卵呈现出一种适应，为陆地繁殖铺平了进化途径。正如无羊膜动物的许多例子所示，陆地繁殖在脊椎动物中多次发生——羊膜动物卵呈现出一种“解决方案”，使它们能够征服陆地进行繁殖。
