Ontogeny of the appendicularian Oikopleura dioica (Tunicata, Chordata) reveals characters similar to ascidian larvae with sessile adults
Ontogeny of the appendicularian Oikopleura dioica (Tunicata, Chordata) reveals characters similar to ascidian larvae with sessile adults
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阑尾纲 Oikopleura dioica（被囊动物、脊索动物）的个体发育揭示了与无柄成虫的海鞘幼虫相似的特征
DOI：
10.1007/s00435-007-0041-5
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发表时间：
2007
期刊：
影响因子：
1
通讯作者：
T. Stach
中科院分区：
文献类型：
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作者：
T. Stach
尾尾虫在被囊动物和脊索动物的进化中一直占据着中心地位。人们提出了两种假设——一种认为尾尾纲是现存被囊动物的姐妹分类群，另一种认为尾尾纲是从类似海鞘的祖先进化而来的。在本研究中，我报告了对被囊鱼幼虫的电镜研究结果，包括对早期个体发育的尾尾鱼“strekform”尾巴的首次电镜研究，并讨论了它们的系统发育意义。Oikopleura dioica Fol， 1872的早期“strerecform”具有细胞外覆盖物，该覆盖物由内部的电子光层和最外层的电子密层组成。此外，细胞外覆盖层形成鳍叶。由于这些特征与不同海鞘幼虫的被膜相似，因此早期尾鞘胚胎的细胞外覆盖物可能与海鞘幼虫的被膜同源。总的来说，尾尾虫的早期发育阶段的尾巴由一种嵌合的半胚和半胚特征组成。直的、连续的内胚层链是从一个共同的脊索动物祖先遗传下来的，而由细胞外物质组成的幼虫小鳍则是从一个共同的被囊动物祖先遗传下来的。尾尾的水平方向整体上继承自尾尾纲和尾尾纲海鞘的最后共同祖先，尾尾后尖端的浮动延伸是全浮游Oikopleura dioica所特有的。这些发现支持了一种假说，即阑尾虫起源于一种无柄的、类似海鞘的祖先。
Appendicularians have always occupied a central role in considerations of tunicate and chordate evolution. Two hypotheses have been proposed – one holds that appendicularia represents the sister taxon to the remaining tunicates, the other suggests that appendicularians were derived from an ascidian-like ancestor. In the present study I report results from electron microscopic investigation of larval tunicates including the first electron microscopic investigation of the tail of the early ontogenetic appendicularian “Streckform” and discuss their phylogenetic implications. The early “Streckform” of Oikopleura dioica Fol, 1872 is invested with an extracellular covering that consists of an inner electron-light layer and an electron-dense outermost layer. In addition, the extracellular covering forms fin blades. Because these traits are shown to be similar to the tunic of different ascidian larvae, the extracellular covering in early appendicularian embryos is suggested to be homologous to the larval tunic of ascidian larvae. Overall, the tail of early developmental stages of appendicularians consists of a mosaic of apomorphic and plesiomorphic features. The straight, continuous endodermal strand was inherited from a common chordate ancestor whereas the finlets of larvae, consisting of extracellular material, were inherited from a common tunicate ancestor. The horizontal orientation of the tail as a whole was inherited from the last common ancestor of appendicularians and aplousobranch ascidians, and the discovered floating extension at the posterior tip of the tail is unique to the holoplanktonic Oikopleura dioica. These findings support the hypothesis that Appendicularia is derived from a sessile, ascidian-like ancestor.