Wanninger Andreas
University of Copenhagen, Department of Biology, Research Group for Comparative Zoology, Universitetsparken 15, DK-2100 Copenhagen, Denmark.
Biol Bull. 2009 Jun;216(3):293-306. doi: 10.1086/BBLv216n3p293.
Despite the large variation in adult bodyplan phenotypes, a worm-shaped morphology is considered plesiomorphic for both Lophotrochozoa and Bilateria. Although almost all larval and adult lophotrochozoan worms have serially arranged ring muscles in their body wall, a comparison of their ontogeny reveals no less than six different developmental pathways that lead to this homogenous arrangement of ring muscles. However, in all taxa, with the exception of chaetodermomorph molluscs and the segmented annelids, ring muscle development starts with synchronous formation of certain pioneer myocytes, which is thus considered basal for Lophotrochozoa. Recent studies on spiralian neurogenesis revealed remnants of ancestral segmentation in echiurans and sipunculans, thus confirming molecular phylogenetic studies that propose a close relationship of these three taxa. Larval entoprocts exhibit a mosaic of larval and adult molluscan characters and, among other apomorphies, share with polyplacophoran Mollusca a complex larval apical organ and a tetraneurous nervous system, strongly suggesting a monophyletic assemblage of Entoprocta and Mollusca. The term Tetraneuralia is proposed herein for this lophotrochozoan clade. Overall, formation of the lophotrochozoan neuromuscular bodyplan appears as a highly dynamic process on both the ontogenetic and the evolutionary timescales, highlighting the importance of insights into these processes for reconstructing ancestral bodyplan features and phylogenetic relationships.
尽管成体体型表型存在很大差异，但蠕虫状形态被认为是冠轮动物门和两侧对称动物的原始形态。虽然几乎所有的冠轮动物门幼虫和成体蠕虫在其体壁中都有呈系列排列的环形肌肉，但对它们个体发育的比较显示，导致这种环形肌肉均匀排列的发育途径不少于六种。然而，在所有类群中，除了毛颚形态的软体动物和分节的环节动物外，环形肌肉的发育始于某些先驱肌细胞的同步形成，因此这被认为是冠轮动物门的基础特征。最近对螺旋动物神经发生的研究揭示了螠虫动物门和星虫动物门中祖先分节的遗迹，从而证实了分子系统发育研究提出的这三个类群密切关系的观点。幼虫内肛动物表现出幼虫和成年软体动物特征的镶嵌，并且在其他衍生特征中，与多板纲软体动物共享一个复杂的幼虫顶端器官和一个四神经的神经系统，这强烈表明内肛动物门和软体动物门是一个单系类群。本文中为这个冠轮动物门分支提出了“四神经动物”这一术语。总体而言，冠轮动物门神经肌肉体型的形成在个体发育和进化时间尺度上都显得是一个高度动态的过程，突出了深入了解这些过程对于重建祖先体型特征和系统发育关系的重要性。
