Zattara Eduardo E, Bely Alexandra E
Department of Biology, University of Maryland, College Park, MD 20740 USA ; Current address: Department of Biology, Indiana University, 915 E. Third Street, Myers Hall 150, Bloomington, IN 47405-7107 USA.
Department of Biology, University of Maryland, College Park, MD 20740 USA.
Front Zool. 2015 Apr 18;12:8. doi: 10.1186/s12983-015-0100-6. eCollection 2015.
An important goal for understanding how animals have evolved is to reconstruct the ancestral features and evolution of the nervous system. Many inferences about nervous system evolution are weak because of sparse taxonomic sampling and deep phylogenetic distances among species compared. Increasing sampling within clades can strengthen inferences by revealing which features are conserved and which are variable within them. Among the Annelida, the segmented worms, the Clitellata are typically considered as having a largely conserved neural architecture, though this view is based on limited sampling.
To gain better understanding of nervous system evolution within Clitellata, we used immunohistochemistry and confocal laser scanning microscopy to describe the nervous system architecture of 12 species of the basally branching family Naididae. Although we found considerable similarity in the nervous system architecture of naidids and that of other clitellate groups, our study identified a number of features that are variable within this family, including some that are variable even among relatively closely related species. Variable features include the position of the brain, the number of ciliary sense organs, the presence of septate ventral nerve cord ganglia, the distribution of serotonergic cells in the brain and ventral ganglia, and the number of peripheral segmental nerves.
Our analysis of patterns of serotonin immunoreactive perikarya in the central nervous system indicates that segmental units are not structurally homogeneous, and preliminary homology assessments suggest that whole sets of serotonin immunoreactive cells have been gained and lost across the Clitellata. We also found that the relative position of neuroectodermal and mesodermal segmental components is surprisingly evolutionarily labile; in turn, this revealed that scoring segmental nerves by their position relative to segmental ganglia rather than to segmental septa clarifies their homologies across Annelida. We conclude that fine taxonomic sampling in comparative studies aimed at elucidating the evolution of morphological diversity is fundamental for proper assessment of trait variability.
理解动物如何进化的一个重要目标是重建神经系统的祖先特征和进化过程。由于分类采样稀疏以及所比较物种之间的系统发育距离较远，许多关于神经系统进化的推断都很薄弱。增加类群内的采样可以通过揭示哪些特征是保守的以及哪些是可变的来加强推断。在环节动物门（Annelida）的分节蠕虫中，寡毛纲（Clitellata）通常被认为具有基本保守的神经结构，尽管这一观点是基于有限的采样得出的。
为了更好地理解寡毛纲内神经系统的进化，我们使用免疫组织化学和共聚焦激光扫描显微镜来描述基部分支的仙女虫科（Naididae）12个物种的神经系统结构。尽管我们发现仙女虫科的神经系统结构与其他寡毛类群有相当大的相似性，但我们的研究确定了该科内一些可变的特征，包括一些即使在亲缘关系相对较近的物种之间也存在差异的特征。可变特征包括脑的位置、纤毛感觉器官的数量、有隔膜的腹神经索神经节的存在、5-羟色胺能细胞在脑和腹神经节中的分布以及外周节段神经的数量。
我们对中枢神经系统中5-羟色胺免疫反应性核周体模式的分析表明，节段单元在结构上并非同质，初步的同源性评估表明，整个5-羟色胺免疫反应性细胞群在寡毛纲中有所增减。我们还发现，神经外胚层和中胚层节段成分的相对位置在进化上出人意料地不稳定；反过来，这表明根据节段神经相对于节段神经节而非节段隔膜的位置来划分节段神经，能够更清晰地阐明它们在整个环节动物门中的同源性。我们得出结论，在旨在阐明形态多样性进化的比较研究中，精细的分类采样对于正确评估性状变异性至关重要。
