Division of Cell Biology, University of Veterinary Medicine Hannover, Bischhofsholer Damm 15, 30173, Hannover, Germany.
BMC Evol Biol. 2013 Jun 10;13:119. doi: 10.1186/1471-2148-13-119.
Remipedia were initially seen as a primitive taxon within Pancrustacea based on characters considered ancestral, such as the homonomously segmented trunk. Meanwhile, several morphological and molecular studies proposed a more derived position of Remipedia within Pancrustacea, including a sister group relationship to Hexapoda. Because of these conflicting hypotheses, fresh data are crucial to contribute new insights into euarthropod phylogeny. The architecture of individually identifiable serotonin-immunoreactive neurons has successfully been used for phylogenetic considerations in Euarthropoda. Here, we identified neurons in three species of Remipedia with an antiserum against serotonin and compared our findings to reconstructed ground patterns in other euarthropod taxa. Additionally, we traced neurite connectivity and neuropil outlines using antisera against acetylated α-tubulin and synapsin.
The ventral nerve cord of Remipedia displays a typical rope-ladder-like arrangement of separate metameric ganglia linked by paired longitudinally projecting connectives. The peripheral projections comprise an intersegmental nerve, consisting of two branches that fuse shortly after exiting the connectives, and the segmental anterior and posterior nerve. The distribution and morphology of serotonin-immunoreactive interneurons in the trunk segments is highly conserved within the remipede species we analyzed, which allows for the reconstruction of a ground pattern: two posterior and one anterior pair of serotonin-immunoreactive neurons that possess a single contralateral projection. Additionally, three pairs of immunoreactive neurons are found in the medial part of each hemiganglion. In one species (Cryptocorynetes haptodiscus), the anterior pair of immunoreactive neurons is missing.
The anatomy of the remipede ventral nerve cord with its separate metameric ganglia mirrors the external morphology of the animal's trunk. The rope-ladder-like structure and principal architecture of the segmental ganglia in Remipedia corresponds closely to that of other Euarthropoda. A comparison of the serotonin-immunoreactive cell arrangement of Remipedia to reconstructed ground patterns of major euarthropod taxa supports a homology of the anterior and posterior neurons in Pancrustacea. These neurons in Remipedia possess unbranched projections across the midline, pointing towards similarities to the hexapod pattern. Our findings are in line with a growing number of phylogenetic investigations proposing Remipedia to be a rather derived crustacean lineage that perhaps has close affinities to Hexapoda.
Remipedia 最初被认为是节肢动物门中的一个原始分类群，其特征为同节的体干，这被认为是祖先特征。同时，一些形态学和分子研究提出 Remipedia 在节肢动物门中的位置更为衍生，包括与六足动物的姐妹群关系。由于这些相互矛盾的假设，新的数据对于深入了解真节肢动物的系统发育至关重要。单独可识别的血清素免疫反应神经元的结构已成功用于真节肢动物的系统发育研究。在这里，我们使用针对血清素的抗血清鉴定了 Remipedia 三个物种中的神经元，并将我们的发现与其他真节肢动物分类群中重建的基础模式进行了比较。此外，我们使用针对乙酰化α-微管蛋白和突触素的抗血清追踪了神经突连接和神经丛轮廓。
Remipedia 的腹神经索显示出一种典型的绳梯状排列，由通过成对的纵向突出连接体连接的独立体节神经节组成。外周投射包括一个节间神经，由两个分支组成，它们在离开连接体后不久融合，以及节段的前神经和后神经。我们分析的 Remipede 物种中，血清素免疫反应性中间神经元在躯干节段中的分布和形态高度保守，这允许重建一个基础模式：两对后对和一对前对具有单侧投射的血清素免疫反应性神经元。此外，在每个半神经节的内侧部分还发现了三对免疫反应性神经元。在一个物种（Cryptocorynetes haptodiscus）中，前对免疫反应性神经元缺失。
Remipede 腹神经索的解剖结构与其独立的体节神经节相匹配，反映了动物躯干的外部形态。Remipedia 节段神经节的绳梯状结构和主要架构与其他真节肢动物非常相似。将 Remipedia 的血清素免疫反应细胞排列与主要真节肢动物分类群的重建基础模式进行比较，支持在泛节肢动物中前对和后对神经元的同源性。这些 Remipede 神经元具有穿过中线的无分支投射，指向与六足动物模式的相似性。我们的发现与越来越多的系统发育研究一致，这些研究提出 Remipedia 是一个相对衍生的甲壳动物谱系，可能与六足动物有密切的亲缘关系。
