Sombke Andy, Stemme Torben
University of Greifswald, Zoological Institute and Museum, Cytology and Evolutionary Biology, Soldmannstrasse 23, 17487 Greifswald, Germany.
Division of Cell Biology, University of Veterinary Medicine Hannover, Bischofsholer Damm 15/102, 30173 Hannover, Germany.
Zoological Lett. 2017 Jul 4;3:9. doi: 10.1186/s40851-017-0070-y. eCollection 2017.
Given the numerous hypotheses concerning arthropod phylogeny, independent data are needed to supplement knowledge based on traditional external morphology and modern molecular sequence information. One promising approach involves comparisons of the structure and development of the nervous system. Along these lines, the morphology of serotonin-immunoreactive neurons in the ventral nerve cord has been investigated in numerous tetraconate taxa (Crustacea and Hexapoda). It has been shown that these neurons can be identified individually due to their comparably low number, characteristic soma position, and neurite morphology, thus making it possible to establish homologies at the single cell level. Within Chilopoda (centipedes), detailed analyses of major branching patterns of serotonin-immunoreactive neurons are missing, but are crucial for developing meaningful conclusions on the homology of single cells.
In the present study, we re-investigated the distribution and projection patterns of serotonin-immunoreactive neurons in the ventral nerve cord of three centipede species: , , and . The centipede serotonergic system in the ventral nerve cord contains defined groups of individually identifiable neurons. An anterior and two posterior immunoreactive neurons per hemiganglion with contralateral projections, a pair of ipsilateral projecting lateral neurons (an autapomorphic character for Chilopoda), as well as a postero-lateral group of an unclear number of cells are present in the ground pattern of Chilopoda.
Comparisons to the patterns of serotonin-immunoreactive neurons of tetraconate taxa support the homology of anterior and posterior neurons. Our results thus support a sister group relationship of Myriapoda and Tetraconata and, further, a mandibulate ground pattern of individually identifiable serotonin-immunoreactive neurons in the ventral nerve cord. Medial neurons are not considered to be part of the tetraconate ground pattern, but could favor the 'Miracrustacea hypothesis', uniting Remipedia, Cephalocarida, and Hexapoda.
鉴于关于节肢动物系统发育有众多假说，需要独立的数据来补充基于传统外部形态学和现代分子序列信息的知识。一种有前景的方法涉及对神经系统结构和发育的比较。沿着这些思路，已经在众多四锥类群（甲壳纲和六足纲）中研究了腹神经索中5-羟色胺免疫反应性神经元的形态。已经表明，由于这些神经元数量相对较少、特征性的胞体位置和神经突形态，可以对其进行个体识别，从而有可能在单细胞水平上建立同源性。在唇足纲（蜈蚣）中，缺少对5-羟色胺免疫反应性神经元主要分支模式的详细分析，但这对于得出关于单细胞同源性的有意义结论至关重要。
在本研究中，我们重新研究了三种蜈蚣物种（具体物种未给出）腹神经索中5-羟色胺免疫反应性神经元的分布和投射模式。腹神经索中的蜈蚣5-羟色胺能系统包含明确的可个体识别的神经元群。每个半神经节有一个前侧和两个后侧免疫反应性神经元，具有对侧投射，一对同侧投射的外侧神经元（唇足纲的一个自近裔性状），以及后侧一组数量不明的细胞，存在于唇足纲的基本模式中。
与四锥类群5-羟色胺免疫反应性神经元模式的比较支持前侧和后侧神经元的同源性。因此，我们的结果支持多足纲和四锥类群的姐妹群关系，进而支持腹神经索中可个体识别的5-羟色胺免疫反应性神经元的有颚类基本模式。内侧神经元不被认为是四锥类基本模式的一部分，但可能支持“微甲壳类假说”，即整合了桨足纲、头虾纲和六足纲。
