Table_1_Comparative development of the serotonin- and FMRFamide-immunoreactive components of the nervous system in two distantly related ribbon worm species (Nemertea, Spiralia).DOCX

IntroductionNeurodevelopment in larval stages of non-model organisms, with a focus on the serotonin- and FMRFamide-immunoreactive components, has been in the focus of research in the recent past. However, some taxonomic groups remain understudied. Nemertea (ribbon worms) represent such an understudied clade with only few reports on nervous system development mostly from phylogenetically or developmentally derived species. It would be insightful to explore neurodevelopment in additional species to be able to document the diversity and deduce common patterns to trace the evolution of nervous system development.MethodsFluorescent immunohistochemical labeling with polyclonal primary antibodies against serotonin and FMRF-amide and a monoclonal antibody against synapsin performed on series of fixed larval stages of two nemertean species Cephalothrix rufifrons (Archinemertea, Palaeonemertea) and Emplectonema gracile (Monostilifera, Hoplonemertea) were analyzed with confocal laser scanning microscopy.ResultsThis contribution gives detailed accounts on the development of the serotonin- and FMRFamide-immunoreactive subsets of the nervous system in two nemertean species from the first appearance of the respective signals. Additionally, data on synapsin-like immunoreactivity illustrates the general structure of neuropil components. Events common to both investigated species are the appearance of serotonin-like immunoreactive signals before the appearance of FMRF-like immunoreactive signals and the strict progression of the development of the lateral nerve cords from the anteriorly located, ring-shaped brain toward the posterior pole of the larva. Notable differences are (1) the proboscis nervous system that is developing much earlier in investigated larval stages of E. gracile and (2) distinct early, but apparently transient, serotonergic neurons on the frontal and caudal pole of the larva in E. gracile that seem to be absent in C. rufifrons.DiscussionAccording to the results from this investigation and in line with previously published accounts on nervous system development, the hypothetical last common ancestor of Nemertea had a ring-shaped brain arranged around the proboscis opening, from which a pair of ventro-lateral nerve cords develops in anterior to posterior progression. Early frontal and caudal serotonergic neurons that later degenerate or cease to express serotonin are an ancestral character of Nemertea that they share with several other spiralian clades.

近年来，对非模式生物幼虫期神经发育的研究焦点集中于血清素和FMRF酰胺免疫反应成分。然而，一些分类群的研究仍显不足。环节动物门（环节虫）便是此类研究不足的类群之一，关于其神经系统发育的研究报告寥寥无几，且主要来自系统发育或发育起源的物种。探索其他物种的神经发育，以便记录多样性并推断出共同模式，从而追溯神经系统发育的演化历程，将具有启发性。研究方法：对两种环节动物物种Cephalothrix rufifrons（基环虫纲、古环虫纲）和Emplectonema gracile（单刺环节纲、多刺环节纲）的固定幼虫阶段，利用针对血清素和FMRF酰胺的多克隆原抗体及针对突触素的单克隆抗体进行的荧光免疫组织化学染色，通过共聚焦激光扫描显微镜进行了分析。研究结果：本文详细描述了两种环节动物物种中血清素和FMRF酰胺免疫反应的神经系统亚群的发育过程，从各自信号首次出现开始。此外，关于突触素样免疫反应性的数据描绘了神经纤维成分的一般结构。两种研究物种的共同事件包括在FMRF样免疫反应信号出现之前出现血清素样免疫反应信号，以及侧神经索从位于幼虫前端的环状脑向后端幼虫极的严格发育进程。显著差异包括：（1）在E. gracile的幼虫发育阶段，吻神经系统的发育开始得较早；（2）在E. gracile的幼虫前极和后极存在早期但明显短暂的血清能神经元，而在C. rufifrons中似乎不存在。讨论：根据本研究结果及与先前关于神经系统发育的出版物一致，环节动物门的假想最后共同祖先具有环绕吻开口的环状脑，由此发育出一对从前端至后端的腹外侧神经索。早期前极和后极的血清能神经元后来发生退化或停止表达血清素，这是环节动物与其他螺旋动物类群共有的祖先特征。