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.

中文翻译：

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两种远缘带状蠕虫物种（纽虫、螺旋虫）神经系统中血清素和 FMRF 酰胺免疫反应成分的比较发育

简介非模式生物幼虫阶段的神经发育，重点是血清素和 FMRF 酰胺免疫反应成分，一直是近年来研究的焦点。然而，一些分类群仍未得到充分研究。 Nemertea（带状蠕虫）代表了这样一个未被充分研究的分支，关于神经系统发育的报道很少，主要来自系统发育或发育衍生的物种。探索其他物种的神经发育，以便能够记录多样性并推断共同模式，以追踪神经系统发育的进化，将是富有洞察力的。方法使用抗血清素和 FMRF-酰胺的多克隆一抗和抗突触蛋白的单克隆抗体进行荧光免疫组织化学标记两种纽虫物种的一系列固定幼虫阶段红丝头丝菌（Archinemertea、Palaeonemertea）和细线藻（Monostilifera，Hoplonemertea）用共焦激光扫描显微镜进行了分析。结果此贡献详细说明了两种纽虫物种神经系统中血清素和 FMRFamide 免疫反应亚群从各自信号首次出现以来的发展情况。此外，突触蛋白样免疫反应性的数据说明了神经纤维成分的一般结构。这两个研究物种的共同事件是在 FMRF 样免疫反应信号出现之前出现 5-羟色胺样免疫反应信号，以及侧神经索从位于前面的环形脑向后极的发育的严格进展。幼虫。显着的差异是（1）在所研究的幼虫阶段，长鼻神经系统发育得更早。细细桉(2)幼虫额叶和尾极上明显的早期但明显短暂的血清素能神经元细细桉似乎不存在于C. rufifrons讨论根据这项调查的结果，并与之前发表的关于神经系统发育的描述一致，纽虫的假设的最后共同祖先有一个围绕长鼻开口排列的环形大脑，一对腹侧神经索从中发育出来从前到后的进展。早期额叶和尾部的血清素能神经元后来退化或停止表达血清素，这是纽虫亚纲的祖先特征，它们与其他几个螺旋分支共有。