Harzsch Steffen, Müller Carsten H G
Max Planck Institute for Chemical Ecology, Department of Evolutionary Neuroethology, Beutenberg Campus, Hans-Knöll-Str, 8, 07745 Jena, Germany.
Front Zool. 2007 May 18;4:14. doi: 10.1186/1742-9994-4-14.
The Chaetognatha (arrow worms) are a group of marine carnivores whose phylogenetic relationships are still vigorously debated. Molecular studies have as yet failed to come up with a stable hypothesis on their phylogenetic position. In a wide range of metazoans, the nervous system has proven to provide a wealth of characters for analysing phylogenetic relationships (neurophylogeny). Therefore, in the present study we explored the structure of the ventral nerve centre ("ventral ganglion") in Sagitta setosa with a set of histochemical and immunohistochemical markers.
In specimens that were immunolabeled for acetylated-alpha tubulin the ventral nerve centre appeared to be a condensed continuation of the peripheral intraepidermal nerve plexus. Yet, synapsin immunolocalization showed that the ventral nerve centre is organized into a highly ordered array of ca. 80 serially arranged microcompartments. Immunohistochemistry against RFamide revealed a set of serially arranged individually identifiable neurons in the ventral nerve centre that we charted in detail.
The new information on the structure of the chaetognath nervous system is compared to previous descriptions of the ventral nerve centre which are critically evaluated. Our findings are discussed with regard to the debate on nervous system organisation in the last common bilaterian ancestor and with regard to the phylogenetic affinities of this Chaetognatha. We suggest to place the Chaetognatha within the Protostomia and argue against hypotheses which propose a deuterostome affinity of Chaetognatha or a sister-group relationship to all other Bilateria.
毛颚动物（箭虫）是一类海洋食肉动物，其系统发育关系仍存在激烈争论。分子研究尚未就其系统发育位置得出稳定的假说。在广泛的后生动物中，神经系统已被证明能为分析系统发育关系（神经系统发育学）提供丰富的特征。因此，在本研究中，我们用一组组织化学和免疫组织化学标记物探究了刚毛箭虫腹侧神经中枢（“腹神经节”）的结构。
在针对乙酰化α微管蛋白进行免疫标记的标本中，腹侧神经中枢似乎是外周表皮内神经丛的浓缩延续。然而，突触素免疫定位显示腹侧神经中枢由约80个串联排列的微区组成高度有序的阵列。针对RFamide的免疫组织化学揭示了腹侧神经中枢中有一组串联排列、可逐个识别的神经元，我们对其进行了详细绘图。
将毛颚动物神经系统结构的新信息与之前对腹侧神经中枢的描述进行了比较，并对这些描述进行了批判性评估。我们结合关于最后一个共同两侧对称动物祖先的神经系统组织的争论以及关于这种毛颚动物的系统发育亲缘关系的讨论了我们的发现。我们建议将毛颚动物置于原口动物中，并反对提出毛颚动物与后口动物有亲缘关系或与所有其他两侧对称动物有姐妹群关系的假说。
