School of Physiology, Pharmacology and Neuroscience, University of Bristol, Bristol, BS8 1TD, UK.
J Exp Biol. 2022 Mar 8;225(Suppl_1). doi: 10.1242/jeb.243382.
Although neural tissues in cnidarian hydroids have a nerve net structure, some cnidarian medusae contain well-defined nerve tracts. As an example, the hydrozoan medusa Aglantha digitale has neural feeding circuits that show an alignment and condensation, which is absent in its relatives Aequorea victoria and Clytia hemisphaerica. In some cases, neural condensations take the form of fast propagating giant axons concerned with escape or evasion. Such giant axons appear to have developed from the fusion of many, much finer units. Ribosomal DNA analysis has identified the lineage leading to giant axon-based escape swimming in Aglantha and other members of the Aglaura clade of trachymedusan jellyfish. The Aglaura, along with sister subclades that include species such as Colobonema sericeum, have the distinctive ability to perform dual swimming, i.e. to swim at either high or low speeds. However, the form of dual swimming exhibited by Colobonema differs both biomechanically and physiologically from that in Aglantha and is not giant axon based. Comparisons between the genomes of such closely related species might provide a means to determine the molecular basis of giant axon formation and other neural condensations. The molecular mechanism responsible may involve 'fusogens', small molecules possibly derived from viruses, which draw membranes together prior to fusion. Identifying these fusogen-based mechanisms using genome analysis may be hindered by the many changes in anatomy and physiology that followed giant axon evolution, but the genomic signal-to-noise ratio may be improved by examining the convergent evolution of giant axons in other hydrozoa, such as the subclass Siphonophora.
虽然刺胞动物水螅的神经组织具有神经网结构，但有些刺胞动物水母含有明确的神经束。例如，水螅水母属的水螅有神经摄食回路，显示出排列和浓缩，而其近亲海月水母和半球海刺水母则没有。在某些情况下，神经浓缩形成与逃避或回避有关的快速传播的巨大轴突。这种巨大轴突似乎是由许多更细的单位融合而成的。核糖体 DNA 分析已经确定了导致基于巨大轴突的逃避游泳的谱系，这种游泳存在于水螅属和其他 Trachymedusae 水母的 Aglaura 分支的成员中。Aglaura 及其姐妹亚群，包括 Colobonema sericeum 等物种，具有独特的双重游泳能力，即可以高速或低速游泳。然而，Colobonema 表现出的双重游泳形式在生物力学和生理学上与水螅属不同，并且不是基于巨大轴突的。对如此密切相关的物种的基因组进行比较可能提供一种确定巨大轴突形成和其他神经浓缩的分子基础的方法。负责的分子机制可能涉及“融合蛋白”，即可能来自病毒的小分子，它们在融合之前将膜拉到一起。使用基因组分析识别这些基于融合蛋白的机制可能会受到巨大轴突进化后解剖和生理学上许多变化的阻碍，但通过检查其他水螅纲动物（如 Siphonophora 亚纲）中巨大轴突的趋同进化，可能会提高基因组的信号与噪声比。
