Bierman Hilary S, Zottoli Steven J, Hale Melina E
Committee on Neurobiology, University of Chicago, Chicago, Ill., USA.
Brain Behav Evol. 2009;73(3):174-87. doi: 10.1159/000222562. Epub 2009 Jun 3.
Studies of vertebrate brain evolution have focused primarily on patterns of gene expression or changes in size and organization of major brain regions. The Mauthner cell, an important reticulospinal neuron that functions in the startle response of many species, provides an opportunity for evolutionary comparisons at the cellular level. Despite broad interspecific similarities in Mauthner cell morphology, the motor patterns and startle behaviors it initiates vary markedly. Response diversity has been hypothesized to result, in part, from differences in the structure and function of the Mauthner cell-associated axon cap. We used light microscopy techniques to compare axon cap morphology across a wide range of species, including all four extant basal actinopterygian orders, representatives of a variety of teleost lineages and lungfishes, and we combined our data with published descriptions of axon cap structure. The 'composite' axon cap, observed in teleosts, is an organized conglomeration of glia and fibers of inhibitory and excitatory interneurons. Lungfish, amphibian tadpoles and several basal actinopterygian fishes have 'simple' axon caps that appear to lack glia and include few fibers. Several other basal actinopterygian fishes have 'simple-dense' caps that include greater numbers of fibers than simple caps, but lack the additional elements and organization of composite caps. Phylogenetic mapping shows that through evolution there are discrete transitions in axon cap morphology occurring at the base of gnathostomes, within basal actinopterygians, and at the base of the teleost radiation. Comparing axon cap evolution to the evolution of startle behavior and motor pattern provides insight into the relationship between Mauthner cell-associated structures and their functions in behavior.
脊椎动物大脑进化的研究主要集中在基因表达模式或主要脑区大小和组织的变化上。莫特纳尔细胞是一种重要的网状脊髓神经元，在许多物种的惊吓反应中发挥作用，为细胞水平的进化比较提供了机会。尽管莫特纳尔细胞形态在种间具有广泛的相似性，但其启动的运动模式和惊吓行为却有显著差异。有假说认为，反应多样性部分源于与莫特纳尔细胞相关的轴突帽的结构和功能差异。我们使用光学显微镜技术比较了广泛物种的轴突帽形态，包括所有四个现存的基干辐鳍鱼目、各种硬骨鱼谱系的代表以及肺鱼，并且我们将我们的数据与已发表的轴突帽结构描述相结合。在硬骨鱼中观察到的“复合”轴突帽是神经胶质以及抑制性和兴奋性中间神经元纤维的有组织聚集体。肺鱼、两栖类蝌蚪和几种基干辐鳍鱼具有“简单”轴突帽，似乎缺乏神经胶质且纤维很少。其他几种基干辐鳍鱼具有“简单致密”轴突帽，其纤维数量比简单轴突帽更多，但缺乏复合轴突帽的其他成分和组织结构。系统发育图谱显示，在进化过程中，轴突帽形态在有颌类动物基部、基干辐鳍鱼类内部以及硬骨鱼辐射基部发生了离散转变。将轴突帽进化与惊吓行为和运动模式的进化进行比较，有助于深入了解与莫特纳尔细胞相关的结构及其在行为中的功能之间的关系。
