Roth G, Nishikawa K C, Wake D B
Brain Research Institute, University of Bremen, Germany.
Brain Behav Evol. 1997 Jul;50(1):50-9. doi: 10.1159/000113321.
Compared to other vertebrates, even including lampreys and hagfishes in some respects, salamanders exhibit a relatively simple organization of brain and sense organs which is illustrated here using the visual system as an example. The greatest simplicity is found in the bolitoglossine salamanders, yet all bolitoglossines possess highly projectile tongues and rely on vision for survival; furthermore, some species are agile and acrobatic. The unusual features of the visual system of salamanders include small numbers of large neurons, a low degree of morphological differentiation among neurons, a small proportion of myelinated axons in the optic nerve, and an optic tectum consisting essentially of a periventricular cellular layer and a superficial fiber layer. Similar features are found throughout the central nervous system of salamanders and in the lateral line, auditory and olfactory systems as well. Phylogenetic analysis shows that the most parsimonious interpretation of these data is that the simple organization of the brain and sense organs of salamanders was derived secondarily from a more complex ancestral state. We hypothesize that increased genome size has led to simplification of the nervous system in salamanders. Increased genome size appears to have had profound effects on neural development in salamanders, leading to paedomorphosis, the retention of juvenile or even embryonic characteristics into adulthood. In particular, large genome size is associated with large cell size and reduced rates of cell proliferation, migration and differentiation. Secondary simplification has constrained the function of the salamanders' visual system, primarily by increasing cell size and decreasing cell numbers. However, it also has provided an opportunity for the evolution of compensating mechanisms, which have helped to restore or even enhance visual function. Most apparent among the compensatory mechanisms of bolitoglossine salamanders is the presence of well developed ipsilateral retinotectal projections, which apparently enhance depth perception. It is difficult to explain the unusual history of the nervous system in salamanders solely in terms of natural selection and adaptation. Increasing genome size through selfish replication appears to have played a major role in the evolution of salamander brains by imposing functional constraints as well as creating opportunities for overcoming them.
与其他脊椎动物相比，即使在某些方面包括七鳃鳗和盲鳗，蝾螈的脑和感觉器官组织相对简单，本文以视觉系统为例进行说明。在箭螈中发现了最大程度的简单性，但所有箭螈都拥有高度可弹射的舌头，并依靠视觉生存；此外，一些物种敏捷且善于杂技。蝾螈视觉系统的不寻常特征包括大神经元数量少、神经元之间形态分化程度低、视神经中有髓轴突比例小，以及视顶盖基本上由室周细胞层和浅层纤维层组成。在蝾螈的整个中枢神经系统以及侧线、听觉和嗅觉系统中也发现了类似特征。系统发育分析表明，对这些数据最简约的解释是，蝾螈脑和感觉器官的简单组织是次生地从更复杂的祖先状态衍生而来的。我们假设基因组大小的增加导致了蝾螈神经系统的简化。基因组大小的增加似乎对蝾螈的神经发育产生了深远影响，导致幼态延续，即幼年甚至胚胎特征保留到成年期。特别是，大基因组大小与大细胞大小以及细胞增殖、迁移和分化速率降低有关。次生简化限制了蝾螈视觉系统的功能，主要是通过增加细胞大小和减少细胞数量。然而，它也为补偿机制的进化提供了机会，这些机制有助于恢复甚至增强视觉功能。箭螈补偿机制中最明显的是存在发达的同侧视网膜顶盖投射，这显然增强了深度感知。仅用自然选择和适应来解释蝾螈神经系统的不寻常历史是困难的。通过自私复制增加基因组大小似乎在蝾螈大脑的进化中发挥了重要作用，既施加了功能限制，也创造了克服这些限制的机会。
