Roth G, Nishikawa K C, Naujoks-Manteuffel C, Schmidt A, Wake D B
Brain Research Institute, University of Bremen, FRG.
Brain Behav Evol. 1993;42(3):137-70. doi: 10.1159/000114147.
Comparative neuroanatomists since Herrick [1914] have been aware of the paradox that the brain of amphibians, especially salamanders, is less complex than one would expect based on their phylogenetic position among the Tetrapoda. Many features of the brain are less differentiated in salamanders than in tetrapod outgroups, including chondrichthyans and bony fishes, and for some brain characters, the salamander brain is even more simple than that of the agnathans. Here, we perform a cladistic analysis on 23 characters of four sensory systems (visual, auditory, lateral line and olfactory) and the brain. Our taxa include myxinoids, lampreys, chondrichthyans, actinopterygians, Latimeria, Neoceratodus and the lepidosirenid lungfishes, amniotes, frogs, caecilians, salamanders and bolitoglossine salamanders. Of the 23 characters we examined, 19 are most parsimoniously interpreted as secondarily simplified in salamanders from a more complex ancestral state, two characters are equally parsimonious under both hypotheses, one character (well developed ipsilateral retinotectal projections) is more complex in bolitoglossine salamanders than in vertebrates generally, and only one character (migration of neurons in the medial pallium) is most parsimoniously interpreted as retention of the plesiomorphically simple condition. Secondary simplification of the salamander brain appears to result from paedomorphosis, or retention of juvenile or embryonic morphology into adulthood. Paedomorphosis is correlated with an increase in genome size, which in turn is positively correlated with cell size, but negatively correlated with cell proliferation and differentiation rates. Available data suggest that, although increasing genome size and paedomorphosis tend to compromise the function of the salamander brain, compensating mechanisms have evolved that may restore or even enhance brain function.
自赫里克（1914年）以来，比较神经解剖学家就已经意识到这样一个矛盾：两栖动物，尤其是蝾螈的大脑，其复杂程度低于根据它们在四足动物中的系统发育位置所预期的水平。蝾螈大脑的许多特征比包括软骨鱼和硬骨鱼在内的四足动物外类群的特征分化程度更低，而且对于某些脑特征来说，蝾螈大脑甚至比无颌类动物的大脑还要简单。在这里，我们对四个感觉系统（视觉、听觉、侧线和嗅觉）以及大脑的23个特征进行了分支分析。我们的分类单元包括盲鳗类、七鳃鳗、软骨鱼、辐鳍鱼、矛尾鱼、澳洲肺鱼和南美肺鱼、羊膜动物、青蛙、蚓螈、蝾螈和疣螈。在我们研究的23个特征中，有19个最简约地解释为蝾螈从更复杂的祖先状态次生简化而来，两个特征在两种假设下同样简约，一个特征（同侧视网膜 - 顶盖投射发达）在疣螈中比一般脊椎动物更复杂，只有一个特征（内侧大脑皮层中神经元的迁移）最简约地解释为保留了近裔共性的简单状态。蝾螈大脑的次生简化似乎是幼态持续的结果，即幼年或胚胎形态保留到成年期。幼态持续与基因组大小增加相关，而基因组大小增加又与细胞大小呈正相关，但与细胞增殖和分化速率呈负相关。现有数据表明，尽管基因组大小增加和幼态持续往往会损害蝾螈大脑的功能，但已经进化出了补偿机制，可能会恢复甚至增强大脑功能。
