Storks Levi, Powell Brian J, Leal Manuel
Division of Biological Sciences, University of Missouri, Columbia, MO, USA.
Integr Comp Biol. 2020 Sep 11. doi: 10.1093/icb/icaa129.
Studies of vertebrate brain evolution have mainly focused on measures of brain size, particularly relative mass and its allometric scaling across lineages, commonly with the goal of identifying the substrates that underly differences in cognition. However, recent studies on birds and mammals have demonstrated that brain size is an imperfect proxy for neuronal parameters that underly function, such as the number of neurons that make up a given brain region. Here we present estimates of neuron numbers and density in two species of lizard, Anolis cristatellus and A. evermanni, representing the first such data from squamate species, and explore its implications for differences in cognitive performance and vertebrate brain evolution. The isotropic fractionator protocol outlined in this article is optimized for the unique challenges that arise when using this technique with lineages having nucleated erythrocytes and relatively small brains. The number and density of neurons and other cells we find in Anolis for the telencephalon, cerebellum, and the rest of the brain (ROB) follow similar patterns as published data from other vertebrate species. Anolis cristatellus and A. evermanni exhibited differences in their performance in a motor task frequently used to evaluate behavioral flexibility, which was not mirrored by differences in the number, density, or proportion of neurons in either the cerebellum, telencephalon, or ROB. However, the brain of A. evermanni had a significantly higher number of nonneurons and a higher nonneuron to neuron ratio across the whole brain, which could contribute to the observed differences in problem solving between A. cristatellus and A. evermanni. Although limited to two species, our findings suggest that neuron number and density in lizard brains scale similarly to endothermic vertebrates in contrast to the differences observed in brain to body mass relationships. Data from a wider range of species are necessary before we can fully understand vertebrate brain evolution at the neuronal level.
脊椎动物大脑进化的研究主要集中在大脑大小的测量上，尤其是相对质量及其在不同谱系间的异速生长缩放，其共同目标是确定认知差异背后的基础。然而，最近对鸟类和哺乳动物的研究表明，大脑大小并非是功能背后神经元参数（如构成给定脑区的神经元数量）的完美指标。在此，我们给出了两种蜥蜴（冠变色龙和埃氏变色龙）的神经元数量和密度估计值，这是有鳞目物种的首批此类数据，并探讨了其对认知表现差异和脊椎动物大脑进化的影响。本文概述的等体积分离法方案针对在对有核红细胞和相对较小大脑谱系使用该技术时出现的独特挑战进行了优化。我们在变色龙的端脑、小脑和大脑其余部分（ROB）中发现的神经元及其他细胞的数量和密度，与其他脊椎动物物种已发表的数据呈现出相似模式。冠变色龙和埃氏变色龙在一项常用于评估行为灵活性的运动任务中的表现存在差异，但小脑、端脑或ROB中神经元的数量、密度或比例差异并未反映出这一点。然而，埃氏变色龙的大脑在整个大脑中具有显著更多的非神经元以及更高的非神经元与神经元比例，这可能导致了冠变色龙和埃氏变色龙在解决问题能力上观察到的差异。尽管仅限于两个物种，但我们的研究结果表明，蜥蜴大脑中的神经元数量和密度与恒温脊椎动物的缩放方式相似，这与在脑体重关系中观察到的差异形成对比。在我们能够全面理解神经元水平上的脊椎动物大脑进化之前，需要来自更广泛物种的数据。
