The Evolutionary Origins of the Vertebrate Brain: Neural Organization and Complexity in Chondrichthyans

“这项奖励是根据2009年美国复苏和再投资法案（公法111-5）资助的。”人们普遍认为鲨鱼是脑容量小的动物，行为能力有限。最近的研究已经打破了这些神话，并发现鲨鱼拥有与鸟类和哺乳动物相当大小的大脑。小脑是一个特别令人感兴趣的大脑结构，主要是由于其广泛的进化历史以及围绕其功能的持续争论。这种结构最初出现在早期的鲨鱼身上，并在脊椎动物的进化过程中从最早的软骨鱼一直延续到人类大脑的基础。然而，从鱼类到人类，小脑及其功能一直是神经生物学家争论的一个领域。大量的研究声称小脑控制和协调运动，而其他研究表明小脑的主要功能不是运动，而是获取和辨别感觉信息。板鳃动物（鲨鱼、鳐鱼和鳐鱼）对我们对人类小脑及其功能起源的理解有什么贡献？在包括鲨鱼在内的软骨鱼类中，小脑的大小和卷曲（或叶状）存在很大的差异。直到最近，小脑复杂性的变化仅通过使用视觉分级指数根据褶皱的长度和深度对表面结构进行评分来定性评估。这些视觉评估的结果表明，生活在开阔海洋（3D）环境中的敏捷捕食者，如短鳍鲭鲨Isurus oxyrinchus，细尾长尾鲨Alopias vulpinus和双髻鲨Sphyrna mokarran，叶子化程度最高，而缓慢移动的被动捕食者，如Orectolobus maculatus（纹须鲨），Squalus acanthias（刺角鲨），Squatina angelus（天使鲨），叶子化程度最低。这些与大脑发育的初步生态学相关性表明了这一特征的功能基础。然而，视觉分类是有限的，因为它不能提供一个定量的方法来表征和比较叶理。这些趋势表明，小脑复杂性与生态因素（原始栖息地、不同的运动方式、繁殖模式和猎物捕获策略）有关，为了证实这些趋势，需要更定量的方法。磁共振成像（MRI）在非侵入性获取软组织结构高分辨率数据的能力上是独一无二的，扩散张量成像（DTI）是一种MRI方法，提供白质纤维束的方向和相干性信息，从而促进神经纤维通路的重建。本项目旨在利用显微解剖MRI和显微DTI结合新颖的分析方法（形状分析和纤维束测绘）来评估小脑的折叠程度，以及这与从大脑结构到其他神经处理中心的纤维束通路的多样化和增殖之间的关系。通过这些方法，该团队将为小脑功能的争论提供信息，为软骨鱼小脑叶面发育的进化提供理论依据，并探索适应、发育和系统发育过程在多大程度上推动神经进化。出版物、结果、数据库和工具将通过现有的数字鱼类图书馆网站（www.digitalfishlibrary.org）向研究界和公众发布。

"This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)."A common perception is that sharks are small-brained animals with a limited behavioral repertoire. Recent research has been dispelling these myths and has found that sharks possess brains that are of comparable size to birds and mammals. One brain structure of particular interest is the cerebellum, largely due to its extensive evolutionary history as well as the ongoing debate surrounding its function. This structure originally appeared in early sharks and has been carried through vertebrate evolution from the earliest cartilaginous fishes to the base of the human brain. However, the cerebellum and its function, from fish through to humans, has been an area of debate among neurobiologists. A large body of research asserts that the cerebellum controls and orchestrates movement while other research suggests that the primary function of the cerebellum is not movement, but is the acquisition and discrimination of sensory information. What can elasmobranchs (sharks, skates, and rays) contribute to our understanding of the human cerebellum and the origin of its function?Large variation exists in the size and convolution (or foliation) of the cerebellum across cartilaginous fishes, including sharks. Until recently, the variation in cerebellar complexity has only been qualitatively assessed by scoring surface structure on the basis of length and depth of the folds using the visual grading index. The output of these visual assessments has shown that the highest levels of foliation are found in agile predators that lived in open ocean (3D) environments, such as Isurus oxyrinchus (shortfin mako shark), Alopias vulpinus (thintail thresher shark), and Sphyrna mokarran (great hammerhead shark) and the lowest levels of foliation occur in slow-moving, passive predators such as Orectolobus maculatus (wobbegong shark), Squalus acanthias (spiny dogfish), Squatina angelus (angel shark). These preliminary ecological correlations with brain development suggest a functional basis for this characteristic. However, visual classification is limited, as it does not provide a quantitative method for characterization and comparison of foliation. To confirm these trends, which show that cerebellar complexity is correlated with ecological factors (primary habitats, with varying locomotory styles, reproductive modes, and prey capture strategies), a more quantitative approach is necessary.Magnetic resonance imaging (MRI) is unique in its ability to non-invasively acquire high-resolution data from soft tissue structure and Diffusion Tensor Imaging (DTI) is an MRI method that provides information on the orientation and coherence of white matter fiber tracts, thereby facilitating the reconstruction of neural fiber pathways. This project aims to utilize microscopic anatomical MRI and microscopic DTI in conjunction with novel analysis methods (shape analysis and fiber tract mapping) to assess the degree of folding in the cerebellum and how this relates to diversification and proliferation of fiber tract pathways from this brain structure to other neural processing centers. Through these methods, the team will inform the debate on cerebellar function, provide rationale for the evolution of cerebellar foliation in cartilaginous fishes, and explore the extent to which adaptive, developmental, and phylogenetic processes are driving neural evolution.Publications, results, databases, and tools will be disseminated to the research community and general public via the existing Digital Fish Library website (www.digitalfishlibrary.org).