EVOLUTION OF NEURODEGENERATION AND OTHER SPECIALIZATIONS IN THE VERTEBRATE VISUAL SYSTEM

脊索眼进化和退化的机制和历史我们最近的焦点是太平洋鳗鱼，以了解脊椎动物眼睛的进化。其他环口动物(七鳃鳗)和所有脊椎动物都有一个相机式的眼睛，因此八目鳗(经常被报道为缺乏眼睛/视力)在进化过程中完美地定位于理解脊椎动物眼睛的外观。我们对很少被研究的太平洋鳗鱼幼鱼的研究揭示了新的令人信服的论点，即鳗鱼拥有脊椎动物眼睛和松果腺的几乎所有特征；这将把八目鳗鱼的眼睛编纂为功能，尽管在个体发育过程中退化(不是足形的)，并改写教科书版本的早期脊索状眼睛进化。受到穴居鱼眼睛退化(回归进化)的启发，我们提出了八目鳗眼睛退化是由于适应原因的理论。我们现在正在使用我们开发的作为人类幼年光感受器退化模型的成年斑马鱼缺乏眼睛的突变体，来测试洞穴鱼和八目鳗视觉系统进化的终极假设。我们将通过评估处于进化的最早阶段的鱼类来回答独特的问题，这些鱼类在盲目表型中存活下来。视锥感光细胞嵌合体的发育和功能-鱼类的视锥感光细胞在形态和功能上与其他脊椎动物相似，但在分布上有显著的模式：四个锥体类别(紫外线、蓝色、绿色和红色敏感)排列在一个类似于精心制作的棋盘的“马赛克”中，让人想起果蝇的小眼。这种引人注目的模式的发展和功能(因此进化)仍然是个谜，在不同的鱼类物种中已经发现了几个变体。到目前为止，正向遗传学发现的途径支持这样的假设，即侧向诱导和差异黏附参与了圆锥花叶的形成。这些假说将在斑马鱼身上利用我们在定向突变方面的世界级专业知识进行分析，并通过特定锥体类别中的蛋白质变体的表达来进行分析。细胞-细胞相互作用在这个系统中的作用将被用新型转基因斑马鱼来询问，该转基因斑马鱼允许有条件地消融锥状光感受器亚型。紫外线敏感度的退化进化--大多数鱼类对紫外线有很强的视觉敏感度。在个体发育过程中，这种敏感性可能会降低，这在鲑鱼和鲑鱼的迁徙过程中已经得到了证实。关于这种丧失的适应价值的主要假说假设在具有不同视觉任务的较大动物中紫外线敏感性降低的功能。斑马鱼已经被设计来测试一个新的替代假说，即紫外线敏感视锥的消失可以提高某些视觉调节任务的表现。斑马鱼已经被改造成可以有条件地消融或灭活光感受器类别。这些鱼将接受电生理学和行为模式的评估，以评估包括波长辨别在内的视觉功能。综合。展望未来，我已经开始在一个重要的保护伞下整合这三个主题。不要问‘为什么鱼可以再生它们的眼睛和中枢神经系统？’但更有成效的问题是，为什么哺乳动物失去了再生眼睛和中枢神经系统的能力？我们假设中枢神经系统的再生是适应性的。我将应用回归进化理论来提供关于失去中枢神经系统再生的适应价值的假设。我的工作假设是，参与干细胞/再生的基因已经在哺乳动物中被增选，以实现新的功能，包括突触可塑性、学习和记忆。在成年基础脊椎动物中实验操作这些基因是我们在斑马鱼中进行定向突变的罕见能力的核心。

MECHANISMS AND HISTORY OF CHORDATE EYE EVOLUTION AND REGRESSION Our recent focus is on Pacific Hagfish to understand evolution of the vertebrate eye. Other cyclostomes (lampreys) and all vertebrates share a camera-style eye, thus the Hagfish (oft-reported to lack of eyes/vision) are positioned perfectly to understand the appearance of the vertebrate eye over evolutionary time. Our study of rarely-examined young Pacific Hagfish reveals novel compelling arguments that hagfish possess almost all features of a vertebrate eye, and a pineal gland; this will codify hagfish eyes as functional, though degenerating during ontogeny (not paedomorphic), and rewrite the textbook version of early chordate eye evolution. Inspired by eye degeneration in cavefish (regressive evolution), we theorize that eyes in hagfish degenerate for adaptive reasons. We are now using adult zebrafish mutants that lack eyes (troglomorphic zebrafish!), that we developed as a model of human juvenile photoreceptor degeneration, to test ultimate hypotheses of cavefish and hagfish visual system evolution. We will answer unique questions by assessing fish that are in the very earliest stages of evolving to survive an eyeless phenotype. DEVELOPMENT and FUNCTION of CONE PHOTORECEPTOR MOSAICS – Cone photoreceptors of fishes are similar in morphology and function compared to other vertebrates, yet have a striking pattern in their distribution: The four cone classes (UV-, blue-, green- & red-sensitive) are arranged in a ‘mosaic’ similar to an elaborate checkerboard and reminiscent of Drosophila ommatidia. The development and function (and thus evolution) of this striking pattern remain mysterious, and several variants have been identified in differing fish species. Pathways identified to date by forward genetics support the hypothesis that lateral induction and differential adhesion are involved in cone mosaic formation. Such hypotheses will be analyzed in zebrafish using our world-class expertise in targeted mutagenesis, and by expression of protein variants in specific cone classes. The roles of cell-cell interactions in this system will be queried using novel transgenic zebrafish allowing conditional ablation of cone photoreceptor subtypes. REGRESSIVE EVOLUTION of UV SENSITIVITY – Most fishes have substantial visual sensitivity to ultraviolet light. This sensitivity can decrease during ontogeny, as has been indentified in salmon and trout associated with their migrations. Leading hypotheses regarding adaptive value of this loss assume decreasing function of UV sensitivity in larger animals with different visual tasks. Zebrafish have been engineered to test a novel alternate hypothesis that the disappearance of UV-sensitive cones enhances performance in some visually-mediated tasks. Zebrafish have been engineered such that photoreceptor classes can be conditionally ablated or inactivated. These fish will be assessed in electrophysiology and behavioural paradigms to evaluate visual functions including wavelength discrimination. SYNTHESIS. Going forward, I have begun to integrate these three themes under an overarching umbrella. Ask not ‘Why can fish regenerate their eyes and CNS?’ but more productively ask ‘Why have mammals lost the capacity to regenerate their eyes and CNS?’. We assume CNS regeneration is adaptive. I will apply Regressive Evolution theory to inform hypotheses on the adaptive value of losing CNS regeneration. My working hypothesis is that genes involved in stem cells/regeneration have been co-opted in mammals for new functions including synaptic plasticity, learning and memory. Experimentally manipulating these genes in adult basal vertebrates is a core of our rare ability to perform targeted mutagenesis in zebrafish.