Table 3_The lamprey habenula provides an extreme example for the temporal regulation of asymmetric development.xlsx

By their phylogenetic position and their marked epithalamic asymmetries, lampreys are relevant models for understanding the formation and evolution of this trait across vertebrates. In this study, we use a transcriptomic approach to identify novel signature markers to characterize the highly asymmetric, bipartite organization of habenulae in lampreys. Lamprey habenulae are subdivided into two complementary subdomains related, respectively, to the lateral/ventral and the medial/dorsal habenulae of jawed vertebrates: a dorsal, right-restricted subdomain and a bilateral subdomain that includes the left habenula as well as its ventral right counterpart. Analysis of the formation of the lamprey habenula at prolarval and larval stages using a combination of morphological, immunohistochemical, and in situ hybridization approaches highlights a marked asymmetric temporal regulation. The dorsal right subdomain forms and already expresses all identified signature markers in prolarval stages. In contrast, the left and ventral right subdomain appears significantly later, with the first indication of neuronal identity elaboration in these territories being observed in larval stages. As in gnathostomes, Wnt signaling may be involved in the regulation of this unique, asymmetric mode of development, since β-catenin shows asymmetric and highly dynamic nuclear distributions both in neural progenitors and differentiated neuronal precursors of the two habenular subdomains. These data confirm the importance of lampreys to unravel the developmental logic underlying the recurrence and variation of habenular asymmetries in vertebrates and pave the way for future functional analyses.

七鳃鳗（lampreys）凭借其系统发育位置与显著的上丘脑不对称性（epithalamic asymmetries），成为解析脊椎动物该性状形成与演化的关键模型。本研究采用转录组学方法（transcriptomic approach），鉴定出全新的特征标志物，用以刻画七鳃鳗缰核（habenulae）高度不对称的二分组织结构。七鳃鳗的缰核可被划分为两个互补的亚结构域，分别对应有颌脊椎动物（jawed vertebrates）的外侧/腹侧缰核与内侧/背侧缰核：一个为局限于右侧的背侧亚结构域，另一个为包含左侧缰核及其右侧腹侧对应结构的双侧亚结构域。本研究结合形态学、免疫组织化学（immunohistochemical）与原位杂交（in situ hybridization）技术，对前幼体与幼体阶段七鳃鳗缰核的形成过程进行分析，结果显示其存在显著的不对称时间调控模式。背侧右侧亚结构域在前幼体阶段即可形成，并已表达所有已鉴定出的特征标志物。与之相反，左侧与腹侧右侧亚结构域的出现时间显著滞后，这类区域的神经元身份特化的首个标志性事件在幼体阶段才被观测到。与有颌脊椎动物（gnathostomes）一致，Wnt信号通路（Wnt signaling）可能参与调控这一独特的不对称发育模式，因为β-连环蛋白（β-catenin）在两个缰核亚结构域的神经前体细胞与已分化神经元前体细胞中，均呈现出不对称且高度动态的核分布特征。本研究数据证实了七鳃鳗在解析脊椎动物缰核不对称性的重现与变异背后的发育逻辑方面的重要价值，同时为后续的功能研究铺平了道路。