Table S1 from The Notch pathway in the annelid <i>Platynereis</i>: insights into chaetogenesis and neurogenesis processes

Notch is a key signalling pathway playing multiple and varied functions during development. Notch regulates the selection of cells with a neurogenic fate and maintains a pool of yet uncommitted precursors through lateral inhibition, both in insects and in vertebrates. Here, we explore the functions of Notch in the annelid <i>Platynereis dumerilii</i> (Lophotrochozoa). Conserved components of the pathway are identified and a scenario for their evolution in metazoans is proposed. Unexpectedly, neither <i>Notch</i> nor its ligands are expressed in the neurogenic epithelia of the larva at the time when massive neurogenesis begins. Using chemical inhibitors and neural markers, we demonstrate that Notch plays no major role in the general neurogenesis of larvae. Instead, we find Notch components expressed in nascent chaetal sacs, the organs that produce the annelid bristles. Impairing Notch signalling induces defects in chaetal sac formation, abnormalities in chaetae producing cells and a change of identity of chaeta growth accessory cells. This is the first bilaterian species in which the early neurogenesis processes appear to occur without a major involvement of the Notch pathway. Instead, Notch is co-opted to pattern annelid-specific organs, likely through a lateral inhibition process. These features reinforce the view that Notch signalling has been recruited multiple times in evolution due to its remarkable ‘toolkit’ nature.

Notch信号通路（Notch）是一类在发育进程中发挥多重复杂功能的关键信号通路。在昆虫与脊椎动物体内，Notch均通过侧抑制（lateral inhibition）调控神经命运细胞的选择，并维持未定型前体细胞库。本研究以环节动物（annelid）杜氏阔沙蚕（Platynereis dumerilii，隶属冠轮动物Lophotrochozoa）为研究模型，探究Notch信号通路的功能。我们鉴定了该通路的保守组分，并提出了其在后生动物（metazoans）中的演化模式。出乎意料的是，在大规模神经发生（neurogenesis）启动阶段，幼虫的神经发生上皮中既未检测到Notch的表达，也未检测到其配体的表达。借助化学抑制剂（chemical inhibitors）与神经标记物（neural markers）实验，我们证实Notch信号通路对幼虫的整体神经发生并无显著调控作用。与之相反，我们发现Notch通路组分在新生毛囊（chaetal sacs）中特异性表达——毛囊是产生环节动物刚毛（chaetae）的器官。阻断Notch信号通路会引发毛囊形成缺陷、刚毛产生细胞异常，以及刚毛生长辅助细胞的身份转变。本研究对象是首个被发现早期神经发生过程不受Notch通路显著调控的两侧对称动物（bilaterian）。取而代之的是，Notch被招募以调控环节动物特异性器官的模式形成，这一过程大概率通过侧抑制途径完成。这些特征进一步佐证了Notch信号通路因其卓越的‘工具箱’特性，在演化历程中被多次招募利用的学术观点。