Long Island University-Post, 720 Northern Blvd,, Brookville, NY 11709, USA.
Evodevo. 2012 Jan 24;3:3. doi: 10.1186/2041-9139-3-3.
Larval features such as the apical organ, apical ciliary tuft, and ciliated bands often complicate the evaluation of hypotheses regarding the origin of the adult bilaterian nervous system. Understanding how neurogenic domains form within the bilaterian head and larval apical organ requires expression data from animals that exhibit aspects of both centralized and diffuse nervous systems at different life history stages. Here, we describe the expression of eight neural-related genes during the larval development of the brachiopod, Terebratalia transversa.
Radially symmetric gastrulae broadly express Tt-Six3/6 and Tt-hbn in the animal cap ectoderm. Tt-NK2.1 and Tt-otp are restricted to a central subset of these cells, and Tt-fez and Tt-FoxQ2 expression domains are already asymmetric at this stage. As gastrulation proceeds, the spatial expression of these genes is split between two anterior ectodermal domains, a more dorsal region comprised of Tt-Six3/6, Tt-fez, Tt-FoxQ2, and Tt-otp expression domains, and an anterior ventral domain demarcated by Tt-hbn and Tt-NK2.1 expression. More posteriorly, the latter domains are bordered by Tt-FoxG expression in the region of the transverse ciliated band. Tt-synaptotagmin 1 is expressed throughout the anterior neural ectoderm. All genes are expressed late into larval development. The basiepithelial larval nervous system includes three neurogenic domains comprised of the more dorsal apical organ and a ventral cell cluster in the apical lobe as well as a mid-ventral band of neurons in the mantle lobe. Tt-otp is the only gene expressed in numerous flask-shaped cells of the apical organ and in a subset of neurons in the mantle lobe.
Our expression data for Tt-Six3/6, Tt-FoxQ2, and Tt-otp confirm some aspects of bilaterian-wide conservation of spatial partitioning within anterior neurogenic domains and also suggest a common origin for central otp-positive cell types within the larval apical organs of spiralians. However, the field of sensory neurons within the larval apical organ of Terebratalia is broader and composed of more cells relative to those of other spiralian larvae. These cellular differences are mirrored in the broader spatial and temporal expression patterns of Tt-FoxQ2 and Tt-otp. Corresponding differences in the expression of Tt-hbn, Tt-NK2.1, and Tt-FoxG are also observed relative to their respective domains within the cerebral ganglia of spiralians. Based on these data we argue that the anterior region of the bilaterian stem species included Six3/6, NK2.1, otp, hbn, fez, and FoxQ2 expression domains that were subsequently modified within larval and adult neural tissues of protostome and deuterostome animals.
幼虫的特征，如顶端器官、顶端纤毛簇和纤毛带，常常使关于成虫两侧对称神经系统起源的假说的评估复杂化。了解神经发生域如何在两侧对称头部和幼虫顶端器官内形成，需要来自在不同生活史阶段表现出集中和弥散神经系统特征的动物的表达数据。在这里，我们描述了腕足动物 Terebratalia transversa 幼虫发育过程中 8 个神经相关基因的表达情况。
辐射对称的原肠胚广泛表达 Tt-Six3/6 和 Tt-hbn 在动物帽外胚层中。Tt-NK2.1 和 Tt-otp 局限于这些细胞的中央亚群，而 Tt-fez 和 Tt-FoxQ2 的表达域在这个阶段已经不对称。随着原肠胚的发育，这些基因的空间表达被分裂到两个前外胚层域之间，一个更背侧的区域由 Tt-Six3/6、Tt-fez、Tt-FoxQ2 和 Tt-otp 表达域组成，一个前腹侧域由 Tt-hbn 和 Tt-NK2.1 表达域划定。更靠后，后一个域被横向纤毛带中 Tt-FoxG 表达的区域所包围。Tt-synaptotagmin 1 在前神经外胚层中表达。所有基因在幼虫发育后期都有表达。基上皮幼虫神经系统包括三个神经发生域，由更背侧的顶端器官和顶端叶中的一个腹侧细胞簇以及中腹侧带的神经元组成。Tt-otp 是唯一在顶端器官的许多瓶状细胞和腕足类幼虫的顶叶中的一些神经元中表达的基因。
我们对 Tt-Six3/6、Tt-FoxQ2 和 Tt-otp 的表达数据证实了在前神经发生域中空间分隔的一些方面在两侧对称生物中是广泛保守的，也表明了在螺旋动物的幼虫顶端器官中中央 otp 阳性细胞类型具有共同的起源。然而，与其他螺旋动物幼虫相比，Terebratalia 幼虫顶端器官中的感觉神经元的范围更广，细胞也更多。Tt-FoxQ2 和 Tt-otp 的更广泛的时空表达模式反映了这些细胞上的差异。在螺旋动物的脑神经节中，也观察到与 Tt-hbn、Tt-NK2.1 和 Tt-FoxG 相对应的表达差异。基于这些数据，我们认为两侧对称物种的前区包括 Six3/6、NK2.1、otp、hbn、fez 和 FoxQ2 的表达域，随后在原肠胚和后肠胚动物的幼虫和成年神经组织中被修饰。
