Universitat de Barcelona, Departament de Genètica, Avda. Diagonal 645, E-08028 Barcelona, Spain.
Dev Biol. 2011 Apr 1;352(1):164-76. doi: 10.1016/j.ydbio.2011.01.032. Epub 2011 Feb 3.
The formation of a through-gut was a key innovation in the evolution of metazoans. There is still controversy regarding the origin of the anus and how it may have been either gained or lost during evolution in different bilaterian taxa. Thus, the study of groups with a blind gut is of great importance for understanding the evolution of this organ system. Here, we describe the morphogenesis and molecular patterning of the blind gut in the sexual triclad Schmidtea polychroa. We identify and analyze the expression of goosecoid, commonly associated with the foregut, and the GATA, ParaHox and T-box genes, members of which commonly are associated with gut regionalization. We show that GATA456a is expressed in the blind gut of triclads, while GATA456b is localized in dorsal parenchymal cells. Goosecoid is expressed in the central nervous system, and the unique ParaHox gene identified, Xlox, is detected in association with the nervous system. We have not isolated any brachyury gene in the T-box complement of S. polychroa, which consists of one tbx1/10, three tbx2/3 and one tbx20. Furthermore, the absence of genes like brachyury and caudal is also present in other groups of Platyhelminthes. This study suggests that GATA456, in combination with foxA, is a gut-specific patterning mechanism conserved in the triclad S. polychroa, while the conserved gut-associated expression of foregut, midgut and hindgut markers is absent. Based on these data and the deviations in spiral cleavage found in more basal flatworms, we propose that the lack of an anus is an innovation of Platyhelminthes. This may be associated with loss of gut gene expression or even gene loss.
体腔的形成是后生动物进化的一个关键创新。关于肛门的起源以及它在不同的两侧动物类群的进化中是如何获得或丢失的，仍然存在争议。因此，研究具有盲肠的群体对于理解这个器官系统的进化非常重要。在这里，我们描述了有性三胚层动物秀丽隐杆线虫盲肠的形态发生和分子模式。我们鉴定和分析了与前肠相关的 goosecoid 的表达，以及 GATA、ParaHox 和 T-box 基因，这些基因通常与肠道区域化有关。我们表明，GATA456a 在三胚层的盲肠中表达，而 GATA456b 定位于背侧实质细胞中。goosecoid 在中枢神经系统中表达，而鉴定出的独特的 ParaHox 基因 Xlox 与神经系统相关。我们在 S. polychroa 的 T-box 补体中没有分离出任何 brachyury 基因，该补体由一个 tbx1/10、三个 tbx2/3 和一个 tbx20 组成。此外，其他扁形动物门的群体也缺乏 brachyury 和 caudal 等基因。这项研究表明，GATA456 与 foxA 结合，是保守在三胚层 S. polychroa 中的肠道特异性模式形成机制，而前肠、中肠和后肠标记物的保守肠道相关表达缺失。基于这些数据和在更基础的扁形动物中发现的螺旋分裂偏差，我们提出缺乏肛门是扁形动物的一个创新。这可能与肠道基因表达的丧失甚至基因丢失有关。
