Biosystematics, The Natural History Museum of Denmark, University of Copenhagen, Copenhagen, Denmark.
European Molecular Biology Laboratory, Developmental Biology Unit, Heidelberg, Germany.
Nat Ecol Evol. 2018 Sep;2(9):1358-1376. doi: 10.1038/s41559-018-0641-0. Epub 2018 Aug 22.
It is widely held that the bilaterian tubular gut with mouth and anus evolved from a simple gut with one major gastric opening. However, there is no consensus on how this happened. Did the single gastric opening evolve into a mouth, with the anus forming elsewhere in the body (protostomy), or did it evolve into an anus, with the mouth forming elsewhere (deuterostomy), or did it evolve into both mouth and anus (amphistomy)? These questions are addressed by the comparison of developmental fates of the blastopore, the opening of the embryonic gut, in diverse animals that live today. Here we review comparative data on the identity and fate of blastoporal tissue, investigate how the formation of the through-gut relates to the major body axes, and discuss to what extent evolutionary scenarios are consistent with these data. Available evidence indicates that stem bilaterians had a slit-like gastric opening that was partially closed in subsequent evolution, leaving open the anus and most likely also the mouth, which would favour amphistomy. We discuss remaining difficulties, and outline directions for future research.
人们普遍认为，具有口和肛门的两侧管状肠道是从具有单一主胃开口的简单肠道进化而来的。然而，对于这种情况是如何发生的，尚无共识。单一的胃开口是进化成口，肛门在身体的其他部位形成（原肛），还是进化成肛门，口在身体的其他部位形成（后口），还是进化成口和肛门（中肛）？这些问题可以通过比较今天生活的不同动物的胚孔（胚胎肠道的开口）的发育命运来解决。在这里，我们回顾了关于胚孔组织的身份和命运的比较数据，研究了贯穿肠道的形成与主要身体轴之间的关系，并讨论了进化情景在多大程度上与这些数据一致。现有证据表明，原始两侧对称动物具有狭缝状的胃开口，随后在进化过程中部分关闭，留下肛门，很可能还有口，这有利于中肛。我们讨论了仍然存在的困难，并概述了未来研究的方向。
