Balavoine Guillaume
Institut Jacques Monod, CNRS / Université Paris Diderot, Paris, France.
Int J Dev Biol. 2014;58(6-8):469-83. doi: 10.1387/ijdb.140148gb.
The debate on the origin of segmentation is a central question in the study of body plan evolution in metazoans. Annelids are the most conspicuously metameric animals as most of the trunk is formed of identical anatomical units. In this paper, I summarize the various patterns of evolution of the metameric body plan in annelids, showing the remarkable evolvability of this trait, similar to what is also found in arthropods. I then review the different modes of segment formation in the annelid tree, taking into account the various processes taking place in the life histories of these animals, including embryogenesis, post-embryonic development, regeneration and asexual reproduction. As an example of the variations that occur at the cellular and genetic level in annelid segment formation, I discuss the processes of teloblastic growth or posterior addition in key groups in the annelid tree. I propose a comprehensive definition for the teloblasts, stem cells that are responsible for sequential segment addition. There are a diversity of different mechanisms used in annelids to produce segments depending on the species, the developmental time and also the life history processes of the worm. A major goal for the future will be to reconstitute an ancestral process (or several ancestral processes) in the ancestor of the whole clade. This in turn will provide key insights in the current debate on ancestral bilaterian segmentation.
关于分节起源的争论是后生动物身体结构进化研究中的核心问题。环节动物是最显著的分节动物，其大部分躯干由相同的解剖单元构成。在本文中，我总结了环节动物分节身体结构的各种进化模式，展示了这一特征显著的可进化性，这与在节肢动物中发现的情况类似。然后，我回顾了环节动物谱系中不同的分节形成模式，同时考虑了这些动物生活史中发生的各种过程，包括胚胎发生、胚后发育、再生和无性繁殖。作为环节动物分节形成过程中细胞和基因水平上发生变化的一个例子，我讨论了环节动物谱系中关键类群的端细胞生长或后端添加过程。我为端细胞提出了一个全面的定义，端细胞是负责依次添加节段的干细胞。根据物种、发育时间以及蠕虫的生活史过程，环节动物采用了多种不同的机制来产生节段。未来的一个主要目标将是在整个类群的祖先中重建一个祖先过程（或几个祖先过程）。这反过来将为当前关于双侧对称动物祖先分节的争论提供关键见解。
