Ota Kinya G, Oisi Yasuhiro, Fujimoto Satoko, Kuratani Shigeru
Laboratory of Aquatic Zoology, Marine Research Station, Institute of Cellular and Organismic Biology, Academia Sinica, No. 23-10, Dawen Road, Jiaoxi, Yilan 26242, Taiwan.
Laboratory for Evolutionary Morphology, RIKEN Center for Developmental Biology, Kobe 650-0047, Japan.
Zoology (Jena). 2014 Feb;117(1):77-80. doi: 10.1016/j.zool.2013.10.010. Epub 2013 Dec 11.
The origins of the vertebral elements and the underlying developmental mechanisms have so far remained unclear, largely due to the unusual axial skeletal morphology of hagfish, one of two extant jawless vertebrate clades. Hagfish axial supporting tissue is generally believed to consist of the notochord and cartilaginous fin rays only. However, careful investigations of whether vertebral elements are truly absent in hagfish are scarce, and it is also unclear whether the axial skeletal morphology of the hagfish is an ancestral or a derived condition. To address these questions, we re-examined the axial skeletal morphology of the Japanese inshore hagfish (Eptatretus burgeri). Based on a report published a century ago which implied the existence of vertebral elements in hagfish, we conducted anatomical and histological analyses of the hagfish axial skeletal systems and their development. Through this analysis, we demonstrate that hagfish possesses sclerotome-derived cartilaginous vertebral elements at the ventral aspect of the notochord. Based on (i) molecular phylogenetic evidence in support of the monophyly of cyclostomes (hagfish and lampreys) and jawed vertebrates (gnathostomes), and (ii) the morphology of the vertebral elements in extant gnathostomes and cyclostomes, we propose that the embryos of the common ancestor of all vertebrates would have possessed sclerotomal cells that formed the segmentally arranged vertebral elements attached to the notochord. We also conclude that the underlying developmental mechanisms are likely to have been conserved among extinct jawless vertebrates and modern gnathostomes.
脊椎动物身体结构单元的起源及其潜在的发育机制至今仍不清楚，这主要是由于盲鳗（现存的两个无颌脊椎动物分支之一）具有不寻常的轴向骨骼形态。一般认为，盲鳗的轴向支撑组织仅由脊索和软骨鳍条组成。然而，关于盲鳗是否真的没有脊椎动物身体结构单元的仔细研究很少，而且盲鳗的轴向骨骼形态是原始状态还是衍生状态也不清楚。为了解决这些问题，我们重新研究了日本近海盲鳗（Eptatretus burgeri）的轴向骨骼形态。基于一个世纪前发表的一份报告，该报告暗示盲鳗中存在脊椎动物身体结构单元，我们对盲鳗的轴向骨骼系统及其发育进行了解剖学和组织学分析。通过这项分析，我们证明盲鳗在脊索腹侧具有源自生骨节的软骨脊椎动物身体结构单元。基于（i）支持圆口类动物（盲鳗和七鳃鳗）和有颌脊椎动物（gnathostomes）单系性的分子系统发育证据，以及（ii）现存gnathostomes和圆口类动物中脊椎动物身体结构单元的形态，我们提出所有脊椎动物共同祖先的胚胎可能具有形成附着在脊索上的节段性排列的脊椎动物身体结构单元的生骨节细胞。我们还得出结论，在已灭绝的无颌脊椎动物和现代gnathostomes中，潜在的发育机制可能是保守的。
