Oisi Yasuhiro, Ota Kinya G, Fujimoto Satoko, Kuratani Shigeru
1 Department of Biology, Graduate School of Science, Kobe University, Kobe 657-8501, Japan.
Zoolog Sci. 2013 Nov;30(11):944-61. doi: 10.2108/zsj.30.944.
Recent molecular phylogenetic analyses have shown that the modern jawless vertebrates, hagfishes and lampreys, are more closely related to each other than to the other vertebrates, constituting a monophyletic group, the cyclostomes. In terms of their developmental morphology as well, it is possible to identify an embryonic pattern in hagfish embryos that is common to cyclostomes but not shared by jawed vertebrate embryos. On the basis of this pan-cyclostome embryonic pattern, we describe the developmental sequence of the chondrocranium and associated structures in the hagfish species Eptatretus burgeri and E. atami. Our aim was to establish homologies of the skeletal elements among cyclostomes by comparison of the developmental patterns with a lamprey, Lethenteron reissneri, to characterize further the cyclostome morphotype and its diversification in early vertebrate evolution. We show that the hagfish and lamprey chondrocrania can be compared perfectly at the level of modules corresponding to the craniofacial primordia constituting the cyclostome morphotype. In the adult anatomy, however, there are many instances in which homology cannot be established at the level of single skeletal elements, mainly because of the apparently highly apomorphic nature of the hagfish cranium. Even at the craniofacial modular level, the chondrocrania of cyclostomes and those of jawed vertebrates display very few primary homologies and are therefore very difficult to compare. We also discuss the problem of the homology of a neurocranial element, the trabecula.
最近的分子系统发育分析表明，现代无颌脊椎动物，即盲鳗和七鳃鳗，彼此之间的亲缘关系比与其他脊椎动物更为密切，构成了一个单系类群，即圆口纲。就其发育形态而言，也有可能在盲鳗胚胎中识别出一种圆口纲共有的胚胎模式，但有颌脊椎动物胚胎并不具备这种模式。基于这种泛圆口纲胚胎模式，我们描述了盲鳗物种柏氏黏盲鳗和阿氏黏盲鳗软骨颅及相关结构的发育序列。我们的目的是通过将发育模式与七鳃鳗（雷氏七鳃鳗）进行比较，来确定圆口纲之间骨骼元素的同源性，以进一步刻画圆口纲形态型及其在早期脊椎动物进化中的多样化。我们表明，在对应于构成圆口纲形态型的颅面原基的模块水平上，盲鳗和七鳃鳗的软骨颅可以完美地进行比较。然而，在成体解剖结构中，有许多情况下无法在单个骨骼元素水平上确定同源性，主要是因为盲鳗颅骨明显具有高度特化的性质。即使在颅面模块水平上，圆口纲的软骨颅和有颌脊椎动物的软骨颅也显示出很少的主要同源性，因此很难进行比较。我们还讨论了神经颅元素小梁的同源性问题。
