Reconstruction of Hexapod Basal Clades : Embryological Study on Protura

“牙颌目-牙颌目系统”，即“牙颌目(=Ellipurata (=Protura + Collembola)+Diplurata (=Diplura))+牙颌目(=Monocondylia （=Archaeognatha)+双牙颌目（=Zygentoma + Pterygota））”，已被广泛接受，几乎没有挑战。然而，牙颌动物分类群的真实性是值得怀疑的，因为它主要是由牙颌动物支持的。最近的比较古生物学、形态学和比较胚胎学的研究都提供了证据，证明了该类群成员的内颌类作为一种突触形态的有效性，并且一些分子进化证据也出现了，表明原tura和Diplura之间有更密切的亲缘关系，而拒绝Ellipurata。此外，节肢动物之间的内部关系仍然存在激烈的争论。这一背景使得原tura在重建六足动物基支关系方面比以往任何时候都更加重要。比较胚胎学方法是解决这类系统发育问题的最有前途的方法之一，但我们对原体胚胎学还不了解。本研究的目的是首先以杆状芽孢菌（Baculentulus densus, Imadate）为材料，阐明原tura的胚胎发育。杆状芽孢菌卵呈球形，长轴约为120 μm，卵表面散布着许多大小和形状各异的突起。原体胚胎发生的特点是：1)全母细胞分裂，2)长胚带型胚胎发生，3)简单的胚分裂，4)神经发生伴随腹侧器官的形成，5)初级背侧器官的形成，这些都代表了六足动物胚胎发生的祖先条件。前原体胚胎发生的一个显著特征是浆膜保留了向尾膜分化或参与最终背侧闭合的能力，这一特征在六足动物中胚胎本体和胚胎膜之间功能特化的进化过渡中表明，前原体可能是六足动物中比以往所阐明的更基础的分支。少

“Entognatha - Ectognatha System", which is formulated as “Entognatha (=Ellipurata (=Protura + Collembola)+Diplurata (=Diplura))+Ectognatha (=Monocondylia (=Archaeognatha)+Dicondylia (=Zygentoma + Pterygota))", has been widely accepted with little challenge. However, the reality of the taxon Entognatha is suspect because it is supported principally only by entognathy. Recent comparative paleontological, morphological and comparative embryological researches have provided evidence discounting the validity of entognathy as a synapomorphy for the members of this group, and some molecular evolutionary evidence has emerged, suggesting a closer affinity between Protura and Diplura, and rejecting Ellipurata. Furthermore, the internal relationships among Arthropoda still remain furiously debated. This background makes Protura more significant in the reconstruction of relationships of hexapod basal clades than ever. The comparative embryological approach is one of the most promising methods for … More solving such phylogenetic problems, but we have no knowledge on proturan embryology. The aim of the present study is to first clarify the embryonic development of Protura, using Baculentulus densus (Imadate) as materials.Eggs of Baculentulus densus are spherical with a long axis of about 120 μm and with numerous, variously sized and shaped protuberances scattered over their surface. The proturan embryogenesis is characterized by 1) holoblastic cleavage, 2) embryogenesis of long germ band type, 3) simple blastokinesis, 4) neurogenesis accompanied by the formation ventral organs, and 5) formation of a primary dorsal organ, all of which represent the ancestral conditions in hexapod embryogenesis.A noticeable feature of proturan embryogenesis is that the serosa retains the ability to differentiate into the tergum or to participate in the definitive dorsal closure, which in light of the evolutionary transition of the functional specialization between the embryo proper and embryonic membranes among Hexapoda, suggests that Protura may represent a more basal clade in Hexapoda than ever elucidated. Less