Reconstruction of Hexapod Basal Clades : Embryological Study on Protura

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“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