Phylogenetic Analysis Of Existing Hexapoda Based On Pleural Sclerites And18S RDNA

This research is focused on the three hot points contentious in insect systematics:1) Hexapoda originated in the Pancrustacean, which group of crustacean is the most relative to Hexapoda?2) Is Hexapoda monophyletic or polyphyletic?3) What is the relationship among the constituent groups of Hexapoda? Comparative morphological and molecular data sets are used to preliminarily answer these three issues.First, the morphological study:Typical species of insects were selected to research and analysis the structure of the pleural sclerites and its evolution in different groups of insects, especially the latterly evolutionary relationships between the pleural sclerites and the wings. The phylogenetic tree shows that the evolutionary direction of pleural sclerites is from the simple to the complex and then return to simple. The anapleurite extends and covers the entire thoracic arthropleure, and the number of the sclerites is from less to more, and finally reduced. The coxopleurite tends to disappear. The phylogenetic tree shows the relationship between Palaeoptera and Neoptera:Ephemeroptera+(Odonata+Neoptera). The phylogenetic relationship of Neoptera is:Plecoptera+(Megaloptera+Neuroptera+(Orthoptera+(Hemiptera+(Coleoptera+(Mecoptera+Lepidoptera+(Hymenoptera+Diptera))))))=Second, molecular taxonomic studies:The18SrDNA datasets were applied to the analysis of the phylogeny and classificational status of Hexapoda. The Phylogeny and classificational status of Holometabola, the phylogeny and classificational status of Hexapoda, as well as the origin and monophyly of Hexapoda were discussed respectively. All the18S rDNA sequences datasets used were acquired from the GenBank, including Myriapoda,1species from Chilopoda; Crustacean,6species from Malacostraca,7species from Cepopoda,7species from Branchiopoda and5species from Ostracoda; Hexapoda,1species from Protura,2species from Collembola,1species from Diplura and56species from Insecta. The phylogenetic trees were reconstructed with NJ, ML and Bayesian methods. The evolutionary relationships among the Hexapoda groups were tentatively revealed as follow.(1) Holymetabola is divided into three branches:1) Diptera+Strepsiptera;2) Coleoptera+(Megaloptera+(Raphidioptera+Neuroptera));3) Hymenoptera+(Siphonaptera+Mecoptera)+(Lepidoptera+Trichoptera). A monophyletic Halteria is supported.(2) The phylogenetic relationships of the three classes of lower Hexapoda are:(Protura+Diplura)+Collembola. The phylogenetic relationships among Insecta groups are:Zygentoma+(Archaeognatha+(Odonata+(Ephemeroptera+(((Thysanoptera+(Hemiptera+(Psocoptera+Phthiraptera)))+((Orthoptera+((Isoptera+(Mantodea+Blattaria))+(Zoraptera+Dermaptera+Plecoptera)))+(Embioptera+Grylloblattodea+Phasmatodea)))+Holometabola). The monophyly of Ellipura or Parainsecta is not supported.(3) The phylogenetic relationships of Pancrustacea are:(Notostraca+Anostraca)+(Myodocopida+(Collembola+Podocopida)+(Halocyprida+(Isopoda+Decapoda)+((Protura+Diplura)+(Thoracica+Amphiopoda)))+((Harpacticoida+(Calanoida+(Poecilostomatoida+Cyclopoida)))+Insecta)). The monophylies of Branchiopoda and Cepopoda are well supported, while Ostracoda and Malacostraca are not. Hexapoda is paraphyletic, but Insecta is monophyletic whose sister group is copepods.