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However, the Zygentoma was found to have a closer relationship with winged insects than has the Archaeognatha 3, 4, 5, 6, 7, 8. Members of two wingless insect orders Archaognatha and Zygentoma are traditionally classified in the order “Thysanura” 1, 2, based on the external morphological similarities. Determining the relationship of Apterygota with Pterygota and the relationship of palaeopteran orders with regard to Neoptera are the keys to understanding evolution of flight and insect diversification.
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Although the monophyly of the Pterygota is well established, the interrelationships among basal orders are ambiguous. All other winged insects form the Neoptera.
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The Palaeoptera includes two extant orders: Odonata (dragonflies and damselflies) and Ephemeroptera (mayflies). According to the character of whether insects can fold back the wings over the abdomen, the Pterygota are subdivided into Palaeoptera and Neoptera. Winged insects, namely the subclass Pterygota occur in the next stage. Yet, it is generally agreed that wingless species comprising the subclass Apterygota, including bristletails (Archaeognatha) and silverfish (Zygentoma), constitute the most primitive lineages in Insecta. The origin of insect wings and flight remain contentious. The acquisition of wings and of ability of flight contribute to the success of insects in the planet. In addition, a sister group relationship between Palaeoptera and Neoptera was supported by the current mitogenomic data. Analyses using site-heterogeneous CAT-GTR model strongly supported the Palaeoptera clade, with the monophyletic Ephemeroptera being sister to the monophyletic Odonata. This confirmed the relatively primitive position of Zygentoma to Ephemeroptera, Odonata and Neoptera. Our results generally recovered Zygentoma as a monophyletic group, which formed a sister group to Pterygota. Phylogenetic analyses were performed based on various data coding schemes, using maximum likelihood and Bayesian inferences under different models of sequence evolution. Combined with published insect mitogenome sequences, we constructed a data matrix with all 37 mitochondrial genes of 85 taxa, which had a sampling concentrating on the palaeopteran lineages. A tRNA gene cluster of IMQM was found in three ephemeropteran species, which may serve as a potential synapomorphy for the family Heptageniidae. We then compared the structures of the newly sequenced mitogenomes. In this study, we used a next-generation sequencing approach to reconstruct new mitochondrial genomes (mitogenomes) from 18 species of basal insects, including six representatives of Ephemeroptera and 11 of Odonata, plus one species belonging to Zygentoma. In particular, the relationships among Ephemeroptera, Odonata and Neoptera are the focus of debate. Phylogenetic relationships of basal insects remain a matter of discussion.