The Complete Mitochondrial Genome Sequence Of Vietnamella Dabieshanensis And Paegniodes Cupulaturs (Insecta: Ephemeroptera)

The mayflies (Insecta:Ephemeroptera) are a group of the oldest living insects and have a long series of plesiomorphic characters which contribute a lot to the origin and evolution of winged insects. In recent years, more and more mitochondrial genome (mtDNA) sequences have been used in insect systematics. But the phylogenetic position of Ephemeroptera within Pterygota, whether based on molecular or morphological characrters, is still controversial. It is believed here that more mitochondrial genome data from more lineages of Ephemeroptera can provide more useful information for reconstruction of the phylogeny and evolution of mayflies as well as insects.In this study, we sequence two mitochondrial genomes of mayfly Vietnamella dabieshanensis (Vietnamellidae) and Paegniodes cupulatus (Heptageniidae). They are used further to detect the phylogenetic relationship of Ephemeroptera in Pterygota and higher phylogeny within the order via maximum likelihood (ML) and Bayesian analyses (BI). And the PCGs value in these kinds of researchs is evaluated as well.The complete mtDNA sequences of V. dabieshanensis and P. cupulatus are circular molecule of 15,761 bp and 15,715 bp respectively. Both of them contain a large noncoding region (control region) and additional typical 37 genes which can be found in common metazoans:13 PCGs,22 tRNA genes and 2 ribosomal genes. The gene orders of both mayfly spieces are identical to other primitive arthropods, which seems to imply that the mayfly mtDNA structure is a plesiomorphic model. Except coxl starting with CCG and nad6 with TTA, the start codons of both Vietnamella dabieshanensis and Paegniodes cupulatus mitochondrial PCGs are ATN. Three genes (cox2, nad5, cob) of Vietnamella dabieshanensis and three(cox2, nad5, nad4) of Paegniodes cupulatus had partial stop codons a single T or TT, and the remaining genes in two species have complete terminate codon TAA or TAG. The average A+T contents in Vietnamella dabieshanensis and Paegniodes cupulatus mitochondrial genomes correspond well to the general A+T bias observed in insect mitochondrial genomes. The strongest A+T biase is found at the A+T riched region, which are 74.9% and 63.1% respectively in two mayflies studied here.The phylogenetic inferences suggest the Ephemeroptera is the sister group of all other winged insects. According to both BI and ML trees, the topology of five mayfly species is((Siphlonurus immanis+Ephemera orientails)+(Parafornurus youi+ Paegniodes cupulatus))+Vietnamella dabieshanensis. Among 5 species, Parafornurus youi and Paegniodes cupulatus belong to Heptageniidae, the other three represent a family respectively. It suggests that the relationship of those four families is ((Siphlonuridae+Ephemeridae)+Heptageniidae)+Vietnamellidae. The result here is very provisional and need further discussion because of our limited taxa in this study. Besides, the phylogeny tree of those taxa based on the 13 mitochondrial PCGs is same as that on 12 mitochondrial PCGs (without atp8), which seems to imply that atp8 gene have no contribution to the phylogenetic topology. Finally, based on the known trees and analysis methord used to discuss the phylogeny of Ephemeroptera, the outgroup, PCGs choice and ingroup number may be the key factors affecting the outcome topology.