The Complete Mitochondrial Genome Sequences Of Drunella Cryptomeria And Potamanthellus Edmundsi Plus A Preliminary Research On The Phylogeny Of Mayflies

In reconstructing origin and evolution of the winged insects (Insecta:Pterygota), the mayflies (Ephemeroptera), which have a long series of plesiomorphic and automorphic characters, always play a key role. So far, although several suggestions have been proposed upon both morphological and molecular evidences, the phylogenetic position of the Ephemeroptera in the Pterygota is still controversial. The phylogenetic hypotheses of the Ephemerellidae and Neoephemeridae in the order are also instable. Mitogenomic data, especially complete mitochondrial genome sequences (mtDNA), have become popular molecular markers in the researchs of insect phylogeny and evolution. In most cases, they can clarify the evolutionary history of given taxa.In this study, the mitogenomes of mayflies Drunella cryptomeria (Ephemerellidae) and Potamanthellus edmundsi (Neoephemeridae) are sequenced. The mitogenome of the former species is a typical circular DNA molecule of 15,379 bp in length, and contains 37 genes (13 PCGs,22 tRNAs,2 rRNAs) and an A+T-rich region, identical to common metazoans. The nearly complete mitochondrial genome of the latter is 14,227 bp in length, the trnM、trnQ、trnl genes and A+T-rich region sequences are not obtained. The gene orders and orientation of both mitogenomes are identical to other primitive hexapodans. This situation may illustrate the primitive status of mayfly. All tRNAs can be folded into the typical cloverleaf-like secondary structure, without gene rearrangement. Compared to other known mayfly mitogenomes, the nucleotide composition of D.cryptomeria mitogenome is biased toward A+T (65.0%), which is the lowest. But that data of the species P.edmundsi is the highest (72.8%). Like other mayflies but unlike other insects, the J-strand of two mayfly mitogenomes here has both negative AT-skew and negative GC-skew. The AT-skew is usually positive in other insects.The nucleotide sequences of 13 PCGs in mitogenomes of 13 mayfly species and other 24 insect species are adopted as in-groups to reconstruct the phylogeny. Bayesian Inference (BI) and Maximum Likelihood (ML) analyses are employed to generate topological trees under five different strategies. Our results show that:1) the Odonata is the first diverged clade of winged insects and "Palaeoptera" does not exist. Ephemeroptera is the sister-group of the Neoptera.2) In the monophyletic order Ephemeroptera, the species Siphluriscus chinensis is the most plesiomorphic.3) The positions of Neoephemeridae and Ephemerellidae are different from previous reports. In our study, the P.edmundsi (Neoephemeridae) is not a sister group of Caenidae, and 4) the position of D.cryptomeria (Ephemerellidae) is within the Haptageniidae.5) In our topological trees, Archaeogatha and Zygentoma are always grouped together as sister-groups, which was also reported before by others. Traditionally, the Zygenoma is regarded as the sister group of the Pterygota. This situation may results from the unstable Tricholepidion gertschi or the exact relationship between Archaeogatha and Zygentoma needs more investigations.6) Comparing obtained trees of different analyses, we find that generally the involved taxon number, different analyzing methods and outgroups can influence the structures of topological trees in molecular systematics. So if possible, in order to get more stable and credible phylogeny, we should use more in-groups, better out-groups and diverse analysis methods and stategies in the research.