The Mitochondrial Genome Sequences Of Isonychia Ignota And Siphluriscus Chinensis

The phylogeny of Ephemeroptera in Pterygota is still controversial. Besides three previous hypotheses of the winged insects (Palaeoptera= Ephemeroptera+odonata, Metapterygota= odonata+Pterygota, and Chiastomyaria= Pterygota+ Ephemeroptera), another point of view has been proposed in recent studies, suggesting that Ephemeroptera is the sister group of Plecoptera. This opinion challenges the widely accepted monophyletic Neoptera. Furthermore in the Ephemeroptera, the phylogenetic positions of the Isonychiidae and Siphluriscidae are not stable either.In this study, we sequenced the mitochondrial genomes of Isonychia ignota (Ephemeroptera:Isonychiidae) and Siphluriscus chinensis (Ephemeroptera: Siphiuriscidae). The circular genome of Isonychia ignota is 15105 bp in size and encodes 37 genes (13 protein-coding genes,2 ribosomal RNAs, and 22 transfer RNAs). The overall A+T content of Isonychia ignota is 61.7%, which is the lowest record in known insect mtDNA genomes. Except a partial control region, the obtained sequence of Siphluriscus chinensis mtDNA genome is 16616 bp in length. Within it, an extra tRNALys (AAA) locates in N-strand between tRNASer (AGN) and tRNAGlu. Most protein coding genes in two species use standard initiation codons. The exceptions are nad2 and nad5 starting with GTG in Isonychia ignota and TTG for nadl in Siphluriscus chinensis. A peculiar strand skew bias is observed in sequenced ephemeropteran species. The PCGs on the J-strand contain more thymines and cytosines than adenines and guanines, unlike the majority of hexapodan species in which adenines outnumber thymines. This phenomenon may result from the lower overall A+T content in Ephemeroptera.The amino acid sequences of 13 PCGs and their nucleotides in mtDNA genome of 29 species (2 mayflies in this study,25 other insects and 2 crustaceans downloaded from GenBank, the latter two as outgroup) are used here to reconstruct the phylogeny of Ephemeroptera. Both maximum likelihood and Bayesian analyses show same topology. It indicates that Ephemeroptera+Plecoptera is the sister group of other winged insects, the traditional "Neoptera" is not a Monophly. In the order Ephemeroptera, the Siphluriscidae diverges first from other mayflies, and Isonychiidae and Heptageniidae are grouped together.