The mitochondrial genome of Sinentomon erythranum (Arthropoda: Hexapoda: Protura): An example of highly divergent evolution

Background The phylogenetic position of the Protura, traditionally considered the most basal hexapod group, is disputed because it has many unique morphological characters compared with other hexapods. Although mitochondrial genome information has been used extensively in phylogenetic studies, such information is not available for the Protura. This has impeded phylogenetic studies on this taxon, as well as the evolution of the arthropod mitochondrial genome. Results In this study, the mitochondrial genome of Sinentomon erythranum was sequenced, as the first proturan species to be reported. The genome contains a number of special features that differ from those of other hexapods and arthropods. As a very small arthropod mitochondrial genome, its 14,491 nucleotides encode 37 typical mitochondrial genes. Compared with other metazoan mtDNA, it has the most biased nucleotide composition with T = 52.4%, an extreme and reversed AT-skew of -0.351 and a GC-skew of 0.350. Two tandemly repeated regions occur in the A+T-rich region, and both could form stable stem-loop structures. Eighteen of the 22 tRNAs are greatly reduced in size with truncated secondary structures. The gene order is novel among available arthropod mitochondrial genomes. Rearrangements have involved in not only small tRNA genes, but also PCGs (protein-coding genes) and ribosome RNA genes. A large block of genes has experienced inversion and another nearby block has been reshuffled, which can be explained by the tandem duplication and random loss model. The most remarkable finding is that trnL2(UUR) is not located between cox1 and cox2 as observed in most hexapod and crustacean groups, but is between rrnL and nad1 as in the ancestral arthropod ground pattern. The "cox1-cox2" pattern was further confirmed in three more representative proturan species. The phylogenetic analyses based on the amino acid sequences of 13 mitochondrial PCGs suggest S. erythranum failed to group with other hexapod groups. Conclusions The mitochondrial genome of S. erythranum shows many different features from other hexapod and arthropod mitochondrial genomes. It underwent highly divergent evolution. The "cox1-cox2" pattern probably represents the ancestral state for all proturan mitogenomes, and suggests a long evolutionary history for the Protura.