The complete mitochondrial DNA sequence of the basal hexapod Tetrodontophora bielanensis: evidence for heteroplasmy and tRNA translocations.

We present the complete 15,455-nt mitochondrial DNA sequence of the springtail Tetrodontophora bielanensis (Arthropoda, Hexapoda, Collembola). The gene content is typical of most metazoans, with 13 protein-coding genes (PCGs), 2 genes encoding for ribosomal RNA subunits, and 22 tRNA genes. The nucleotide sequence shows the well-known A+T bias typical of insect mtDNA; its A+T content is lower (72.7%) than that observed in other insect species, but still higher than that in other arthropodan taxa. The bias appears to be uniform across the whole molecule, unlike other insect taxa, which show increased A+T content in the so-called A+T-rich region. However, the bias is slightly higher in the third codon positions of the PCGs (81.4%). Anomalous initiation codons have been observed in the nad2 and the cox1 genes. In the latter, the ATTTAA hexanucleotide is suggested to be involved in the initiation signaling. All tRNAs could be folded into the typical cloverleaf secondary structure, but the tRNA for cysteine appears to be missing the DHU arm. Long tandemly repeated regions (193 nt) were found in the A+T-rich region, which in turn was shown to have the possibility of forming a complex array of secondary structures. One of these structures encompassed the junction between the repeats. The A+T-rich region was also interesting in that it showed heteroplasmy in the number of repeats. Three haplotypes were found, possessing 2, 3, and 4 identical repeats, respectively. The order of protein coding and rRNA genes in the molecule was determined and was identical to that of all insects studied so far. However, two tRNA translocations were found which were unprecedented among Arthropoda. These involved the trnQ, which was found between the rrnS and the A+T-rich region, and the trnS(ucn), which was located between trnM and trnI. A preliminary phylogenetic analysis based on the amino acid sequence of the PCGs failed to find support for the monophyly of Hexapoda.