The mitochondrial genomes of soft ticks have an arrangement of genes that has remained unchanged for over 400 million years.

There are two major groups of ticks: soft ticks and hard ticks. The hard ticks comprise the prostriate ticks and the metastriate ticks. The mitochondrial (mt) genomes of one species of prostriate tick and two species of metastriate ticks had been sequenced prior to our study. The prostriate tick has the ancestral arrangement of mt genes of arthropods, whereas the two metastriate ticks have rearrangements of eight genes and duplicate control regions. However, the arrangement of genes in the mt genomes of soft ticks had not been studied. We sequenced the mt genomes of two species of soft ticks, Carios capensis and Ornithodoros moubata, and a metastriate tick, Haemaphysalis flava. We found that the soft ticks have the ancestral arrangement of mt genes of arthropods, whereas the metastriate tick, H. flava, shares the rearrangements of mt genes and duplicate control regions with the other two metastriate ticks that have previously been studied. Our study indicates that gene rearrangements and duplicate control regions in mt genomes occurred once in the most recent common ancestor of metastriate ticks, whereas the ancestral arrangement of arthropods has remained unchanged for over 400 million years in the lineages leading to the soft ticks and the prostriate ticks.