Evolution of the Demodex gene arrangement from the ancestral chelicerate. Circular mitochondrial genomes are depicted as linearized, starting at the 5′ end of the COX1 gene. The one letter amino acid code is used to designate the tRNA genes, with the exceptions that L1 = CUN; L2 = UUR; S1 = AGN; and S2 = UCN. The putative control region is designated as A + T to indicate that this has a higher AT% than the rest of the genome. Genes above the medial line are encoded on one strand, while those below the line are encoded on the other strand. As shown, the arrangement of protein-coding plus rRNA genes can be derived via one block interchange, with a minimum of three breakpoints. This could have occurred via a tandem duplication followed by differential loss of genes between the two tandem copies. Alternatively, it could have resulted from direct translocation, either of the 12S gene or of the interval that stretches from ND5 to the 16S gene. The tRNA genes seem to have moved independently of the other genes, since it would require a minimum of 15 breakpoints to explain the evolution of the Demodex arrangement if tRNA genes are included in the analysis.