Comparison of the mitochondrial 12S and 16S ribosomal DNA genes in resolving phylogenetic relationships among hard ticks (Acari: Ixodidae)

Domain III of the mitochondrial 12S rDNA was examined in 51 tick species to test discrepancies between a previously published 16S mitochondrial rDNA phylogeny and phylogenies based on morphology and the 18S nuclear rDNA gene. There were 4 possible hypotheses. First, the 12S and the 16S genes support identical trees. Second, the 12S produces a unique phylogeny. If either hypothesis is valid, then additional genes would need to be examined to test morphological or 18S rDNA phylogenies. Third, the 12S tree, like the 16S tree, provides a weakly resolved phylogeny suggesting that neither gene is useful in resolving higher order relationships. Fourth, the 12S tree supports the morphological and 18S phylogenies, suggesting that the 16S is not useful for resolving higher order relationships. The results of the study support the 3rd hypothesis; both mitochondrial rDNA genes provided weak phylogenetic support among all but the most recently evolved groups of species. Although the 12S and 16S genes resolved intrageneric and intraspecific relationships, the lack of resolution among higher taxonomic groups appears to be caused by homoplasy arising from the high A/T content and a high overall mutation rate in the tick mitochondrial genome.