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 p hylogeny 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 17S produces a unique phylogeny. If either hy pothesis is valid, then additional genes would need to be examined to test morphological or 18S rDNA phylogenies. Third, the 12S tree, like the 16S tr ee, provides a weakly resolved phylogeny suggesting that neither gene is us eful in resolving higher order relationships. Fourth, the 12S tree supports the morphological and 18S phylogenies, suggesting that the 16S is not usef ul for resolving higher ol del relationships. The results of the study supp ort the 3rd hypothesis; both mitochondrial rDNA genes provided weak phyloge netic support among all but the most recently evolved groups of species. Al though the 17S and 16S genes resolved intrageneric and intraspecific relati onships, the lack of resolution among higher taxonomic groups appears to be caused by homoplasy arising from the high AT content and a high overall mu tation rate in the tick mitochondrial genome.