An investigation of the variation in the transition bias among various animal mitochondrial DNA

The transition : transversion ratio (ts / tv) is known to be very high in human mitochondrial DNA, but we have little information about this ratio in other species. Here we investigate the transition bias in animal mitochondrial DNA using single nucleotide polymorphism data at four-fold degenerate sites. We investigate this pattern of polymorphism in the cytochrome b gene (cyt-b) in 70 species using a total of 1823 mutations. We show that most species show a bias towards transitions but that the ratio varies significantly between species. There is little evidence for variation within orders or genera and between closely related species such as the great apes. The majority of the variation appears to be at a higher phylogenetic levels: between orders and classes. We test whether the variation in ts / tv ratio could be due to variation in the metabolic rate by considering whether the ratio is correlated to base composition. We find no evidence that the metabolic rate affects the ts / tv ratio. We also investigate the relative frequencies of C to T or T to C (C ¿ T) mutations and A to G or G to A (A ¿ G) mutations. We show that overall they occur at significantly different frequencies, and that there is significant variation in their relative frequency between species and between classes. We find no evidence in support of the hypothesis that this variation could be due to different metabolic rates.