Mito-communications.

It has long been known that the mitochondrial genome normally mutates at a higher rate than the nuclear genome. Direct estimates of mitochondrial mutation rates can be made by sequencing individuals from mutation-accumulation lines, for which the evolutionary timescale is known exactly. Using such an approach, estimates of mutation rates have been made for a small number of species (e.g. Denver et al. 2000; Haag-Liautard et al. 2008; Howe et al. 2010). Xu et al. (2012) have estimated mitochondrial mutation rates in the water flea, Daphnia pulex, using mutation-accumulation lines. They found no significant difference between the rates in asexual and sexual lineages, which had respective mean rates of 1.37 £ 10 and 1.73 £ 10 mutations/site/generation. Assuming a generation time of 10–13 days, these translate into rates of about 4.35 £ 10 and 5.49 £ 10 mutations/site/year, respectively. These rate estimates are higher than those from Caenorhabditis and Drosophila. The 23 observed mutations comprised 17 indels and 6 nucleotide replacements. As with previous direct estimates of mitochondrial mutation rates, Xu et al. (2012) found that the rates in Daphnia greatly exceeded those obtained in phylogenetic studies. For example, their estimates are nearly 400 times higher than the ‘standard’ evolutionary rate for arthropods (Brower 1994). This disparity in rates has been observed in a range of organisms and can have a substantial impact on the analyses of evolutionary timescales (Howell et al. 2003; Ho and Larson 2006). The use of next-generation sequencing techniques will enable the production of large datasets for estimating mutation rates. This will improve estimation precision, allowing better characterisation of the patterns and rates of mutation in different species.