DNA sequence evolution in fast evolving mitochondrial DNA nad1 exons in Geraniaceae and Plantaginaceae

Previously, nucleotide substitution rates in mitochondrial DNA of Geraniaceae and Plantaginaceae have been shown to be exceptionally high compared with other angiosperm mtDNA lineages. It has also been shown that mtDNA introns were lost in Geraniaceae and Plantaginaceae. In this study we compile 127 DNA sequences from two partial exons of the mtDNA nad1 gene in Geraniaceae, Plantaginaceae, and other angiosperm groups for which rate accelerations have not been reported, to assess the extent and nature of the nucleotide substitution rate acceleration. Whereas Litorella appears to have undergone a rate acceleration comparable to that observed in Plantago, the Geraniacean sister group representative Hypseocharis biloba has not, indicating that the rate change has occurred between the split of Hypseocharis and the rest of the Geraniaceae. Silent/non-silent rate ratios ¿ have decreased threefold in the "fast mtDNA" clades compared with other angiosperms, whereas their codon usage bias is around 20% lower. Absence of RNA editing in Geraniacean and Plantago mtDNA genes is confirmed. Possible causes for the exceptional substitution rate accelerations observed in these lineages are discussed in terms of the retroprocessing process or the possibility of affected mitochondrial DNA polymerase ¿ proofreading accuracy control