Large dataset of octocoral mitochondrial genomes provides new insights into mt-mutS evolution and function

All studied octocoral mitochondrial genomes contain a gene from the MutS family, whose members code for proteins involved in DNA mismatch repair, other types of DNA repair, meiotic recombination, and other functions. Although mutS homologues are found in all domains of life as well as viruses, octocoral mt-mutS is the only such gene encoded in an organellar genome. While the function of mtMutS is not known, its domain architecture, conserved sequence, and presence of some characteristic residues suggest its involvement in mitochondrial DNA repair. This inference is supported by exceptionally low rates of mt-sequence evolution observed in octocorals. Previous studies of mt-mutS have been limited by the small number of octocoral mt-genomes available. We utilized sequence-capture data from the recent Quattrini et al. study to assemble complete mitochondrial genomes for 97 species of octocorals. Combined with sequences publicly available in GenBank, this resulted in a dataset of 184 complete mitochondrial genomes, which we used to re-analyze the conservation and evolution of mt-mutS. We discovered the first case of mt-mutS loss among octocorals in one of the two Pseudoanthomastus sp. assembled from Quattrini et al. data. This species displayed accelerated rate and and changed patterns of nucleotide substitutions in mt-genome, which we argue provide additional evidence for the role of mtMutS in DNA repair. In addition, we found accelerated mt-sequence evolution in the presence of mt-mutS in several octocoral lineages. This accelerated evolution did not appear to be the result of relaxed selection pressure and did not entail changes in patterns of nucleotide substitutions. Overall, our results support previously reported patterns of conservation in mt-mutS and suggest that mtMutS is involved in DNA repair in octocoral mitochondria. They also indicate that the presence of mt-mutS contributes to, but does not fully explain, the low rates of sequence evolution in octocorals