Excess of non-synonymous mutations and structural variations in mitogenomes of freshwater snails in the genus <i>Semisulcospira</i>

Mitochondrial DNA has unique biological characteristics, such as a high mutation rate and clonal inheritance, which make it an appropriate marker in molecular biology. However, exceptionally high mitochondrial divergences have been reported in terrestrial and marine invertebrates, plants, and certain vertebrate lineages, which complicates the interpretation of the results in DNA barcoding and evolutionary studies. The freshwater snails in the genus <i>Semisulcospira</i> have two mitochondrial DNA lineages: a normal mitochondrial lineage (mt-A type) and a highly divergent 'enigmatic' lineage (mt-B type). We assembled thirty-two mitogenomes of <i>Semisulcospira</i> to understand the molecular evolution of the mt-B type. We found that the mt-B type evolves faster than the mt-A type, has more non-synonymous mutations, and exhibits structural variations with truncated <i>cob</i> and <i>cox3</i> genes. Our results showed that the observed molecular evolution of the mt-B type could result from a decreased efficiency of natural selection due to small population size and/or the accumulation of slightly deleterious mutations caused by mitochondrial sex ratio distortion.