Adaptive evolution characteristics of mitochondrial genomes in genus Aparapotamon (Brachyura, Potamidae) of freshwater crabs.

BACKGROUND: Aparapotamon, a freshwater crab genus endemic to China, includes 13 species. The distribution of Aparapotamon spans the first and second tiers of China's terrain ladder, showing great altitudinal differences. To study the molecular mechanisms of adaptive evolution in Aparapotamon, we performed evolutionary analyses, including morphological, geographical, and phylogenetic analyses and divergence time estimation. We sequenced the mitogenomes of Aparapotamon binchuanense and Aparapotamon huizeense for the first time and resequenced three other mitogenomes of Aparapotamon grahami and Aparapotamon gracilipedum. These sequences were combined with NCBI sequences to perform comparative mitogenome analysis of all 13 Aparapotamon species, revealing mitogenome arrangement and the characteristics of protein-coding and tRNA genes. RESULTS: A new species classification scheme of the genus Aparapotamon has been detected and verified by different aspects, including geographical, morphological, phylogenetics and comparative mitogenome analyses. Imprints from adaptive evolution were discovered in the mitochondrial genomes of group A, including the same codon loss at position 416 of the ND6 gene and the unique arrangement pattern of the tRNA-Ile gene. Multiple tRNA genes conserved or involved in adaptive evolution were detected. Two genes associated with altitudinal adaptation, ATP8 and ND6, which experienced positive selection, were identified for the first time in freshwater crabs. CONCLUSIONS: Geological movements of the Qinghai-Tibet Plateau and Hengduan Mountains likely strongly impacted the speciation and differentiation of the four Aparapotamon groups. After some group A species dispersed from the Hengduan Mountain Range, new evolutionary characteristics emerged in their mitochondrial genomes, facilitating adaptation to the low-altitude environment of China's second terrain tier. Ultimately, group A species spread to high latitudes along the upper reaches of the Yangtze River, showing faster evolutionary rates, higher species diversity and the widest distribution.