Comparative analysis of mitochondrial genome of a deep-sea crab Chaceon granulates reveals positive selection and novel genetic features

Deep-sea organisms survive in an extremely harsh environment. There must be some genetic adaptation mechanisms for them. We systematically characterized and compared the complete mitochondrial genome (mitogenome) of a deep-sea crab (Chaceon granulates) with those of shallow crabs. The mitogenome of the crab was 16 126 bp in length, and encoded 37 genes as most of a metazoan mitogenome, including 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes, and 2 ribosomal RNA (rRNA) genes. The gene arrangement and orientation was conserved in the crabs. However, a unique mitogenome element regulator, the origin of light-strand replication (OL), was firstly predicted in the present crab mitogenome. In addition, further positive selection analysis showed that two residues (33S in ND3 and 502I in ND5) in C. granulates mitogenome were positively selected, indicated the selective evolution of the deep-sea crab. Therefore, the mitogenome of deep-sea C. granulates showed a unique OL element and positive selection. These special features would influence the mitochondrial energy metabolism, and be involved in the adaptation of deep-sea environment, such as oxygen deficits and low temperatures.