Molecular phylogenetic relationship based on mitochondrial genomes within novel deep-sea corals (Octocorallia: Alcyonacea) insights into the slow evolution and adaptation of the extreme deep-sea environment

A total of 10 Alcyonacea corals were collected at depths ranging from 900 m to 1640 m by the manned submersible Shenhai Yongshi during two cruises in the South China Sea (SCS). Phylogenetic distance and average nucleotide identity (ANI) analyses of mitochondrial genomes combined with morphology examination and sclerite scanning showed that the collected samples could be assigned to four suborders - Calcaxonia, Holaxonia, Scleraxonia, and Stolonifera - which might represent 10 novel deep-sea species. The analyses of the dissimilarity of GC skew, phylogenetic distance, and ANI indicated that the evolution of Octocorallia mitochondrial sequences was slow. The nonsynonymous (Ka) and synonymous (Ks) substitution (Ka/Ks) ratios indicated that 14 protein-coding genes (PCGs) were undergoing purifying selection and that the selection pressures might be from specific deep-sea environments. The correlation analysis of median values of Ka/Ks ratio of five gene families and environmental factors showed that the genes encoding cytochrome b (cob) and DNA mismatch repair protein (mutS) might be driven by environmental factors to format deep-sea species. This study highlighted the slow evolution and adaptative mechanism of deep-sea corals in the deep ocean.