The Complete Mitochondrial Genomes of Two Octopods Cistopus chinensis and Cistopus taiwanicus: Revealing the Phylogenetic Position of the Genus Cistopus within the Order Octopoda

In the present study, we determined the complete mitochondrial DNA (mtDNA) sequences of two species of Cistopus, namely C. chinensis and C. taiwanicus, and conducted a comparative mt genome analysis across the class Cephalopoda. The mtDNA length of C. chinensis and C. taiwanicus are 15706 and 15793 nucleotides with an AT content of 76.21% and 76.5%, respectively. The sequence identity of mtDNA between C. chinensis and C. taiwanicus was 88%, suggesting a close relationship. Compared with C. taiwanicus and other octopods, C. chinensis encoded two additional tRNA genes, showing a novel gene arrangement. In addition, an unusual 23 poly (A) signal structure is found in the ATP8 coding region of C. chinensis. The entire genome and each protein coding gene of the two Cistopus species displayed notable levels of AT and GC skews. Based on sliding window analysis among Octopodiformes, ND1 and DN5 were considered to be more reliable molecular beacons. Phylogenetic analyses based on the 13 protein-coding genes revealed that C. chinensis and C. taiwanicus form a monophyletic group with high statistical support, consistent with previous studies based on morphological characteristics. Our results also indicated that the phylogenetic position of the genus Cistopus is closer to Octopus than to Amphioctopus and Callistoctopus. The complete mtDNA sequence of C. chinensis and C. taiwanicus represent the first whole mt genomes in the genus Cistopus. These novel mtDNA data will be important in refining the phylogenetic relationships within Octopodiformes and enriching the resource of markers for systematic, population genetic and evolutionary biological studies of Cephalopoda.