Phylogenetic relationships of the lancelets of the genus Branchiostoma in China inferred from mitochondrial genome analysis

It is commonly accepted that the Branchiostoma lancelets in South and North China belong to Branchiostoma belcheri (Gray, 1847) and Branchiostoma belcheri tsingtauense (Tchang and Koo, 1934), respectively. Three partial mitochondrial DNA (mtDNA) fragments of cytochrome oxidase c subunit I (COI), cytochrome b (Cytb), and 16S ribosomal RNA (16S rRNA) genes were sequenced to analyze phylogenetic relationships of the Branchiostoma lancelets from South (Xiamen) and North (Qingdao and Rizhao) China, and phylogenetic trees constructed also included the existing data from Japanese waters. The genetic distances of the lancelets between South and North China averaged 0.19, 0.21, and 0.17 based on partial sequences analysis of COI, Cytb, and 16S rRNA genes, respectively, which were much higher than those observed in other intraspecific variations. However, the value between North China and Japanese waters was only 0.01 based on partial sequences analysis of three mtDNA genes, which indicated low intraspecific genetic divergence existed in the two areas. The results also clearly indicated two monophyletic clades (clade A (North China and Japanese waters), clade B (South China)) existed in the specimens, corresponding to the South and North China, respectively. Above all, our results indicate that the Branchiostoma lancelets in South and North China should belong to different species, and the original subspecies B. belcheri tsingtauense together with the lancelets in most Japanese waters is an independent species. According to the rule of priority and present studies, the B. lancelets in North China and most Japanese waters should be revised to Branchiostoma japonicus. The divergence time between B. belcheri and B. japonicus was estimated at about 39.90 - 43.24 million years ago.