Morphological and molecular comparisons of dominant amphioxus populations in the China Seas

The amphioxi Branchiostoma belcheri, Branchiostoma japonicum and Branchiostoma malayanum are recorded from the coast of China Seas. Six amphioxus populations comprising three Branchiostoma species collected from Hong Kong (southern China), Xiamen (south-eastern China) and Qingdao (northern China) were compared at the morphological and molecular levels. Phylogenetic separations among the species and geographic populations were evaluated by the analysis of 12S ribosomal RNA gene sequences and amplified fragment length polymorphism (AFLP) markers. From morphological characters, B. belcheri is more closely related to B. japonicum than to B. malayanum. However, phylogenetic affinities indicated by both 12S rRNA gene sequences and AFLP analysis showed a more recent phylogenetic splitting of B. belcheri and B. malayanum than that of B. belcheri and B. japonicum. In the AFLP similarity tree, geographic populations of B. japonicum were clustered into different clades. The AFLP data also showed that both B. belcheri and B. japonicum populations in Hong Kong have the highest levels of within-population genetic diversity as compared to that in Xiamen and Qingdao, suggesting that genetic diversity of Branchiostoma decreases from low to high latitudes. Results of hierarchical analysis of molecular variance (AMOVA) also revealed a high level of genetic diversity either for the three B. japonicum populations or the two B. belcheri populations in China Seas. However, genetic variation among the three B. japonicum populations was insignificant, indicating that these populations are genetically connected.