Phylogeny and Geographical Derivation of Genus Hildenbrandia Based on Morphological and Molecular Evidence

The marine and freshwater Hildenbrandia samples were collected from diverse locations of China and Japan, and morphological characterization and molecular phylogenetic analysis were conducted on these samples. Morphological measurements of freshwater specimens were consistent with the results of H. jigongshanensis, whereas cell dimensions of marine specimens were slightly larger than those of widely distributed H. rubra and H. crouanii. Phylogenetic trees based on rbcL, psbA and UPA sequences were consistent. Freshwater specimens collected in this study formed an independent clade with H. jigongshanensis supported by high values. Species attribution was confirmed further by the similar ITS2 secondary structures among the samples of H. jigongshanensis. It was found that the intraspecific divergence of rbcL gene in H. jigongshanensis was smaller than other two freshwater species H. rivularis and H. angolensis. Phylogenetic trees showed that marine specimens in this study grouped together with H. rubra samples of North America, and were different with the H. rubra samples of Europe. Combining the comparison results of CBC numbers in ITS2 secondary structure, we propose the marine samples collected in this study to be a new species Hildenbrandia qingdaoensis sp. nov. The separation of the marine and freshwater specimens was supported by the rbcL, UPA and psbA phylogeny as well as ITS2 secondary structure inference. Biogeographical reconstruction showed the ancestor of freshwater Hildenbrandia might derive from South America and dispersed from North America to Europe and then to Asian countries, which needs to be verified further with more samples and molecular evidences from South America.