Genetic Differentiation, Molecular Phylogenetic Analysis, and Ethnobotanical Study of Eutrema japonicum and E. tenue in Japan and E. yunnanense in China

This paper reports the level of genetic differentiation between two Japanese and one Chinese species of Eutrema: E. japonicum, "wasabi"; its wild relative in Japan, E. tenue; and their wild relative in China, E. yunnanense. Phylogenetic analyses were based on the DNA sequence of the chloroplast trnK/matK region of 16 Brassicaceae and an outgroup species. Neighbor joining (NJ) and maximum parsimony (MP) trees were constructed, revealing that the three Eutrema species form a single Glade clearly separated from other Brassicaceae species. The two Japanese Eutrema species are highly differentiated from Chinese E. yunnanense, and it is estimated that they diverged from E. yunnanense approximately 5 million years ago. An ethnobotanical survey was conducted among ethnic Chinese in Yunnan Province, and the results indicate that E. yunnanense is not perceived as "hot" in taste, while a pungent flavor is associated with wasabi; in addition, no evidence was found for the domestication of E. yunnanense. On the basis of the present molecular phylogenetic study and the ethnobotanical survey, we conclude that wasabi acquired its specific pungent flavor during its long botanical history in Japan, and that its subsequent domestication in Japan was because of this acquired pungent flavor. The culinary habit of using wasabi with raw fish has since become an important feature of Japanese cuisine and culture.