Complete chloroplast genome sequence and phylogenetic analysis of wasabi (Eutrema japonicum) and its relatives

In Japan, two Eutrema species, wasabi (Eutrema japonicum, the important traditional Japanese condiment) and yuriwasabi (E. tenue), have been recognized as endemic species. We sequenced complete chloroplast (cp) genomes of seven wasabi and yuriwasabi accessions from Japan to study their phylogeny and evolution, using molecular dating of species divergence. Phylogenetic analyses of the complete cp DNA of these two Japanese species and five other Eurasian Eutrema species revealed that wasabi and yuriwasabi did not form a monophyletic group. One yuriwasabi accession (Gifu) formed a Glade with E. yunnanense from China, indicating that this accession should be considered as a different species from the other yuriwasabi accessions. We reveal that Japanese Eutrema species diverged from the 'E. yunnanense-yuriwasabi (Gifu)' Glade approximately 1.3 million years ago (Mya), suggesting that the connection between Japan and the Eurasian continent has existed more recently than the Quaternary period. The abundance of cp sequence data in this study also allowed the detection of genetic differentiation among wasabi cultivars. The two polymorphic sites detected between Tujidaruma' and `Shimane No.3' were used to develop genotyping markers. The cp genome information provided here will thus inform the evolutionary histories of Japanese Eutrema species and help in genotyping wasabi cultivars.