Molecular identification of alien species of Vallisneria (Hydrocharitaceae) species in Japan with a special emphasis on the commercially traded accessions and the discovery of hybrid between nonindigenous V. spiralis and native V. denseserrulata

The aquatic plant genus Vallisneria, which includes several invasive species, is frequently used as ornamental plants in aquaria. Naturalized populations of Vallisneria have been discovered in Japan and are believed to have escaped from aquaria. We identified these naturalized Vallisneria using the internal transcribed spacer regions of nuclear ribosomal DNA (nrITS). Two accessions of nonnative Vallisneria were determined to have invaded into Japan: one is a hybrid between Vallisneria spiralis native to Eurasia and Vallisneria denseserrulata native to China and Japan, and another is Vallisneria australis native to Australia. The invasive nature of the former accession may have resulted from hybrid vigor. We also sequenced the nrITS regions of the accessions commercially traded in Japan. The two naturalized accessions were genetically identical to two of those circulating in the Japanese market as ornamental plants for aquaria. These invasive accessions propagate vegetatively and would profoundly influence native water ecosystems in Japan. We seek to arouse attention regarding the risks posed by these invasive Vallisneria in Japan, as well as other areas.