Abstract: Exotic plants can become invasive by acquiring genetic diversity and novel traits through hybridization with native species. The spread of Taraxacum officinale in Japan might be an example of this. T. officinale generally reproduce by apomixis, but it produces pollen and rarely hybridizes with native Japanese diploid dandelions. The hybrid dandelions have spread throughout Japan and now outnumber T. officinale. However, where hybrid dandelions originated and how they spread are unknown. Previous studies have shown that there are two types of ploidy in hybrid dandelions: triploid and tetraploid, and triploid hybrids can backcross with native dandelions.
In this study, we investigated the origin of Japanese hybrid dandelions and how they spread in both triploid and tetraploid hybrids. We collected samples from all over Japan and determined the chloroplast haplotype to estimate the origin of the hybrids. Since the mother of hybrid dandelions is always native species, the chloroplasts of hybrids are derived from native species. Therefore, the origin of a hybrid can be inferred by examining which native species the haplotypes of the hybrid match. Next, we used genome-wide SNPs obtained by the GRAS-Di method to determine hybrid clones' diversity and spatial distribution. Since hybrid dandelions generally reproduce by apomixis, the frequency of hybrid formation and the range of migration can be estimated from the clone diversity and distribution range of each clone.
Triploid and tetraploid hybrids differed in origin and the extent of migration. The triploid hybrids had numerous clones, and their distribution ranges were relatively narrow. The triploid hybrids in eastern and western Japan had genes derived from native species of eastern and western Japan, respectively. These results and backcrossing ability in triploid hybrids suggest that triploid hybrids, including backcrosses, were repeatedly formed in different regions and did not spread far from their origin. They may have acquired genes from native species that are adapted to the environment of each region and thus cannot migrate to regions with different environments. In contrast, almost a single clone was dominant throughout Japan in the tetraploid hybrids. No native dandelion was found to have a chloroplast haplotype in common with this clone. This super-clone, which may have originated outside Japan and adapted to the wider environment, may have expanded its distribution. Alternatively, this clone may have been introduced to Japan long before the other hybrids were formed. Further studies are needed to clarify the ecology of each clone.
