Rehmannia japonica (Thunb.) Makino ex T. Yamaz. is an endangered perennial herb species in Japan. Although earlier the Japanese considered it a variety of R. glutinosa, recent Japanese taxonomists have consistently regarded it as an independent species. According to the historical literature, Rehmannia japonica seems to have been known in China and Japan in the past. However, Chinese taxonomists do not recognize R. japonica at present. In Japan, only two populations are known, and although these populations flower every year, seed reproduction has not been observed. In this study, we aimed to reveal the phylogenetic relationships and levels of genetic diversity of R. japonica. A haplotype network based on two chloroplast DNA regions (trnL-trnF and rps16) showed that the sequences of R. japonica were distinguishable by three or four sites of indels from the most closely related species, R. chingii, consistent with the separate species status of R. japonica. An analysis of genetic diversity using twelve microsatellite loci showed that all of the ramets of R. japonica collected from two geographically isolated populations had an identical multilocus genotype, including identical heterozygous genotypes at six loci. This result indicated asexual origin of all sampled ramets. This study also suggests that the absence of sexual reproduction of R. japonica is explained by self-incompatibility combined with only a single genet remaining in the R. japonica populations.
Keywords: Clonal plant; Conservation; Historical transplantation; Microsatellite; Rehmannia chingii; Rehmannia japonica.