New Sedum species discovered on Tokara Islands and Yoron-jima Island

Assistant Professor Takuro Ito and Professor Masayuki Maki of the Botanical Gardens of the Center for Academic Resources and Archives, Tohoku University; Professor Emeritus Masatsugu Yokota of the University of the Ryukyus; Professor Akitoshi Iwamoto and Graduate Student Mizuho Endo of Kanagawa University; and Senior Curator Goro Kokubugata of the National Museum of Nature and Science have documented that plants distributed on the Tokara Islands of Akuseki-jima, Gaja-jima, and Naka-no-shima and Yoron-jima Island in the Satsunan Islands, previously identified as Sedum formosanum of the family Crassulaceae, genus Sedum, are a new species endemic to the region, which they have named Satsunan-mannen-gusa (Sedum diversiflorum). This discovery represents an important finding indicating the potential presence of previously unrecognized taxonomic groups in the island regions of the Japanese Archipelago. The results were published online in the Nordic Journal of Botany.

The genus Sedum (family Crassulaceae), a succulent plant widely distributed primarily in the Northern Hemisphere, is known as one of the plant groups that is difficult to classify because it contains many species with similar morphology, and the morphology of the same species can change significantly depending on the growing environment.

The research group conducted field surveys of the plants identified as Sedum formosanum distributed on Akuseki-jima Island and Yoron-jima Island, and performed taxonomic re-examination based on morphology, flowering period, and genetic information.

Comparison of morphology and flowering period revealed that this plant, in addition to its autumn-flowering characteristic of blooming from October to December, differs markedly from S. formosanum and related species distributed in other regions in the shape of its flowers, leaves, and fruits, as well as in the color of its anthers and stems, and branching patterns. Furthermore, it was confirmed to possess extremely rare characteristics among Sedum species, including instability in merosity with flowers showing variation in the number of pistils, stamens, petals, and sepals within the same individual, and the presence of functional female and male flowers lacking stamens or pistils, respectively.

Genetic analysis revealed that this plant has nuclear DNA sequences phylogenetically close to S. danjoense and maternally inherited chloroplast DNA sequences close to S. formosanum subsp. miyakojimense. This pattern, showing different phylogenetic positions between nuclear and chloroplast DNA sequences, suggests the possibility that a phenomenon called chloroplast capture occurred, in which the chloroplast genome was replaced from one species to another through past hybridization between the two species. Morphologically, this plant also exhibits combined characteristics of the two species considered to be the hybrid parents, including the autumn-flowering trait of S. danjoense and the yellow anthers and stem color observed in S. formosanum subsp. miyakojimense. Regarding floral structure, while S. danjoense is fundamentally tetramerous (4 pistils, petals, and sepals; 8 stamens) and S. formosanum subsp. miyakojimense is pentamerous (5 pistils, petals, and sepals; 10 stamens), the new species exhibits a mixture of irregular merosity in its flowers, potentially reflecting trait instability derived from hybridization. Such phenomena, where hybridization between species with different merosity brings instability to floral merosity, have been similarly noted in Saxifragaceae plants, and the new species may be another example.

In addition to Akuseki-jima Island and Yoron-jima Island, field surveys were similarly conducted on Naka-no-shima Island, where specimens of similar plants had actually been collected, but the new species could not be confirmed. Additionally, field surveys on Gaja-jima Island were abandoned because it is an uninhabited island with difficult access. However, the morphological characteristics of specimens collected from these islands matched those of individuals obtained from Yoron-jima Island and Akuseki-jima Island, suggesting they are the same plant. It was also confirmed that S. formosanum distributed on other islands in the Satsunan Islands is a known species with wide distribution.

Based on these comprehensive results, the researchers determined that these plants distributed in the Tokara Islands (Akuseki-jima, Gaja-jima, and Naka-no-shima) and Yoron-jima Island in the Satsunan Islands represent a previously unrecognized, undescribed taxonomic group, and described them as the new species Satsunan-mannen-gusa (scientific name: Sedum diversiflorum).

Journal Information
Publication: Nordic Journal of Botany
Title: Sedum diversiflorum sp. nov. (Crassulaceae), a new species with variable merosity from the Satsunan Islands, Kagoshima, Japan
DOI: 10.1002/njb.04877