The Thismiaceae are a special family of plants that have ceased photosynthesis and therefore do not have green leaves. They also produce strange small flowers near the ground that could be mistaken for mushrooms.
A research group comprising Professor Kenji Suetsugu of the Graduate School of Science at Kobe University, Mr. Yasunori Nakamura, a resident living in Fukuoka Prefecture, Associate Professor Takafumi Nakano of the Graduate School of Science at Kyoto University, and Associate Professor Shuichiro Tagane of Kagoshima University Museum at Kagoshima University discovered a novel plant of the Thismiaceae family in the Kimotsuki Mountains of the Osumi Peninsula (Kagoshima Prefecture). This plant has characteristics different from any known genus. Consequently, a new genus, Relictithismia, was established, and a new species, Relictithismia kimotsukiensis, was described.
The Japanese name "Mujina-no-shokudai" was given to this species, because although it appears to be a species of the genus Thismia, detailed examination has revealed that they are not similar. The last plant that was recognized as a new genus soon after its discovery in Japan and whose genus name is still recognized today is the one found in 1930. R. kimotsukiensis is the discovery of the century in this field. The study findings were published in the Journal of Plant Research.
Two genera (with six species in total) of the family Thismiaceae are known to be native to Japan: T. abei, T. kobensis, and T. tuberculata of the genus Thismia, and O. hyodoi, O. shinzatoi, and O. yamashitae of the genus Oxygyne. All are listed as endangered, and T. tuberculata has been declared extinct. Nakamura, a plant enthusiast living in Fukuoka Prefecture, accidentally discovered a single plant of the family Thismiaceae on the Osumi Peninsula in early June 2022. The plant differed in morphology from known species of this family. He prepared a specimen of this plant and sent it to Tagane at the Kagoshima University Museum.
At first, Tagane thought the plant was an undescribed Thismia species, and for confirmation, sent a photo of it to Suetsugu, an expert on the genus. Suetsugu's study of this plant revealed the possibility that it did not belong to any existing genus, because its six stamens were separate without cylindrical fusion.
An additional survey was carefully carried out around the native habitat, but no similar plants were found. However, without giving up, Nakamura and Tagane re-surveyed the area the following year and found several additional plants in June 2023.
Based on closer examination of the morphology of these specimens, the plants found in the survey confirmed to have short, tuberous, and bead-like roots; one flower per inflorescence; radially symmetrical flowers; a floral tube not divided into multiple chambers; a ring-like structure inside the floral tube; and six separate stamens hanging from it and in contact with the pistil. The combination of these characteristics is not found in any of the five previously known genera of the family Thismiaceae, and it is clear that the plants discovered in this study are distinct from the known plants in existence.
Additionally, phylogenetic analysis of the genomic DNA, performed in collaboration with Nakano of Kyoto University, suggested that the plants discovered in this study are clearly distinct from plants of the genus Thismia distributed in the Old World and is genetically close to plants of the genus Haplothismia found in India. Both have one morphological feature in common; namely, they are the only family Thismiaceae members that have roots resembling tubers. However, their genomes and other data led to the conclusion that the plants found in this survey cannot be considered a new Haplothismia species. Its morphological characteristics suggest that it is a highly distinct species, with features intermediate between a lineage that diverged at the base of the family Thismiaceae and the Old World Thismia genus that had diversified relatively recently within the family. Taking all of this information together, Suetsugu et al. established a new genus, Relictithismia, and named the unknown plant R. kimotsukiensis.
Because of this discovery, Japan has been found to have the genus Oxygyne (the earliest divergent taxon of the family Thismiaceae), Thismia abei and T. kobensis (belonging to the earliest divergent group of the Old World Thismia genus), and the genus Relictithismia.
Japan is the only area in the world where three genera of the family Thismiaceae are distributed and is thus an important region for clarifying the evolutionary history of this family. In particular, the discovery of R. kimotsukiensis, which has features intermediate between the Thismia and other genera, advances our understanding of the enigmatic Thismiaceae family as a whole. Since the Thismicaea live on nutrients from fungi, they are easily affected by the surrounding environment. Indeed, almost all species are threatened with extinction owing to land alteration without their presence being noticed, and appropriate protection measures are required from the point of view of protecting biodiversity.
Journal Information
Publication: Journal of Plant Research
Title: Relictithismia kimotsukiensis, a new genus and species of Thismiaceae from southern Japan with discussions on its phylogenetic relationship
DOI: 10.1007/s10265-024-01532-5
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.