Departmental Bulletin Paper 〈Original Papers〉和歌山県固有植物 キイシモツケ の蛇紋岩土壌への適応と分子系統学によるイワシモツケおよびトサシモツケとの比較

明渡, 絵里朱  ,  平田, 智子  ,  上井, 和幸  ,  髙木, 祐子  ,  水野, 隆文  ,  水野, 直治  ,  小林, 真  ,  小池, 孝良  ,  大和, 勝幸  ,  秋田, 求  ,  泉井, 桂

バラ科シモツケ属のキイシモツケ(Spiraea nipponica Maxim. var. ogawae( Nakai) Yamanaka)は、蛇紋岩地帯にて自生する和歌山県固有の植物である。県北部に位置する龍門山の山頂付近の群落は県の天然記念物に指定されている。キイシモツケは、かつては新種とされSpiraea ogawai Nakai と命名され登録されていた。しかし近年は形態学的比較に基づいて、高知県に自生するトサシモツケ(Spiraea nipponica Maxim. var. tosaensis( Yatabe) Makino)と共に、近畿以北に自生するイワシモツケ(Spiraea nipponica Maxim.)の変種とされ、学名も上記のように改められている。本研究では、キイシモツケがトサシモツケおよびイワシモツケなどの近縁種とは顕著に異なるのかどうかを初めて分子系統分類学の立場から検討した。同時に、キイシモツケの蛇紋岩土壌との関わりについて若干の検討を行った。まず、3 種類それぞれに特徴的な葉の形態は、種子から同一の環境条件下で生育させても再現され、生来の形質であることがわかった。次に、龍門山の土壌の交換性金属イオンの元素組成を調べ、蛇紋岩土壌特有の組成をもつことが確かめられた。キイシモツケの植物体について、金属イオンの元素分析を行った結果、蛇紋岩土壌の元素の中で最も植物に対する毒性が強いとされるニッケル(Ni)が植物体内に取り込まれていることを認めた。さらにキイシモツケの種子は発芽時に3 者の中で最も強いNi 耐性を示し、若い苗では10 mmol L - 1 のNi 2 + に対しても耐性を示した。葉緑体DNA のtRNA のtrnL – trnF 領域および核ゲノムDNA の5S RNA 遺伝子とその両端のスペーサー配列を含むITS 領域の塩基配列を解析した。その結果、葉緑体のtRNA 領域(約1000 塩基対)は完全に一致し、ITS 領域(約640 塩基対)では3 種は互いに数塩基(1%以下)異なるのみであり、分子系統学的比較においてもキイシモツケはイワシモツケおよびトサシモツケと極めて近縁であることが証明された。したがって、キイシモツケは日本の近畿以北に広く分布するイワシモツケを起源とし、和歌山の限られた蛇紋岩地帯に適応して、地理的に隔絶して自生するにいたったエコタイプであると推測された。A plant called Kiishimotsuke( in Japanese) is indigenous to Wakayama prefecture in Japan. The plant vegetates only in the three narrow mountain areas consisting of serpentine soils. The largest colony of this plant in the area of Mt. Ryumon higher than 600 m above sea level had been designated as a prefectural natural treasure since May 1971. The plant was first discovered by Y. Ogawa, a botanist in Iwade city, and registered as a new species with a name of Spiraea ogawai Nakai in 1928. However, the validity of this registration was questioned later, because there were two kinds of plants, called Iwashimotsuke and Tosashimotsuke, which were morphologically similar to Kiishimotuke except for the shape of leaves. Iwashimotsuke is vegetating in northern area than Kinki district, and Tosashimotsuke indigenously in Kochi prefecture. Thus at present Iwashimotsuke is named as Spiraea nipponica Maxim, Tosashimotsuke as S. nipponica Maxim var. tosaensis( Yatabe) Makino, and Kiishimotsuke as S. nipponica Maxim var. ogawae (Nakai) Yamanaka. In view of recent progress of molecular phylogenetics, the conventional method was employed to examine whether Kiishimotsuke is a species intimately related to the other two plants or not. The nucleotide sequences were determined for the regions of trn L – trn F of chloroplast DNA and ITS (Internal Transcribed Spacer) of 5S RNA coding region of nuclear DNA. The results showed that the differences in the sequences were less than 1% among the three, demonstrating their close evolutionary relationships. Thus it is reasonable to conclude that Kiishimotsuke is a variety or ecotype of S. nipponica based not only on morphological comparison but also on molecular phylogenetic analysis. Unexpectedly significant difference in the sequence of the ITS for Iwashimotsuke( Spiraea nipponica Maxim) was found between the one published by Potter et al.( DQ897622) and ours( LC133174 and LC13375). The difference remains to be elucidated. Since Kiishimotsuke is endemic to serpentine soils in Wakayama, the interaction of the plant with serpentine soils was also studied. When these three plants were raised from seeds on non-serpentine cultivation soils, all of them grew normally and the differences in the shape of the leaves were retained, indicating the shapes of leaves were not affected by the kind of soil but determined inherently. Nikkel ion is known to be one of the most toxic metal ions in serpentine soils. When the effects of increasing concentrations of Ni 2 + on the sprouting of seeds were tested, Kiishimotsuke was most tolerant among others. In the aboveground parts of Kiishimotsuke plants grown on serpentine soils of Mt. Ryumon, accumulation of Ni 2 + was confirmed and its possible significance of the plant survival was discussed.

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