西南日本の植物雑記II. : 霧島山系におけるミヤマキリシマ, キリシマツツジ, ヤマツツジ諸集団の形質変異  [in Japanese] Taxonomical notes on plants of southern Japan II. Variation in the Rhododendron obtusum group in the Kirishima Mis. southern Kytushu.  [in Japanese]

1.霧島山系におけるミヤマキリシマ, キリシマツツジ, ヤマツツジの形質変異と訪花昆虫相について調査を行った。2.諸形質の解析と生育地域の空間構造からキリシマツツジ集団はミヤマキリシマ集団やヤマツツジ集団からは, 区別する事が出来る。3.ヤマツツジ(南九州型)は標高800mまでの低地の林縁沿いに生育し, ミヤマキリシマは火山性山岳の標高1000m以上の比較的開けた斜面に生育する。一方, キリシマツツジはヤマツツジとミヤマキリシマの中間ゾーンに分布し, 形態的にはややヤマツツジに近いながらも花色に著しい変異をもつ自然雑種起源と推定される集団である。4.キリシマツツジ集団の成立にはミヤマキリシマとヤマツツジの交配親和性の高さが原因となっていると推定される。5.ヤマツツジ集団とミヤマキリシマ集団の訪花昆虫相は, 送粉者として適合的な種では, 互いに異なっており, 自然状態では両種間の生殖隔離は一応保たれている。6.一方, キリシマツツジを含む2集団間, あるいは3集団間に共通する訪花昆虫も存在し, これらによってヤマツツジ集団とミヤマキリシマ集団間の遺伝的隔離が部分的に破られ, 雑種集団のキリシマツツジが成立し, この集団を通してさらに遺伝子の浸透性交雑が進行していると推定される。

Morphological variation of natural populations of the Rhododendron obtusum group (R. obtusum, R. kaempferi and R. kiusianum) were studied at 19 sites in the Kirishima Mts., southern Kyushu. In this part of southern Japan R. kaempferi (southern type) grows wide on rocky places and along forest edges on lower mountain slopes up to 800m alt. Rhododendron kiusianum occurs higher on open slopes from ca. 1000m alt. to the summits of volcanic mountains and usually forms large populations. In the zone between these two species polymorphic R. obtusum occurs in open forests, around forest edges and on roadsides at 800-1000m alt. Many morphological characteristics of this species are intermediate between R. kaempferi and R. kiusianum, and flower color varies immensely. Undoubtedly, this species originated through natural hybridization of the two species. Rhododendron kaempferi and R. kiusianum are crossed easily and freely and produce viable F_1,F_2 and further generations in cultivation. In the field, however, these two species have different pollinator syndromes. The main visitors for R. kaempferi are Lepidoptera (Byasa alcinous and Papilio spp.) and the honey bee Apis mellifera. The large red flowers are sometimes also visited by Diptera and small bees. In contrast, the small purplish flowers of R. kiusianum are visited by various species of Diptera and small bees. Large butterflies usually did not visit. Different pollinator assemblages and habitat segregation may isolate these two species. Rhododendron obtusum is visited by all the common pollinators of R. kiusianum and R. kaempferi, e.g., Lepidoptera (Byasa alcinous and Pieris melete), Diptera (Eristalis tenax and Phytomia zonata), and Hymenoptera (Bombus ardens and Apis mellifera). Through this pollinator assemblage, R. obtusum seems to be an effective route of gene introgression between R. kaempferi and R. kiusianum.