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| the Rhododendron keiskei complex. B–D, Morphological comparisons of: B, Rhododendron keiskei Suzuki (1932) C, Rhododendron kantoense S.Sakag. & Y.Watan., nom. & stat. nov. D, Rhododendron tyaihyonii S.Sakag., H.J.Choi & S.C.Kim, sp. nov. in Sakaguchi, H.-J. Choi, Yoichi, Takahashi, ..., H.-J. Choi et S.-C. Kim, 2024. — Photos: B, Shota Sakaguchi; C, Watanabe Yoichi; D, J.-C. Yang. |
Abstract
Unraveling species boundaries is pivotal for evolutionary biology and conservation endeavors. However, it proves challenging in instances where recent speciation is intertwined with complex demographic histories and natural selection processes. The Rhododendron keiskei complex, an evergreen rhododendron distributed in East Asia, consists of a widespread variety (R. keiskei var. keiskei) and a more restricted R. keiskei var. hypoglaucum. Intriguingly, the latter is exceptionally rare yet displays a disjunction that spans approximately 1100 km. This study aimed to elucidate the evolutionary backgrounds of the enigmatic disjunctions of R. keiskei var. hypoglaucum and to propose species delimitation within the species complex. An integrative approach, combining genomic data (MIG-seq and GBS-derived SNPs) with Scanning Electron Microscopy analysis of leaf microstructures was adopted in this study. Phylogenetic analyses revealed significant divergence among the studied rhododendrons. Genetic demographic analyses favored the population models that assumed non-monophyly of two disjunct populations of R. keiskei var. hypoglaucum indicating their independent origins. Recent gene flow between the widespread R. keiskei var. keiskei and “var. hypoglaucum” populations were limited due to geographic and habitat isolation factors, even in areas where their distributions overlap. Detailed morphological assessments detected distinctions between morphologically similar “var. hypoglaucum” populations based on leaf microstructures and flowering habits. Our study has shown that the apparent disjunctions of rare rhododendrons are more likely attributed to morphological convergence, possibly due to similar environmental selections in unrelated taxa. The finding highlights the importance of an integrative approach for resolving taxonomic challenges in plant species complexes.
Keywords: demographic modeling, disjunct distribution, East Asia, endangered species, phylogeny, species delimitation
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| A) Rhododendron tyaihyonii; B) R. kantoense; C) R. keiskei |
R. keiskei var. keiskei
R. keiskei var. hypoglaucum (R. kantoense nom. & stat. nov.)
Rhododendron sp. (R. tyaihyonii sp. nov.; formerly identified as var. hypoglaucum)
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| A, The geographic distribution of the Rhododendron keiskei complex. B–D, Morphological comparisons of: B, R. keiskei var. keiskei; C, R. keiskei var. hypoglaucum; D, Rhododendron sp. E, Comparison of leaf blade morphology of three taxa, based on the voucher specimens (suppl. Table S1). Note that the leaf morphological variation of R. keiskei var. hypoglaucum and Rhododendron sp. overlap. — Photos: B, Shota Sakaguchi; C, Watanabe Yoichi; D, J.-C. Yang. |
Rhododendron tyaihyonii S.Sakag., H.J.Choi & S.C.Kim, sp. nov.
Distribution: SOUTH KOREA. Jeollanam-do, Yeosu-si, Samseon-myeon. Narrowly endemic to Daesambudo Island.
Ecology: The plants are found on the northern slope of Daesambudo Island. They inhabit coastal rocky places with the evergreen woody species of Rhaphiolepis indica var. umbellata (Thunb.) H.Ohashi, Ligustrum japonicum Thunb., Camellia japonica L, Litsea japonica (Thunb.) Juss.
Etymology: The specific epithet, “tyaihyonii”, is named in honor of Chung Tyaihyon (1883–1971), known as the father of plant taxonomy in Korea and the first curator of Sungkyunkwan University Herbarium (Ha Eun Herbarium, SKK). We have a better understanding of Korean flora thanks to his work in the early to mid-1900s.
Korean name: Seom-Jin-Dal-Rae.
Japanese name: Chosen-Hikage-Tsutsuji.
English name: Tyaihyon's rhododendron.
Rhododendron kantoense S.Sakag. & Y.Watan., nom. & stat. nov.
≡ Rhododendron keiskei var. hypoglaucum Suto & T.Suzuki in Trans. Nat. Hist. Soc. Formosa 22: 23. 1932
– Holotype: JAPAN. Kanto District, Tochigi (Shimotsuke), Kanuma, Mt. Ozaku, 1 May 1931, E. Kitamura ST3902 (TAI barcode TAI 119040 [image!] [fl.]; isotype: TNS barcode TNS 55292 [image!]).
Note: Rhododendron kantoense is similar to R. tyaihyonii, the differences being: the latter has petiole 4–7 mm long; stomata type anomocytic; inflorescence with 2–6 flowers.
Distribution: JAPAN. Kanto, Prefectures of Tochigi, Gunma, Saitama and Tokyo. Endemic to the chert and limestone mountain ridges of Kanto District.
Etymology: The specific epithet “kantoense” is derived from the Kanto District, where this species is distributed.
Japanese name: Urajiro-Hikage-Tsutsuji.
Korean name: Il-bon-Huin-Jin-Dal-Rae.
English name: Kanto rhododendron.
Shota Sakaguchi, Hee-Joo Choi, Watanabe Yoichi, Daiki Takahashi, Shun K. Hirota, Masayuki Maki, Shoki Murakami, Taichi Harada, Nobuo Kobayashi, Yuji Kurashige, Jun-Ho Song, Hyeok-Jae Choi and Seung-Chul Kim. 2024. Unraveling Enigmatic Disjunctions: Population Genetic Analysis Points to Independent Origins of Rare Rhododendrons in the Rhododendron keiskei complex (Ericaceae). TAXON. DOI: doi.org/10.1002/tax.13288
