Plant speciation in the Karst—Phylogenetics of limestone Begonia in China

中越邊境喀斯特岩溶地貌蘊含了十分豐富的植物多樣性及特有度，為了解石 灰岩植物多樣性的生成機制，本論文以分布於中越邊境石灰岩地貌的秋海棠來探 討遺傳分化。中越邊境石灰岩秋海棠包含側膜組秋海棠、棒果組秋海棠以及四種 秋海棠組秋海棠，物種數高達七十種。因大部分的石灰岩物種傳播能力不佳且在 高度破碎化的石灰岩生境多呈現狹域分布的格局，推論基因交流的受限可能為造 成其遺傳分化之主因；形態差異主要在營養器官且與環境無明顯相關性，暗示形 態的變異可能為非適應性變化。本研究利用兩段葉綠體 DNA 片段 (trnC-ycf6、 petB-petD) 重建 61 種中越邊境石灰岩秋海棠的親緣關係，並根據親緣關係分析結 果選取四組近緣種支序 (A-D 支序) 以及三種廣泛分布種進行遺傳、地理以及形態 距離三方的相關性檢測。研究結果顯示，中越邊境石灰岩秋海棠形成一支持度不 高的單系群，其內包括十一支亞支序群；種內和種間層級的遺傳距離與地理距離 之相關性檢測，除了 D 支序外，皆具有顯著相關性；葉部形態變異與遺傳及地理 距離的相關性檢測在所有的檢測單位皆不具相關性；花部形態距離與遺傳及地理 距離的相關性檢測在所有的檢測，除了 C 支序外，皆不具相關性。根據分析結果 推論，中越邊境石灰岩秋海棠的高度物種多樣性為輻射演化的結果，傳播能力的 受限以及地理上的隔離為造成種化的重要推手，形態變異依據目前檢測結果傾向 為天擇造成，推論此結果可能與物種因具有棲位保守性相關。V Abstract The Karst terrains across the Sino-Vietnamese border are known for its extremely high plant diversity and endemism. To understand mechanisms that had generated its remarkable biodiversity, we study patterns of genetic diversification of the Sino-Vietnamese limestone Begonia. About 70 Begonia species are found exclusively in the limestone habitats of the region, including all members of sect. Coelocentrum and sect. Leprosae and four species of sect. Diploclinium. Because most of these limestone species are poor dispersers and have narrow and restricted distributions on the highly fragmented limestone habitats, limited gene flow might have been the major contributor to their genetic diversification. Morphological variation of Sino-Vietnamese limestone Begonia is mainly manifested in vegetative traits that have no apparent correlation to their environments, implying that morphological variation could have been primarily non-adaptive. We collected DNA sequences of two chloroplast intergenic spacers (trnC-ycf6 and petB-petD) of 61 Sino-Vietnamese limestone Begonia species to reconstruct their phylogenetic relationships. Mantel tests between genetic, geographical, and morphological distances were performed for three widespread species and four groups (clade A-D) of closely related species revealed by the phylogenetic trees. Results show that Sino-Vietnamese limestone Begonia group forms a poorly supported clade composed of 11 subclades. A significant positive correlation between genetic and geographic distance was observed in both inter- and intraspecific tests except for clade D. Correlations between morphological distance of leaf traits and genetic distance were not significant in all tests. Correlations between leaf morphological distance and geographic distance were also not significant in all tests. Except for clade C, VI correlations between floral morphological distance and genetic distance were not significant. Correlations between morphological distance of floral traits and geographic distance were not significant except for clade C. Based on these analyses and inferences, the high species diversity of Sino-Vietnamese limestone Begonia appears to be results of species radiation. Limited dispersability and geographic isolation should have played an important role in spawning its species diversity. Morphological variation of these limestone Begonia species appears to be resulted from natural selection, likely related to the strong niche conservatism to their habitats