臺灣產高粱屬分類群之地理分布、遺傳多樣性及親緣關係

Abstract

高粱屬 (Sorghum) 總共有25個物種，包含第五大禾穀類作物的栽培種高粱 (S. bicolor (L.) Moench) 和世界惡名昭彰的雜草詹森草 (S. halepense)。由於臺灣地理位置特殊，造就多變的氣候和多樣的環境，非常適合許多生物物種生長。臺灣野外可發現四個高粱屬類群，包含栽培高粱、掃帚高粱 (S. bicolor var. technicum)、葦狀高粱 (S. bicolor ssp. verticilliflorum) 和詹森草。這四個高粱類群在臺灣各地的分布具明顯差異，係因每個高粱類群不同之特性與人類活動導致其侵略程度相異。此外，野外觀察到不同高粱類群共同生長的情形，增加基因流佈或基因漸滲之可能性，進而造成分類鑑定上的困難。因此，更精準的物種鑑定結果進一步則以倍體數測定確認。最終，本研究根據五年之田野調查、形態鑑定和倍體數測定結果，認為前人文獻提及的擬高粱 (S. propinquum) 目前不存在於臺灣野外。另一方面，本研究發現近年來野生高粱族群有增加和擴大的趨勢，根據Kolmogorov–Smirnov 試驗結果指出五年間葦狀高粱 (p = 0.004) 和詹森草 (p = 0.035) 族群有顯著擴散的趨勢。對於正在擴大的野生高粱族群須特別小心其成為臺灣野外具侵略性之危害雜草。而棲地調查結果顯示，栽培型高粱偏好與人為和農業活動相關的棲地，反之，野生型高粱則偏好屬於自然環境的棲地。由於野生型高粱在農業區易被農民剷除，且相較栽培型高粱，野生型高粱具耐旱特性和休眠性的種子，因此其在野外環境具有較高的生存率。值得注意的是，栽培型高粱的棲地五年間從農業類型棲地轉換為自然類型棲地，表示栽培種子逸散至臺灣野外的情形變多。本研究以25個簡單重複性序列 (Simple sequence repeat, SSR) 分析2014-2015年間蒐集之121個野生高粱蒐集系，結果發現，具有地下走莖的詹森草之遺傳歧異度最高 (He > 0.776)；而近年來歸化臺灣的葦狀高粱遺傳歧異度最低 (He < 0.272)。此外，分別自屏東和高雄蒐集的葦狀高粱族群間具有高度的遺傳分化 (FST = 0.5207)，顯示此兩族群受天然地理隔離之影響。而2018-2019年間共248個野生高粱蒐集系則以2,751個高品質的單一核甘酸多型性 (Single nucleotide polymorphism, SNP) 評估遺傳歧異度，同樣發現四個高粱類群的歧異度由高至低分別是詹森草 > 栽培高粱 > 掃帚高粱 > 葦狀高粱。值得注意的是，5個蒐集自桃園和彰化的中間型高粱在主座標分析和親緣樹結果皆被獨立出來，在族群結構結果分群至較複雜的次族群中且為混合型個體，顯示這些中間型高粱之間可能發生基因流佈或基因漸滲的現象。此外，栽培高粱族群與掃帚高粱 (Nm = 1.443) 和詹森草 (Nm = 0.98) 族群間可能有較高程度的基因流佈的之現象。臺灣多變的環境與豐富的生態多樣性提供這些高粱屬族群適合的生長條件，對於族群和演化遺傳的研究是絕佳的試驗材料。同時，不同類群間複雜的「作物-雜草-野生」關係也進一步藉由地理分布和棲地調查，以及SSR和SNP分子標誌之分析深入淺出的探究與解開。

The genus Sorghum consists of 25 species, including S. bicolor (L.) Moench, one of the top five cereal crops, and S. halepense, one of the most noxious weeds. The geographic location of Taiwan is unique, creating diverse climates and environments suitable for many species to live, including four Sorghum taxa, S. bicolor ssp. bicolor, var. technicum, ssp. verticilliflorum, and S. halepense, identified in the field. The four Sorghum taxa showed distinct geographic distributions, revealing that invasiveness was influenced by their characteristics and human activities. Besides, the observed co-existence of different taxa would increase the possibility of gene flow or introgression, causing troubles with the taxonomic identifications. The precise classifications for these four Sorghum taxa were further examined by the ploidy test. It was concluded that S. propinquum does not exist in Taiwan based on thorough field investigations, taxonomic identifications, and ploidy test over five years. On the other hand, the increasing abundance and dispersal of wild sorghum in Taiwan have caught our attention. S. bicolor ssp. verticilliflorum (p = 0.004) and S. halepense (p = 0.035) significantly expanded over five years according to the Kolmogorov–Smirnov test. More attention should be paid to these expanding wild sorghum populations becoming invasive and noxious weeds in Taiwan. The habitat preferences of feral cultivated sorghum were human-disturbed and agricultural areas, on the other hand, the wild relatives were natural habitats. The wild and weedy sorghum will not be eliminated by farmers and possess higher survival rates in the wild compared to feral cultivated ones due to drought tolerance and small dormant seeds. Moreover, the habitat switch of feral cultivated sorghum was observed, indicating that the ferality of cultivated sorghum took place more often in Taiwan recently. In this study, 121 Sorghum collections collected in Taiwan in 2014-2015 were evaluated by using 25 high polymorphic SSRs. The rhizomatous S. halepense showed the highest genetic diversity (He > 0.776); in contrast, the newly naturalized S. bicolor ssp. verticilliflorum was confined to and dominant in southern Taiwan, with the lowest genetic diversity (He < 0.272). Significant genetic differentiation (FST = 0.5207) between the two ssp. verticilliflorum subpopulations assessed by population structure was associated with natural geographic isolation. On the other hand, 248 Sorghum collections in 2018 and 2019 were evaluated by 2,751 high-quality pruned SNPs obtained from GBS. Similarly, the genetic diversity of these four Sorghum taxa was S. halepense > S. bicolor ssp. bicolor > S. bicolor var. technicum > S. bicolor ssp. verticilliflorum. Notably, five intermediate types of sorghum collected in Taoyuan and Changhua were obviously separated in PCA and phylogenetic tree. In addition, these five intermediates were grouped in the complex subpopulation (Pop 5) and also defined as admixed individuals, providing the evidence that gene flow or introgression may have occurred in Taiwan. Besides, higher number of migrants were detected between S. bicolor ssp. bicolor and var. technicum (Nm = 1.443), and ssp. bicolor and S. halepense (Nm = 0.98), indicating more possible gene flow occurred among them. The heterogeneous environments and abundant biodiversity of Sorghum in Taiwan provided advantages for conducting genetic and evolutionary studies for feral and wild sorghum, including the sympatric interaction of a crop-weed-wild complex for a species in agroecosystems. Meanwhile, the in situ geographic and habitat investigations combined with SSRs and SNPs assessment of these four Sorghum taxa in Taiwan delivered new insights into their favored habitats, proliferation, genetic diversity, relatedness, and differentiation.