樓梯草屬植物系統分類、生物地理與無融合生殖之研究

Systematics, biogeography and apomixis in Elatostema (Urticaceae)

曾妤馨(Yu-Hsin Tseng)

指導教授 ： 胡哲明

國立臺灣大學/生命科學院/生態學與演化生物學研究所/博士(2016年)

https://doi.org/10.6342/NTU201610347

摘要

樓梯草屬植物(蕁麻科)包含約500種，廣泛分布於熱帶及亞熱帶地區，主要的分布中心位於熱帶亞洲。由於此群植物的形態特徵具有高度的非同源相似性，再加上物種數眾多且分布非常廣泛，以及複雜的性別系統等特性，樓梯草屬與擬樓梯草、赤車使者、烏來麻的屬間分類，及其屬內的分群問題，一直尚待解決。
在本篇論文中，利用野外觀察、標本研究及分子譜系關係，針對台灣產樓梯草屬進行分類修訂。同時在台灣發現了蕁麻科的第一個天然雜交種–雜交樓梯草，此雜交種的母本為冷清草，而父本是巒大樓梯草。依照結果統計，認定台灣所產的樓梯草共有15種，再加上一種雜交種。
以樓梯草及其相近的分類群（擬樓梯草、赤車使者、烏來麻）進行完整的譜系關係分析，其結果顯示：廣義的樓梯草包含三個高支持度的支序群，可分別獨立為三個屬：烏來麻屬、擬樓梯草屬與樓梯草屬。其中，此重新定義的樓梯草屬包含了四個次支序群：小葉樓梯草、赤車使者、非洲以及核心樓梯草。因形態特徵的非同源相似性，所以無法使用共衍徵來區別各類群，但結合數個形態特徵，可用於廣義樓梯草的分群。
樓梯草屬的演化歷史與許多生物地理的事件有關。其中包含兩次非洲與亞洲間的傳播事件，可能分別與發生在新生代的兩次全球高溫傳播假說有關。另外，在核心樓梯草支序群中，至少發生過五次主要的傳播事件，包含兩次由東亞擴散起源，藉由東南亞至大洋洲的傳播途徑。在核心樓梯草支序群與非洲支序群中，分子譜系關係與地理分布有著非常緊密的關係，可能起因於樓梯草植物有限的傳播機制，而此有限的基因交流，也可能是導致樓梯草屬種化與演化的重要因素。
另外，本研究證實台灣所產的銳齒樓梯草、長圓樓梯草與溪澗樓梯草僅具有雌性或偏雌性的族群，這些物種亦可成功產生種子。此三個物種應屬於多倍體的無融合生殖植物。透過流式細胞儀種子篩選與胚胎發育學更進一步證實，這些物種屬於非常少見的絕對自發性無融合生殖植物。

關鍵字

無融合生殖 ； 樓梯草屬 ； 演化歷史 ； 屬間及屬內關係 ； 分子譜系關係 ； 形態學 ； 分類修訂 ； 蕁麻科

並列摘要

Elatostema (Urticaceae), consisting of about 500 species, is mainly distributed in the tropical to subtropical areas with a diversity center in tropical Asia. Elatostema has long been a taxonomically difficult group due to frequent homoplasy of morphological characters and numerous species with a wide distribution and complex sexual systems. The circumstances of generic delimitation among Elatostema, Elatostematoides, Pellionia and Procris are quite controversial, and the infrageneric grouping within Elatostema is also uncertain.
A taxonomic revision of Elatostema in Taiwan was carried out based on field observation, careful examination of specimens and molecular phylogeny. The first natural hybrid in Urticaceae, E. ×hybrida, was confirmed, which is derived from natural hybridization events between E. lineolatum var. majus (♀) and E. platyphyllum (♂). So far, we recognize 15 species and one natural hybrid in Taiwan.
A comprehensive phylogenetic analysis, comprising Elatostema and related Urticaceae taxa (Elatostematoides, Pellionia, and Procris), was conducted. The results showed that Elatostema s.l. can be divided into three well-supported genera: Procris, Elatostematoides and Elatostema s.a. Four strongly support subclades within Elatostema s.a. can be recognized, including Weddellia clade, Pellionia clade, Afroelatostema clade and core Elatostema clade. Homoplasy amongst the morphological characters used in this study makes it impossible to circumscribe genera using synapomorphies. Combined suites of characters, however, do enable the morphological diagnosis of Elatostema s.l.
The evolutionary history of Elatostema s.a. involves in a variety of biogeographical scenarios. Two intercontinental dispersal events from Asia to Africa were identified, which are probably correlated with two warm phases peaking during Cenozoic Era, respectively. There are at least five major dispersal events within core Elatostema clade, including the two routes of origin from East Asia through Malesia to Australasia. Besides, a strong association between geography and phylogenetic relationship was recovered within Afroelatostema and core Elatostema clades, which may attribute to limited dispersal mechanism. Furthermore, limited gene flow may be an important feature in speciation and evolution in Elatostema s.a.
In addition, based on previous field observation, E. cyrtandrifolium, E. oblongifolium and E. rivulare only have female individuals or female-biased species and still can produce seeds. In this study, these species are confirmed to be apomictic plants with polyploidy. The results of flow cytometric seed screen and embryogenesis further demonstrated that these species should belong to an infrequent case of obligate autonomous apomixis.

並列關鍵字

apomixis ； Elatostema ； evolutionary history ； inter- and infrageneric relationship ； molecular phylogeny ； morphology ； taxonomic revision ； Urticaceae

參考文獻

Lin, Q., & Dong, D.L. 2003. Taxonomic notes on five species of Elatostema (Urticaceae) from China. Acta Botanica Yunnanica 25: 633–638.

Tseng, Y.H., & Hu, J.M. 2015. Taxonomic revision of Elatostema J. R. Forst. & G. Forst. (Urticaceae) in Taiwan. Taiwania 60: 23–32.

Yang, Y.P., Shih, B.L., & Liu, H.Y. 1995. A revision of Elatostema (Urticaceae) of Taiwan. Botanical Bulletin of Academia Sinica 36: 259–279.

Albini, S.M. 1994. A karyotype of the Arabidopsis thaliana genome derived from synaptonemal complex analysis at prophase I of meiosis. Plant Journal 5: 665–672.

Ali, J.R., & Aitchison, J.C. 2008. Gondwana to Asia: plate tectonics, paleogeography and the biological connectivity of the Indian sub-continent from the Middle Jurassic through latest Eocene (166–35 Ma). Earth-Science Reviews 88: 145–166.