臺灣杉屬的親緣地理研究

Abstract

臺灣杉屬（Taiwania）與著名的加州紅木和水杉相同，屬於第三紀孓遺裸子植物。化石與花粉的紀錄顯示臺灣杉屬的族群在第三紀時曾廣泛分布於歐亞大陸的溫帶地區，範圍甚至到達北美，然而現存的臺灣杉屬族群範圍僅零星分布於臺灣、中國華南、緬甸北部與越南北部。由於臺灣杉屬的現存族群在亞洲呈現間斷分布模式，這可能暗示著亞洲臺灣杉屬植物在第三紀時期全球變冷後族群向南遷徙、並在第四紀冰河期時植物遷徙來臺灣、最後這些族群孑遺於現地可能的冰河期避難所位置。過去的研究曾經分析過部分孑遺臺灣杉屬族群間的遺傳變異，試圖對臺灣及中國族群是否歸屬同種做過討論，然在缺乏缺乏全面採樣及親緣地理分析之下，這些研究無法完整解釋臺灣杉屬在亞洲的演化歷史。本研究使用五個cpDNA序列片段（petG-trnP、trnH-psbA、trnV-trnM、trnC-ycf6 與trnL-trnF）分析臺灣與中國、以及最近在越南新發現的臺灣杉屬族群間的遺傳變異，藉以探究亞洲臺灣杉屬族群的親緣演化。結果發現全亞洲所有孑遺族群只存在八種cpDNA 單套型，顯示亞洲現存的臺灣杉屬族群的cpDNA 遺傳多樣性極低。所有臺灣的族群都只屬於同一種cpDNA 單套型A，且單套型網狀圖結構顯示此單套型與亞洲大陸（中國與越南）所有族群普遍存在的的單套型B 間存在八個突變步驟的差異。這表示臺灣與亞洲大陸間的臺灣杉屬族群存在著極大的分化。相對而言，成對族群間的FST、Nm 與AMOVA 分析結果則顯示中國與越南的族群間在cpDNA 的遺傳上較為接近。分歧時間的估算結果顯示臺灣與亞洲大陸的臺灣杉屬族群分化時間平均為1.9-2.5 mya，位於上新世（Pliocene）的晚期。葉綠體單套型的分布模式顯示臺灣的臺灣杉屬族群間可能在遷徙至臺灣之前，便已與亞洲大陸族群發生分歧演化。亞洲大陸現生臺灣杉屬族群所呈現的零星分布與cpDNA的遺傳變異情形則可能是南方的殘餘族群在冰河期結束後發生破碎化，並退縮至數個避難所的結果。

Taiwania is one of the Tertiary relic gymnosperms like the well-known Sequoia and Metasequoia. Fossil and pollen record evidence indicated that it was once widespread in the temperate region of Eurasia and even in North America but now only disperses in Taiwan, Southern China, northern Myanmar and northern Vietnam. Because the living populations of Taiwania have a scattered distribution pattern in Asia, these scattered populations may indicate information about the southward dispersal of Asian plants in the Tertiary, the dispersal of plants to Taiwan during the Quaternary glacial periods, and the formation of glacial refugia in Asia. Previous studies had analyzed the morphological and genetic variation in some relic Taiwania populations in Taiwan and China, trying to solve taxonomic problems between Taiwanese and Chinese populations. The evolutionary history of Taiwania populations in Asia remained unsolved without extensive sampling and phylogeographic analyses. In this study, five cpDNA regions (petG-trnP, trnH-psbA, trnV-trnM, trnC-ycf6 and trnL-trnF) were sequenced to analyze the genetic variation among Taiwania populations in Taiwan, China and a recently discovered population in Vietnam. Only eight haplotypes were distinguished, implying low genetic variations in all extant Taiwania populations. All Taiwanese populations consist only the haplotype A, which was separated by an eight-mutational-step difference to the widely-distributed haplotype B in the Asiatic mainland. Such pattern suggested a strong genetic differentiation between Taiwania populations in Taiwan and the Asiatic mainland. In contrast, the results of pairwise FST, Nm, and AMOVA implied a closer relationship between the Chinese and Vietnamese populations. The results of divergent time estimation suggest that the average differentiation time between the populations in Taiwan and the Asiatic mainland was ca. 1.9-2.5 mya, which was during the late Pliocene. The distribution pattern of cpDNA haplotypes suggested that Taiwanese populations was diverged from populations in the Asiatic mainland before their dispersal to Taiwan. The scattered distribution pattern and the cpDNA variation of living Taiwania populations in Asia may be the result of fragmentation and retreat into several refugia, which was happened after the end of Quaternary glacial cycles.