台灣稜果榕授粉蜂的末次冰期高峰後族群擴張與低族群分化之遺傳證據 Genetic evidences of post last glacial expansion and low population differentiation for a fig-pollinating wasp of Ficus septica in Taiwan

Abstract
發生於末次冰河時期的氣候波動,被認為對於現今生物的族群變化以及遺傳多樣性的程度上,有很深遠的影響。直至目前為止,這類相關的研究大都以歐洲或北美洲的溫帶物種為主,相較之下,對於亞熱帶以及熱帶地區的生物如何受到冰河作用的影響,我們在這方面的知識是較為缺乏的。因此,本研究以亞熱帶地區的一種稜果榕授粉蜂(Ceratosolen sp. 1)為研究對象,藉由探討稜果榕授粉蜂的族群變動歷史、長期以及近期的基因交流,以瞭解最近的冰河作用如何影響一個亞熱帶物種的族群變化和當代的遺傳結構。我們使用來自於219個體的粒腺體COI基因序列片段 (1052 bp)以及398個體在15個微衛星體基因座的基因型等遺傳資料,進行分析。族群變動歷史的重建結果顯示,稜果榕授粉蜂曾經歷過一次顯著的族群成長,發生在距今大約兩萬七千年前;接著在兩萬一千年前左右,也就是最近的冰期高峰之後,此成長幅度開始劇烈地增加,直到距今約一萬年前,族群趨於穩定。此結果說明,稜果榕授粉蜂的族群變動歷史與發生在台灣的末次冰期氣候變動具有緊密的相連性,並且對於末次冰期如何影響亞熱帶物種的議題上,提供了一個明確的例子。在族群數量的變化大小方面,我們發現冰河時期前後,稜果榕授粉蜂的族群大小差異,將近一千倍左右。此結果顯示了在最近一次冰河時期發生時,稜果榕授粉蜂與其宿主的族群是高度被壓縮以及破碎化的。冰河時期後的族群數量與分布空間的擴張也可能是造成稜果榕授粉蜂族群間並不存在遺傳分化(FST< 0.02)的部分原因。根據現生個體的來源族群推測,發現稜果榕授粉蜂具有長距離擴散的可能性以及族群間低比例的第一代遷入者。綜合我們的結果推測,現今觀察到稜果榕授粉蜂的族群結構是同時受到最近一次冰河時期結束之後氣候變動以及授粉蜂擴散能力的影響。除此之外,本研究亦觀察到稜果榕擁有多種授粉蜂,而關於此現象在演化上的意涵,也於此篇報告中一併陳述。
The climate oscillations during the last glacial period have profound influences on the demography and levels of genetic diversity of species. Molecular evidences of glacial effects on temperate species in Europe and North America have been well documented, whereas little is known regarding that on subtropical and tropical species. In this study, we investigated the demographic history, long term and ongoing gene flow of a subtropical fig-pollinating wasp (Ceratosolen sp. 1) of Ficus septicas in Taiwan and provided insights into how its demography and contemporary genetic structure were shaped by climatic changes since the most recent ice age. The analysis was conducted based on mitochondrial sequences of COI gene (1052 bp) from 219 individuals and genotypes of 398 individuals at 15 microsatellite loci. Reconstructed historical demography suggested that the population of C. sp. 1 has undergone a drastic population expansion that began around 27,000 years ago and then grew rapidly around 21,000 years ago, right after the last glacial maximum (LGM) in Taiwan; finally, the population of C. sp. 1 reached stability approximately 10,000 years ago. It revealed that the demographic history of C. sp. 1 was tightly coupled with the climate change since the LGM in Taiwan and provided an unambiguous case for the effects of the last glacial cycle on the demography of a subtropical species. The magnitude of the population size change was huge, approximately 1000 times larger than the size before expansion, which indicated that the population sizes of C. sp. 1 and its host might have been highly compressed and fragmented during the LGM. The postglacial demographic and spatial expansion might partially contribute to the lack of significant genetic differentiation among populations of C. sp. 1 as revealed by the FST statistics (< 0.02) and mismatch distribution of mithochondrial sequences, which implied high level of long term gene flow. Results of population assignment test revealed the potential of long distance dispersal and low frequency of first generation migrants among local populations as well. Our findings clearly demonstrated how recent climate changes since the LGM and dispersal ability of an organism shaped the genetic composition of a subtropical fig-pollinating wasp. In addition, we found multiple pollinators associated with F. septica in Taiwan, and the evolutionary implications of such phenomenon were addressed.
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Keywords
榕果授粉蜂, 族群變動歷史, 冰期後族群擴張, 族群結構, 共存, 稜果榕, fig-pollinating wasp, demographic history, postglacial population expansion, population structure, coexistence, Ficus septica
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