琵鷺的演化與保育遺傳研究

Evolutionary and Conservation Genetics Studies of the Spoonbills (Platalea spp.)

楊愷樂(Carol K. L. Yeung)

指導教授 ： 李壽先

國立臺灣師範大學/理學院/生命科學研究所/博士(2009年)

摘要

長久以來，鳥類的種化被認為是由族群在空間上的隔離所啟動，空間隔離所導致的族群間基因交流中斷及族群量的縮減，會進一步促使各分隔族群內的遺傳漂變，而造成族群分化，最終以致種化。然而，對於陸生脊椎動物中運動能力最強的鳥類而言，地理屏障對其族群間基因交流所形成的障礙，是不近情理的假設，卻少被質疑。在我論文的第二章，我以核基因片段為材料，說明分布於澳洲的皇家琵鷺（Platalea regia)之物種形成，無法由前提是起始於少數奠基者及物種形成過程中完全缺乏與其姊妹種黑面琵鷺（P. minor)間進行“分化後基因交流“（post-divergence gene flow）的奠基者種化（founder speciation）模式加以解釋；相對的，我的遺傳證據較支持包含“分化後基因交流“的種化模式； 我的結果也顯示 ，奠基者種化所需的瓶頸事件(bottleneck)即便存在，所歷時間應極為短暫，或者所謂奠基者的數目會遠大於一般所設想。我同時也發現皇家琵鷺與黑面琵鷺的祖先在兩者開始分化之後，彼此間仍有相當程度的基因交流，而這樣交流所歷時間，更佔兩者分化至今一半以上的時間。在第三章中，以核基因序列所推估的遺傳有效族群量（effective population size）及普查族群量（census population size）之比例，作為評估物種內族群結構化（population structuring）的指標，我的結果顯示了部份琵鷺的祖先物種可能是由數量大且不具明顯結構的族群所構成。我也發現所有琵鷺的共祖及琵鷺與紅鹮（Eudocimus ruber）的共祖之有效族群量都出奇的大，這現象可能是由於這兩者的族群結構是更近似於Wright所提出的島嶼模式，也就是物種由幾個分化但彼此間持續進行基因交流的族群所構成。這些結果都指出選汰壓力在物種的分化上，較阻斷基因交流（地理隔離）扮演更重要的角色。最後在第四章中，我應用近似貝氏計算（approximate Bayesian calculation)分析黑面琵鷺的微衛星多態性數據，發現這個瀕危物種的有效族群量縮減，受到最近一次冰期啟動之影響，遠勝於近期人為干擾的效應；而根據核基因座及微衛星多態性所估出的現時黑面琵鷺有效族群量並不大，因此持續以人為努力減少環境波動對其族群數量的影響，應為維持黑面琵鷺長期存續之必要措施。在最末章中，我針對以上的結果作出綜合討論，並提出未來研究的展望。

關鍵字

地理種化模式 ； 琵鷺 ； 奠基者種化 ； 近似貝氏計算 ； 分化後基因交流 ； 島嶼模式 ； 瓶頸效應 ； 有效族群量

並列摘要

Avian speciation has long been attributed to spatial isolation that prohibits gene flow and that is accompanied by population size reduction, which enhances genetic drift. However, given that birds are the most mobile of terrestrial vertebrates, that geographic barrier should pose such impediment to gene flow is rather counterintuitive and yet seldom questioned. In Chapter 2 of my dissertation, I show that founder effect model, which assumes a small founder population size and the absence of post-divergence gene flow, failed to apply to the speciation of the royal spoonbill (Platalea regia) from the black-faced spoonbill (P. minor); rather, models that considered post-divergence gene flow was much more probable for these species, and speciational bottleneck, if any, would have been very brief or the founder size would have needed to be much larger. Furthermore, post-divergence gene flow was estimated to be quite substantial and span well over half of the divergence history. In Chapter 3, using the ratio of effective to census population size as a proxy for the level of population structuring, I showed that ancestral populations of the most recent common ancestors of some spoonbill species (Platalea spp.) may be large and panmictic. Furthermore, I found that population size of the common ancestor of all spoonbills and the common ancestor of the spoonbills and the scarlet ibis (Eudocimus ruber) was surprisingly large, and reasoned that this may be due to population structuring which can be depicted by a Wrightian island model- that is, one in which populations exchange migrants persistently but remain genetically differentiated. Taken together, these results suggest a more important role for selection in promoting divergence despite gene flow. Finally, applying the ABC method to mirosatellite genotype data of the endangered black-faced spoonbill, I found that rather than anthropogenic disturbances, climatic changes at the onset of the last glacial period have had the greatest impact in reducing the species' effective population size. Moreover, current effective population size of black-faced spoonbill was estimated to be small, and calls for continuous human efforts to reduce environmental stochasticity in the hope of bettering the species' chances of persisting in the long run.

並列關鍵字

geographic speciation modes ； spoonbills ； Platalea ； founder speciation ； approximate bayesian computation ； post-divergence gene flow ； island model ； bottleneck ； effective population size

參考文獻

Price T (2008) Speciation in Birds. Roberts & Company, Greenwood Village, CO.

Price T (2008) Speciation in Birds. Roberts & Company, Greenwood

Chapter 1

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