臺灣杉遺傳及表觀遺傳分歧與適應性演化

Abstract

遺傳漂變及天擇間的交互作用一直是族群遺傳學家深感興趣的議題，兩者不僅能影響族群間的分歧及種化，還能影響物種本身既有的遺傳及表觀遺傳變異，使物種快速回應外在環境壓力產生適應性。臺灣杉（Taiwania cryptomerioides Hayata）目前僅分布於臺灣、中國雲緬邊境及越南北部的中高海拔山區霧林帶。我們利用AFLP及MSAP兩種顯性分子標誌工具分別評估8個臺灣杉現存族群的遺傳及表觀遺傳變異，一共得到1413個AFLP基因座及462個MSAP基因座，而且臺灣杉臺灣親緣支系群相較於亞洲大陸支系群擁有較高的多型性基因座百分比及遺傳、表觀遺傳多樣性。pairwise FST及STRUCTURE的結果顯示臺灣杉臺灣族群與亞洲大陸族群已有顯著的遺傳及表觀遺傳分化，且臺灣親緣支系群在親緣關係上與中國及越南族群相距較遠，可明顯辨認出兩個歸群（K=2）。在可能受潛在天擇作用的基因座檢測中，臺灣杉臺灣親緣支系群內因中性遺傳漂變導致臺灣族群產生分歧，使族群內並無發現任何可能受天擇作用的適應性基因座。在臺灣杉三個親緣支系群內共找到32個可能受天擇作用的outlier loci與環境因子有顯著相關，包含季節性溫度（BIO4）、季節性雨量（BIO15）及NDVI，這些環境變因不僅影響臺灣杉遺傳及表觀遺傳變異，也與遺傳漂變一同影響臺灣杉親緣支系群間的適應性分歧與族群間的分化。因此，藉由天擇保留下來或遺傳漂變所累積的遺傳及表觀遺傳變異，皆有助於臺灣杉適應因氣候變遷或環境快速改變所帶來的選汰壓力，維持臺灣杉族群的存續。The interactions of neutral drift and naturel selection have received much attention from population geneticists because they may increase the level of differentiation between populations and potentially facilitate biological speciation. Taiwania cryptomerioides Hayata whose populations are now only distributed within mountainous cloud forest regions of northern Vietnam, Yunnan-Myanmar border of China, and Taiwan Island, ranging from 1600 – 2500 m a.s.l. We surveyed genetic and epigenetic variations of eight extant populations of Taiwania through two dominant markers, AFLP and MSAP, and then we finally obtained 1413 AFLP loci and 462 MSAP loci. The results suggested that large proportions of polymorphism loci and relatively high levels of genetic and epigenetic diversity in populations of Taiwanese lineages of Taiwania than those of Chinese and Vietnamese lineages. We also found there were significant levels of genetic and epigenetic differentiation between Taiwanese and mainland Asia lineages revealed by pairwise FST and STRUCTURE. Neighbor-Net phylogenetic network demonstrated the close relationship between Yunnan-Myanmar and Vietnamese lineages differentiated from Taiwanese populations, both results suggested Taiwania populations can be devided into two distinctly distinguishing clusters when K=2. In DFDIST and BAYESCAN analyses, no potential selective outliers were found within Taiwanese populations due to strongly neutral drift induced divergence. 32 loci were identified as potentially selective ouliers associated strongly with environmental variables including seasonary temperature (BIO4), seasonary precipitation (BIO15), and NDVI in either global or pairwise lineage comparisons. Not only had these important environmental variables influenced genetic and epigenetic variations, but they also had driven adaptive divergence and population differentiation of Taiwania with random drift. Therefore, the standing genetic and epigenetic variations either determined by selection or fixed by neutral drift, they both may ensure the capacity of Taiwania for adaptations to rapidly varing environment or climate change, and ability to survive and reproduce in the future.