天擇作用於台灣繡眼畫眉族群MHC class I 基因上的證據

Abstract

生物因局部性適應而造成族群間的分化，是演化生物學中的重要議題。而瘧疾對生物在不同海拔高度族群的選汰壓力差異，正提供了廣分布種免疫相關基因產生局部性適應的機會。本研究以廣泛分布於台灣低至高海拔闊葉林緣的繡眼畫眉(Alcippe morrisonia morrisonia)為模式，探討分布於低(200-400公尺)、中(1000-1250公尺)、高(1940-2200公尺)海拔族群的主要組織相容性複合體基因(major histocompatibility complex, MHC)是否受到不同程度禽瘧疾的選汰壓力而產生局部性適應。本研究以毛細管電泳-單股構形多型性(CE-SSCP, capillary electrophoresis-single strand conformation polymorphism,)技術自繡眼畫眉分離出共28個MHC class I對偶基因。在潛在的瘧疾疫區中(中、低海拔族群)，發現MHC對偶基因Almo*05與個體染病與否有顯著關聯 (p=0.0062)：帶有MHC對偶基因Almo*05的個體感染禽瘧疾的比例顯著較未攜帶者為高(p=0.0091)，顯示對偶基因Almo*05對禽瘧疾有較佳的感受力。再者，此對偶基因頻率在非疫區的高海拔族群較中低海拔族群顯著為高(p=0.0282)；此結果顯示Almo*05在高海拔地區有選汰壓力減輕的現象，這間接支持禽瘧疾相關的MHC對偶基因頻率分布，在不同海拔高度的族群產生局部性適應現象。另以AMOVA (Analysis of Molecular Variance,分子變異分析)檢測MHC基因及五個微衛星體基因座(microsatellite loci)於不同海拔族群間的遺傳分化情形；結果顯示，微衛星體基因在不同海拔族群間呈現些微但顯著分化(FST=0.00057, p=0.04692)，但MHC基因在不同海拔族群間並無分化(FST= -0.00032, p=0.45064)。這結果表示MHC對偶基因在三個海拔高度族群間的頻率分布，比中性預期下更為平均，符合不同海拔繡眼畫眉MHC的整體多樣性可能是藉由平衡式天擇來維持的假說。綜合上述結果，本研究證明不同海拔高度間，病原天擇壓力可造成族群MHC對偶基因的局部性適應，而另一方面，不同海拔繡眼畫眉族群MHC基因多樣性可能可藉由平衡式天擇來維持。Local adaptation leading to population differentiation is a fundamental question in evolutionary biology. Altitudinal variation in malarial pressure across altitudes provides opportunities for resistance-related genes of widespread species to adapt locally. Here I used the gray-cheeked fulvetta (Alcippe morrisonia morrisonia) populations in central Taiwan as a model to study whether divergent malarial pressure at low (200-400 m), intermediate (1000-1250 m) and high (1940-2200 m) altitudes leads to local adaptation of host’s MHC (major histocompatibility complex) genes. Twenty-eight alleles of MHC class I genes were identified using CE-SSCP (capillary electrophoresis-single strand conformation polymorphism) method in this study. In malarial endemic region (i.e. low and intermediate altitudes), MHC allele Almo* 05 was identified as a determinant factor in malarial infection (forward stepwise regression analysis, p=0.0062); significantly higher ratio of individuals carrying Almo* 05 were infected with malaria than those without the allele, indicating Almo* 05 was susceptible to malarial infection. Furthermore, Almo* 05 was of significantly higher frequency at high altitudes than at low and intermediate altitudes. My results suggested that allele frequency of specific MHC allele could be modulated by altitudinal pressure gradient of malaria. In addition, results of AMOVA (Analysis of Molecular Variance) test suggested that alleles of five microsatellite loci showed slight but significant differentiation of allelic frequency among three altitudinal populations (FST=0.00057, p=0.04692), whereas MHC alleles did not (FST= -0.00032, p=0.45064). The observation of a more even allele distribution among altitudes at MHC is concordant with the expectation of balancing selection maintaining MHC diversity. In sum, this study demonstrates that, across altitudes, specific MHC allele may adapt to divergent pathogenic selection, while balancing selection may play a key role in maintaining MHC polymorphism in the gray-cheeked fulvetta populations among different altitudes.