利用多基因座遺傳資訊與溯祖理論推論種化模式:以草蜥屬為例

Abstract

異域種化描述兩族群在受到地理阻隔之後，由於基因交流中斷，變異在兩族群內逐漸累積，因而導致種化的發生。過去認為基因交流的存在會弭平兩個族群的差異，因此異域種化一直被視為物種形成的典型模式。然而近年來的證據顯示種化的過程在有少量基因交流的情況下仍然可能發生，這一類非典型的種化模式(如鄰域種化及同域種化)也因此逐漸受到演化學家的重視。本研究旨在檢測翠斑草蜥（Takydromus viridipunctatus）、鹿野草蜥（T. luyeanus）、雪山草蜥（T. hsuehshanensis）三種草蜥分化期間的基因交流量，以推論此三種草蜥的種化模式。我們利用454高通量定序技術取得翠班及鹿野草蜥的cDNA序列，篩選出20組核基因的分子標記，並利用Isolation with migration analysis（IMA2）程式與approximate Bayesian computation（ABC）方法進行分析，以推估三種草蜥種化過程中的基因交流量，並釐清基因交流發生的時間點。結果發現三種草蜥在分化過程中的基因交流值顯著偏離零，顯示種化過程中保有一定程度的基因交流，典型的異域種化模型被拒絕。而ABC的分析結果則顯示翠斑草蜥及鹿野草蜥基因交流的時間發生在分化之後，並持續了一段時間，之後基因交流停止至現今。這個結果顯示IMA2所偵測的基因交流來自早期基因交流的模式，而非因為二次接觸造成的近期交流。總結來說，鄰域種化模式比異域種化模式更適合描述這三個物種的種化模式。Allopatric speciation is the dominant view of speciation among evolutionists. It is the process, where an ancestral population splits into two populations because of a geographic barrier. Genetic divergence gradually accumulates between the two descendant populations and ultimately results in two reproductively isolated entities. Under this scenario, speciation seems unlikely to proceed with the existence of gene flow. However, alternative geographic speciation modes allowing certain amount of gene flow (i.e. parapatic and sympatric modes) during speciation have gained more attention and related studies been published in recent years. In this study, we aim to test the existence of historical gene flow to evaluate the predominance of allopatric isolation during the speciation process across three reciprocally monophyletic species: Takydromus viridipunctatus, T. luyeanus and T. hsuehshanensis. The EST database of Takydromus spp. was constructed by using 454 sequencing, and 20 nuclear loci were successfully identified and amplified from the lizards for genetic investigations. IMA2 analyses revealed significant gene flows among the three species disfavouring a strictly allopatric speciation model. An approximate Bayesian computation (ABC) model choice procedure further indicated that an “early gene flow model” but not an “secondary contact model” fits better with the empirical data (T. viridipunctatus and T. luyeanus). Hence, historical but contemporary gene flow has occurred between T. viridipunctatus and T. luyeanus. We concluded that “parapatric” rather than “allopatric” is the predominant mode of speciation across Takydromus species.