以核內之內轉錄區間序列探討珊瑚共生藻系群的遺傳變異與演化

Genetic Variation and Evolution of Symbiodinium in Corals Based on Phylogeny Reconstructed from the Nuclear Internal Transcribed Spacer (ITS rDNA) Sequences

共生藻（Symbiodinium）與珊瑚所形成的互利共生（mutualism）在珊瑚生態系扮演著重要的角色。在珊瑚中常見的共生藻有4種系群（clade），分別爲A、B、C及D系群。本研究以珊瑚主要共生藻系群的細胞核之內轉錄區間（internal transcribed spacer; ITS rDNA）序列，探討共生藻基因組成上的差異、親緣關係與演化路徑。結果顯示，共生藻ITS rDNA序列平均總長度介於526-576 bp，G+C百分比介於47.5-53.8%，而A系群在序列長度及G+C百分比均低於4種共生藻系群的平均值。共生藻ITS rDNA區段在系群內的遺傳距離爲0.00-0.052，系群間的遺傳距離爲0.283-0.677，由親緣樹狀圖呈現系群間有明顯的分群現象，且A系群位於共生藻的最外群，而A系群在親緣網狀圖具有古老單型（haplotype），顯示出在共生藻的分歧演化（divergent evolution）中，A系群與其他系群有特殊的演化軌跡，而這可能與共生藻從寄生（parasitism）到互利共生（mutualism）的演化過程有關。

Mutualistic relationship between Symbiodinium and corals is crucial importance for the coral reef ecosystem. Using ITS rDNA sequences, we reconstructed the phylogeny of the Symbiodinium. The lengths of the sequences were 526 to 576 bp and the G+C contents were 47.5 to 53.8%. There were four major clades A to D with the genetic distances of 0.283 to 0.677 among the clades and 0.00 to 0.052 among the species within the clades, indicating that there was long divergence among the four clades. A phylogenetic neighbor-joining tree displayed a step-wise differentiation from Clade A, the basal lineage, through Clades D and B to Clade C. The minimum spanning network also revealed that Clade A was ancestral form and clades B, C and D were its evolutionary trajectories. The phylogenetic relationship of the four clades expressed the divergent evolution of Symbiodinium from parasitism to mutualism in corals.