入侵紅火蟻族群遺傳初探及入侵源之分析－微隨體基因座之應用

Abstract

入侵紅火蟻（Solenopsis invicta Buren）於2003年10月，首度於桃園及嘉義地區被發現，並分別成功定殖了約26000及8000公頃之面積。為了減少入侵紅火蟻造成之損失擴大，大規模之滅絕計畫（eradication program）迅速地展開。以滅絕的角度而言，立即判定其入侵來源為此計畫成功與否的關鍵部分，因為於確定入侵來源後，可作為檢疫措施的依據，加強檢測由特定管道所進口之物品，防止再次入侵的可能性。本研究採用6個微隨體基因座（microsatellite loci）進行台灣地區內火蟻族群遺傳學之初步研究及世界各主要火蟻發生地之親緣關係之探討。以多重引子聚合連鎖反應（multiplex PCR）增幅各族群內火蟻個體特定對基因，並經由一系列的統計分析後結果顯示，桃園及嘉義族群間之對偶子組成及分子變異度（molecular variance）有相當顯著的差異，且嘉義族群出現了約兩倍於桃園之獨特對偶子（private allele），指出兩地可能為不同之入侵源。就異型合子比例而言，嘉義族群相對於桃園擁有較高的遺傳變異（HO = 0.6902）；另一方面，桃園族群呈現較低之遺傳變異（HO = 0.5642），並存在距離隔離模式（isolation by distance），推測可能由單源（次）入侵事件所造成。若進一步以TPM模式（two-phased model）檢測，此族群的確遭受近期瓶頸效應之影響。由親緣關係樹分析之結果得知，桃園及嘉義兩入侵地最有可能皆是由美國所入侵，但推測可能為美國境內不同區域或是來自遺傳組成相異之族群。族群分化指數及遺傳距離皆顯示台灣兩地之火蟻族群只呈現微弱的基因交流，足以視為遺傳上隔離之族群。此現象對於提升整體滅絕計畫之成功率有相當大的助益，並可將兩地火蟻族群各視為單一管理單元（management unit）。此外，根據兩地遺傳多樣性所呈現之不同結果顯示，無論由單次入侵伴隨瓶頸效應之影響或是由多源（次）之入侵，皆能被視為於台灣兩個發生地內火蟻族群入侵成功的重要因素。除了遺傳組成之因素外，以生態觀點而言，未能於入侵紅火蟻體內偵測到潛在天敵Wolbachia之感染，也符合敵解假說（natural enemy-release hypothesis）之論點，即缺少共演化之天敵可能促使入侵紅火蟻於台灣之族群擴張。

Red imported fire ant (Solenopsis invicta Buren) has been discovered in two areas within Taiwan, Taoyuan and Chiayi Counties, in October, 2003 and occupied approximately 26000 and 8000 ha territory, respectively. In order to minimize the loss caused by fire ants, large-scale eradication program should immediately be implemented. For the prospect of eradication, immediately determining the geographic source of invasion becomes one of the most important tasks. It will lead the success of eradication program, because the further introductions or reinvasions will be avoided by applying the intensive quarantine regulations on the given possible invading routes based on the exploration on the origin of invasion. Herein, I applied six microsatellite loci by means of multiplex PCR to investigate the phylogenetic relationship and population structure of S. invicta populations that will suggest the origin of invasion and contribute to the eradication efforts. The Chiayi population possesses significant molecular variance and twofold the number of private alleles as compared with the Taoyuan poopulation, suggesting that both populations were possibly initiated from independent invasions. Taking the genetic diversity into account, the Chiayi population presents relatively higher genetic diversity (HO = 0.6902) and is accordingly presumed to establish by multiple introductions; while the Taoyuan population displays lower genetic diversity (HO = 0.5642) and significant isolation by distance, coupled with remarkable heterozygosity excess under the two-phased model (TPM), the population is suggested to initialize with a single introduction. The combination of allelic distribution and phylogenetic analysis suggests that the two areas most probably experience colonization from different regions or genetically distinct populations of the USA. Furthermore, slight gene flow and pronounced genetic differentiation between infested areas suggest the two populations are quite genetically isolated. Coupled with geographical isolation, it is relatively effective to carry out the eradication program at each small population with negligible connectivity. In addition, either genetic departure from the presumed source population accompanied by bottleneck effect or high genetic variability originating from multiple introductions can be concluded as the crucial characteristic contributing to the successful invasiveness of S. invicta based on the alternative patterns of genetic diversity shown in the two infested areas. The absence of Wolbachia infections corresponds with the natural enemy-release hypothesis and assists the successful invasions of S. invicta in Taiwan.