- 學位論文
以繁殖體壓力及遺傳多樣性探討臺灣入侵紅火蟻成功入侵模式
The role of propagule pressure and genetic diversity in successful invasion of the red imported fire ant (Solenopsis invicta) in Taiwan
摘要
遺傳多樣性是物種生存與演化的根基，當入侵生物抵達新的環境時，僅有少數奠基者 (founder) 得以成功定殖而導致遺傳多樣性降低，提昇自交衰亡 (inbreeding depression) 或累積致死因子 (deletrous) 而滅絕的機率，卻依然成功入侵新環境並對當地生物產生危害，降低當地生物多樣性。其中社會性昆蟲僅需少量奠基者即可成功定殖，使得拓殖初期難以偵測其入侵與否直至產生穩定族群，加上其繁殖力強、擴散能力高以及形成「超級個體 (super individual)」之高競爭力導致社會性昆蟲如螞蟻、黃蜂、以及白蟻成為近年來新興的入侵生物。源自南美之入侵紅火蟻 (Solenopsis invicta) 於近幾年證實入侵臺灣、中國及澳洲等國家，提供一個了解入侵生物成功關鍵相當好之題材。入侵紅火蟻之性別是由單一基因座互補性別決定系統 (single locus complementary sex determination [sl-CSD] system) 決定；理論上，入侵紅火蟻會因喪失遺傳多樣性而大量產生二倍體不孕雄蟲進而產生嚴重衰亡。本研究利用族群遺傳的方式來了解入侵紅火蟻如何克服此項困難。我們計算配對交配蟻后 (match mated queen) 之比例來回推性別決定基因之等位基因數，估算臺灣火蟻之性別決定基因擁有12.5個不同等位基因，此結果與臺灣火蟻之入侵來源並無明顯差別。接著利用12個微隨體基因座 (microsatellite) 於桃園120巢火蟻中進行分析，結果顯示兩個不同基因群 (genetic cluster) 混雜分布於其中，代表至少發生兩次以上不同來源的入侵事件。這些跡象顯示入侵紅火蟻可能因為多次入侵臺灣而維持其遺傳多樣性，進而逃離性別決定基因之遺傳負荷 (genetic load) 成為一成功之入侵生物。本研究不僅發現需要大量遺傳標記才可發現的隱藏入侵事件，更提供了遺傳多樣性與多次入侵成功重要關聯的證據。
並列摘要
Genetic diversity is one of the key determinants for species’ survival and evolution. From the conservation biology perspective, a small population is prone to extinction by inbreeding depression or accumulation of deleterious mutations. Invasive species, however, are generally successful in introduced ranges despite loss of genetic diversity due to small founder population sizes, which gives rise to a paradox between invasion and conservation biology in term of genetic diversity. Social insects, such as ants, wasps, and termites, represent a highly successful group of invasive species not only because their small founder population sizes often lack immediately recognizable signature but also their great dispersal power, high reproductive rate and the competitive ability of “super individuals”. One of the notorious ant invaders, the red imported fire ant (Solenopsis invicta), originating from South America, has been accidentally introduced into the USA, Australia, Taiwan and China. Invasions by fire ants in these areas provide an excellent framework to investigate the factors contributing to the successful establishment of social insects especially since this species utilizes a single locus complementary sex determination (sl-CSD) system. Theoretically, the genetic load is predicted to be much more profound for S. invicta due to the mass production of sterile diploid males imposed by loss of genetic diversity. This study therefore aimed to determine how fire ants overcome such a disadvantage using population genetic approaches. An assessment of genetic variation at 12 microsatellite loci in 120 fire ant colonies in northern Taiwan (Taoyuan) revealed that Taoyuan was colonized by ants on a minimum of two occasions. Furthermore, estimates of genetic diversity at the CSD locus by calculating the proportion of match matings (queens mated with males harboring identical CSD allele) do not differ significantly between fire ant populations in Taiwan and the putative source, the USA, suggesting that the successful invasion of fire ants is likely attributable to adequate genetic diversity maintained by larger propagule number (e.g, multiple invasions). In summary, this study not only highlights that cryptic invasions might be undetected until the deep genetic structure of focal populations has been revealed but also signifies the importance of genetic diversity in shaping the invasion success of fire ants.