Nicotianamine synthase 是植物體內金屬運輸機制的關鍵酵素，而 nicotianamine synthase (NAS)基因則是此蛋白的編碼基因。根據前人研究，NAS基因過表現可提昇植物累積重金屬的能力，作為高聚積植物調控體內重金屬含量重要的遺傳基礎。因此，在植物遭受環境過量重金屬的毒害時，NAS基因的遺傳多樣性可能有助於族群適應重金屬污染。本篇論文以NAS基因的遺傳歧異度探討鋅污染對碎米薺族群造成的天擇作用與族群結構上的影響。

首先以微衛星序列分析族群間遺傳差異，發現族群間存在中度或高度分化，顯示族群間缺乏基因交流。接著進一步分析三組NAS基因在有污染和無污染族群的差異，但是族群間呈現之遺傳差異並非來自重金屬汙染的天擇壓力，因此碎米薺在各地族群的NAS基因差異可能是由其他的環境因子造成影響。最後分析NAS基因家族自身的演化歷史，發現三組基因中都發現有方向性天擇作用，且皆為負向天擇，顯示雖然NAS基因在族群與鋅污染之間無法觀察到明顯的關聯，但仍在碎米薺扮演重要的生理功能，未來針對天擇來源作更深入的探討可能有助於發現 nicotianamine synthase 的新功能。

Nicotianamine synthase is a major enzyme involved in transport and accumulation of heavy metals in plants. The over-expression of nicotianamine synthase (NAS) has been shown to improve the capabilities of heavy metal accumulation in plants. The purposes of this thesis are to examine the role of NAS genes and the impacts on genetic diversity across Cardamine flexuosa populations.

Microsatellite fingerprinting revealed a medium or high level of genetic differentiation among populations. There were high genetic polymorphism on three NAS genes (π = 0.01824~0.02269). However, no differentiation was detected between polluted and unpolluted populations at the NAS loci. When examined the mode of natural selection, the Tajima’s D values remained negative but not significant. Nevertheless, directional selection was detected in three paralogs of the NAS gene family for Fu and Li’s D* values which were significantly negative, as dN/dS < 1 suggested that NAS genes were under negative selection.

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