臺灣杜鵑之低變異Non-TIR-NBS-LRR基因亞家族族群遺傳與演化模式

Abstract

植物抗病基因適合使用於分子族群遺傳的處理，以偵測並了解分子層次的天擇作用。而植物的抗病基因家族中以NBS-LRR (nucleotide-binding site plus Leucine-rich repeats)為最大成員。目前用來解釋植物與病原體交互作用之共演化的模式，以軍備競賽(arms race)以及壕溝戰(trench warfare)為主軸。這兩種模式分別是受到正向天擇(positive selection)和均衡天擇(balancing selection)影響。本研究選殖了先前研究得知為低度遺傳變異之Non-TIR-NBS-LRR基因亞家族之抗病基因成員。我們調查了臺灣杜鵑的10個野生族群在NBS譯碼區的歧異度，發現NBS序列的大多變異發生在族群內，顯示族群之間沒有明顯分化。此外利用CODEML分析位點模式的比較結果，偵測到NBS序列受到天擇的位點，這些區域顯示可能影響NBS與ATP結合能力，進而提高抵抗疾病的訊息傳遞和辨識病原的能力或效率有關。在本研究中支持了軍備競賽的假說，而非只是族群動態變化所致，因此在族群中抗病基因之變異是暫時的，對偶基因是因為selective sweeps而呈現高頻率。Plant disease resistance genes are promising candidates for using molecular population genetic approaches to detect and understand natural selection at the molecular level. The nucleotide-binding site plus Leucine-rich repeats (NBS-LRR) gene family is one of the major plant resistance genes. Two evolutionary models, an ‘arms race’ and a ‘trench warfare’, are often invoked to explain co-evolution of host-pathogen interactions. The selective forces act on these two models positive selection and balancing selection, respectively. We cloned NBS sequences which were considered low genetic variation in previous research. Then individuals from each of ten natural populations of plant Rhododendron formosanum Hemsl. were examined for the patterns of nucleotide diversity at NBS-encoding genes. Most variation of NBS sequences were found within populations indicated low population differentiation. Results from site model comparisons of the CODEML analyses detected positively selected codon sites. Regions with positively selected were might be related to ATP binding ability which promoted resistance signal transduction and enhanced pathogen recognition efficiency. This research supports an ‘arms race’ hypothesis, in which variation for resistance will be transient, and that alleles spread to high frequency because of selective sweeps.