台灣稻熱病菌之族群結構

Population structure of Pyricularia oryzae from rice in Taiwan

稻熱病為臺灣水稻重要病害，常對稻米產量及品質造成損失。本研究為探討臺灣各地稻熱病菌族群結構及族群間的基因流動，試將全島各縣市劃分為6區（A～F），每區逢機選取32株單孢菌株，共計192株用於本研究。為菌株基因分型，乃選用非致病基因Avr-Piz-t、簡單重複序列G5及5種Pyrms以及散置重複序列Pot2 rep-PCR等分子標記進行試驗。多型性訊息指數（PIC）顯示，除Pyrms77B及Pyrms409低於0.1，Pot2 rep-PCR及G5約為0.4，其他均達0.6以上，多屬資訊豐富的分子標記。另發現近1/3菌株同時兼具原Avr-Piz-t及插有Pot2轉座子之雙重等位基因。整合上述8種分子標記進行多基因座基因型分析，共得102種組合，亦即有102種基因型，其中75種基因型各僅有1支菌株，另27種基因型分別有2至15支菌株。各地區族群（A-F）分別有10、18、14、11、17及13種獨有基因型（佔比55.0～66.7%），但無任何基因型遍布6個地區。根據上述8種分子標記建構全台6個地區族群的遺傳距離樹狀圖顯示，花蓮台東族群迥異於其他族群，而西部各族群也與宜蘭族群有別。稻熱病菌族群遺傳多樣性指數分析顯示，苗栗－台中－南投族群遺傳變異最大，其次是宜蘭族群。本研究結論為，台灣稻熱病菌已在不同地區演化出基因型特異之族群結構，而不同地區族群間的基因流動率低，可能藉由人為輸送罹病種子、秧苗或稻殼之傳播。

Rice blast disease caused by Pyricularia oryzae is one of the most devastating diseases of rice in Taiwan and worldwide. Despite the importance of rice blast disease, little is known about its diversity and population structure in Taiwan. The present study intended to understand the population structure of P. oryzae and gene flow between six regional populations in Taiwan. A total of 192 isolates (32 from each population) were collected from the whole island and genotyped using different molecular markers: Avr-Piz-t avirulence gene, Pot2-based repetitive element polymerase chain reaction (rep-PCR), and simple sequence repeat markers: G5 and pyrms. Except for pyrms77B and pyrms409, all markers showed the polymorphic information content values between 0.41 and 0.75, suggesting that most of the markers were highly informative to capture the population variances. Interestingly, we found that 61 isolates had the intact Avr-Piz-t allele coexisting with the gene inserted with Pot2 transposon (double-allele). In population structure analyses based on the aforementioned markers, 102 multilocus genotypes were identified and none of them was found in all 6 regional populations. Populations A to F had 10, 18, 14, 11, 17, and 13 unique genotypes (55.0~66.7%), respectively, indicating that each population was very differentiated. Based on Nei's genetic distance, the six regional populations were divided into two clusters. The populations from four western regions and Yilan County were grouped as one cluster, while the Hualien-Taitung population was grouped as another. Genetic diversity indexes showed that population variation is the highest in central Taiwan (Miaoli-Taichung-Nantou region) followed by Yilan. We concluded that the population structure of P. oryzae is closely associated with the geographical location and that gene flow between regions is limited. A few cases of gene flow could be attributed to the transport of infected seeds, seedlings or husks.