矮南瓜黃化嵌紋病毒分離株致病性變異導致抗病性評估的偏差

Resistance evaluation bias due to isolate variation of Zucchini yellow mosaic virus

矮南瓜黃化嵌紋病毒冬瓜分離株（ZYMV_31B05）與傳統的胡瓜分離株（ZYMV-7），以溫室苗期法接種試驗接種瓜類栽培種質，經抗病性評估發現ZYMV_31B05對冬瓜及胡瓜的致病性及致病力都明顯高於ZYMV-7，對西瓜、甜瓜及南瓜的結果則相反。以接種的胡瓜樣本ELISA反應值之頻率分佈來比較分析2株病毒，可見ZYMV_31B05在上位葉的值偏高，表示接種感染的ZYMV_31B05都能移行到上位葉，此為ZYMV-7所不及，因此在這些胡瓜栽培種質中ZYMV_31B05致病力高於ZYMV-7。利用Y. Park等人於2004年所設計的胡瓜抗病分子標誌來進行聚合酶連鎖反應，可得到完全重現性的結果，但是部份外表型明顯抗病或感病的樣本，以分子標誌來鑑別卻得到相反的結果。利用馬少芹等人於2006年所報告的西瓜抗病分子標誌來進行的試驗，結果也是如此。原因可能在我們接種所用的病毒分離株，或是對抗病或感病的定義都與前人不同所致。試驗中以ELISA反應值可找出無病徵感染的樣本，但若完全以ELISA反應值來評估抗病性，這將使一些與病毒較親和的耐病栽培種質被評估為感病性，這樣的結果和育種實務以病徵的嚴重性作為評估標準的結果將會有很大的偏差。

"Zucchini yellow mosaic virus wax gourd isolate (ZYMV_31B05) and the typical cucumber isolate (ZYMV-7) were used to inoculate cucurbit cultigens by greenhouse seedling inoculation method. Based on host reaction in inoculation test, pathogenicity and virulence of ZYMV_31B05 to wax gourd and cucumber were evaluated higher than those of ZYMV-7, but to watermelon, melon and pumpkin, the reverse was observed. Analysis of the frequency distribution of ELISA values derived from cucumber plant inoculated with ZYMV_31B05 or ZYMV-7 revealed the further differentiation of both isolates. In the upper leaves of plants inoculated with ZYMV_31B05, the ELISA values of were higher than those inoculated with ZYMV-7. Evidently, the ZYMV_31B05 had mostly migrated into the upper leaves, which is not the ZYMV-7 could reach. Molecular marker designed by Park et al. 2004 for selecting cucumbers with ZYMV resistance were adopted in the experiment, and the results of polymerase chain reaction (PCR) were completely repeatable as reported previously; however some cultigens with clear resistant or susceptible phenotypes generated the opposite results when tested for the presence of the markers. Using ZYMV-resistance marker for watermelon as reported by Ma et al. 2006 also resulted in the similar situation. The contradiction may due to the virus isolates used for inoculation are varied, or the definitions for resistance or susceptibility are different from the previous reports. When the virus-resistance was evaluated by ELISA values, some of symptomless virus-infected samples were discovered in this study. However, some of the tolerant cultigens with virus-compatibility were evaluated as susceptible if the judgment was absolutely dependent on ELISA values. Consequently, the outcome will be a great deviation from breeding practices in which symptom severity is the main criterion for disease-resistance evaluation."