大菜粉蝶根虫瘟霉菌株转染小菜蛾后侵染力变化及其与寄主血淋巴酚氧化酶活性的关系

源于大菜粉蝶Pieris brassicae的根虫瘟霉Zoophthora radicans菌株R₀通过反复转染小菜蛾Plutella xylostella而分别获得转染菌株R₁、R₃和R₅。用这些菌株对小菜蛾2龄幼虫进行生物测定，发现菌株对寄主的侵染力有随转染次数增加而增强的趋势。接种后第1～6 天，R₀的LC₂₀（孢子数/mm²）分别为14.7、14.5、9.0、7.1、6.0和5.5；R₁的LC₂₀分别为9.6、5.0、4.2、3.6、3.1和3.0；R₃的LC₂₀分别为4.6、2.9、2.8、2.5、2.4和2.2； R₅的LC₂₀分别为5.2、3.7、3.2、2.8、2.6和2.6，接种后同一天菌株 R₃的LC₂₀值最小即侵染力最强。各菌株感染小菜蛾幼虫后可显著激活寄主血淋巴中的酚氧化酶活性，但R₁、R₃和R₅对酚氧化酶的激活程度显著低于原始菌株R₀。各菌株对小菜蛾的侵染力强弱指标log₁₀ (LC₂₀)与其侵染后寄主血淋巴酚氧化酶活性呈明显正相关（0.852<0.95），表明R0在对新寄主转染过程中逐渐获得了逃避或克服新寄主免疫防御的能力，从而增强对新寄主的侵染力。

Infectivity of a Pieris brassicae-derived Zoophthora radicans isolate and its new host-passage isolates against Plutella xylostella in relation to the phenoloxidase activity in the new host hemolymph after infection

A Pieris brassicae-derived Zoophthora radicans isolate, R_0, and its host-passage isolates R_1-R_5, obtained by repeatedly passing through Plutella xylostella larvae, were bioassayed against the second instar larvae of the new host in the laboratory. The host-passage isolates tended to exhibit higher infectivity to the new host than the original isolate as the number of host passages increased. Based on the time-concentration-mortality modeling, the LC_(20) (a lethal concentration to cause 20% mortality) on days 1-6 after exposure to spore showers was estimated as 14.7, 14.5, 9.0, 7.1, 6.0 and 5.5 spores/mm~2 respectively for R_0; 9.6, 5.0, 4.2, 3.6, 3.1 and 3.0 spores/mm~2 for R_1; 4.6, 2.9, 2.8, 2.5, 2.4 and 2.2 spores/mm~2 for R_3; and 5.2, 3.7, 3.2, 2.8, 2.6 and 2.6 sproes/mm~2 for R_5.The phenoloxidase activities were significantly lower in the larval hemolymph stimulated by the infection of the host-passage isolates. There was also a positive correlation between the log_(10) (LC_(20)) and the detected phenoloxidase activities among the isolates (0.85