| The prophenoloxidase-activating system (ProPO-AS) is an efficient non-self recognition system which involves in the generation of superoxide, melanin synthesis, and subsequent sequestration of foreign matter entering the hemocoel of the insect. The entomophthorales is one of the most promising entomopathogenic fungi, while few studies have been made on insect immunity which was activated by entomopathogenic fungi and there is no report in China. This thesis is a preliminary study on immune interaction between Zoophthora radicans and Plutella xylostella. It presents the results on the virulence of Z. radicans against P. xylostella, changes of insect phenoloxidase (PO) during invasion by the Z. radicans, location and characteristics of ProPO, purification and immunological characteristics of ji-l,3-glucan binding protein (PGRP), and effect of Z. radicans and (5GRP on PO activities.Virulence of original-host-different Z. radicans isolates against P. xylostella larvae Four Z. radicans isolates from different original hosts were bioassayed against the second instar larvae of the diamondback moth, P. xylostella in laboratory. Each isolate included eight dosages resulting from time-varying spore shower. On day 8 after exposure to spore shower for inoculation, a P. xylostella-derived isolate, ARSEF 1100, caused a cumulative mortality of 2.4-97.4% at 0.5-319 conidia/mm2 with all cadavers exhibiting typical Zoophthora syndrome. Two isolates derived from Empoascaflavescens, ARSEF 2699 and F99101, killed 2.4-50% and 2.4-60.5% at 1.6-315 and 1.8-484 conidia/mm2, respectively, whereas the isolate ARSEF 1342 from Pieris brassicae killed only 6.52-13.63% at 3.5-633 conidia/mm2. Typical Zoophthora syndrome was observed only in approximately one third of cadavers killed by the latter three isolates. Fitting the data to a time-dose-mortality model generated the estimates of parameters for time and dose effects of each isolate. The parameter estimate for dose effect (B) was 1.89 for ARSEF 1100, 1.48 for F99101, 1.23 for ARSEF 2699, and 0.37 for ARSEF 1342, being significantly different from one to another. The LD50 values estimated for ARSEF 1100 were 232, 113, 71, 41 and 35 conidia/mm2 on days 4-8 after exposure, respectively. The corresponding estimates were 1344, 922, 556, 410 and 397 conidia/mm2 for ARSEF 2699, and 666, 452, 414, 351 and 333 conidia/mm2 for F99101. The virulence for ARSEF 1342 was too weak for its LD50 to be estimated. Conclusively, the isolate ARSEF 1100 was most virulent to P. xylostella, and can escape from immune system of the host.Changes of PO activity in host hemolymph during Z. radicans invasion Afterinfection by different Z. radicans isolates, the PO activities in the hemolymph of the P. xylostella larvae infected was significantly higher than that of the uncontaminated hosts. Different original-host isolates induced different changes of PO activities. PO activities activated by two isolates from Homoptera is much higher than thoes by two isolates from Lepidoptera, the highest PO activity was happened at ARSEF2699 isolate, which was 7-16 folds higher than that of uncontaminated hosts. In addition, The PO activities induced Z. radicans was changed from low to high and then to low. The peaks of PO activitise for F99101, ARSEF1100, ARSEF2699 and ARSEF 1342 was happened on the dl, d2, d3, and d4 respectively, Which may be corresponded to their aggressive speed for each isolate. Then, PO activities began to decrease, especially those induced by ARSEF99101 and ARSEF 1100 on the forth day after inoculation, they are even lower than that of the uncontaminated hosts. The results suggested that the PO activity in hemolymph of host was controlled at later periods due to the fungus infection. Meanwhile, there is the negative relation between virulence of original-host-different Z. radicans isolates against P. xylostella and PO activity induced by them. This maybe indicates that the isolates from similar original-hosts will easily conquer and escape from immune system. But, there is an exception. PO...
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