Abstract
Insect host/parasitoid interactions are co-evolved systems in which host defenses are balanced by parasitoid mechanisms to disable or hide from host immune effectors. Here, we report that Pteromalus puparum venom impairs the antimicrobial activity of its host Pieris rapae. Inhibition zone results showed that bead injection induced the antimicrobial activity of the host hemolymph but that venom inhibited it. The cDNAs encoding cecropin and lysozyme were screened. Relative quantitative PCR results indicated that all of the microorganisms and bead injections up-regulated the transcript levels of the two genes but that venom down-regulated them. At 8 h post bead challenge, there was a peak in the transcript level of the cecropin gene, whereas the peak of lysozyme gene occurred at 24 h. The transcripts levels of the two genes were higher in the granulocytes and fat body than in other tissues. RNA interference decreased the transcript levels of the two genes and the antimicrobial activity of the pupal hemolymph. Venom injections similarly silenced the expression of the two genes during the first 8 h post-treatment in time- and dose-dependent manners, after which the silence effects abated. Additionally, recombinant cecropin and lysozyme had no significant effect on the emergence rate of pupae that were parasitized by P. puparum females. These findings suggest one mechanism of impairing host antimicrobial activity by parasitoid venom.
Highlights
At all developmental stages, most insects are and potentially subject to different infections by a broad range of foreign invaders, including viruses, bacteria, fungi, protozoans, and parasitoids [1,2]
The zone dia. of hemolymph that was extracted from pupae that were treated with the injection of beads plus venom was only 3.0 ̆ 0.2 mm (n = 5), which was not remarkably different from that of the immunologically naïve hemolymph according to the multiple comparison results (F5, 24 = 372.68, P < 0.0001 after Bonferroni-correction for multiple testing). These findings indicate that parasitization and bead injection induce the antimicrobial activity of the host hemolymph, while parasitoid venom treatment decreases this activity very significantly
Venom of the endoparasitoid P. puparum impairs the ability of P. rapae hemocytes to adhere to the surfaces of invaders, which diminishes the encapsulation of the eggs, allowing them to develop in host hemocoels [32]
Summary
Most insects are and potentially subject to different infections by a broad range of foreign invaders, including viruses, bacteria, fungi, protozoans, and parasitoids [1,2]. Insects lack an adaptive immune system and depend on the innate system to overcome invader infections [3]. Insects activate their innate immune responses, which consist of cellular and humoral responses [4]. Nodulation and encapsulation in the case of large invaders, such as the eggs of the parasitoid wasps. Cellular responses are characterized by interactions between hemocytes and invaders [11,12].
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