菜蛾盘绒茧蜂卵携带的免疫抑制因子

Immunosuppressive factors carried by eggs of the parasitoid wasp Cotesia vestalis(Hymenoptera:Braconidae)

抑制寄主昆虫的免疫反应是内寄生蜂存活的关键。菜蛾盘绒茧蜂Cotesia vestalis(Haliday)寄生小菜蛾Plutellaxylostella(L.)幼虫后,蜂卵如何逃避和抑制寄主的免疫攻击,尚未得到全面揭示。本文采用电镜技术系统观察了菜蛾盘绒茧蜂卵表面的超微结构。结果显示:蜂卵表面覆盖有纤维层和絮状的类病毒样纤丝(VLFs),同时携带了含多分DNA病毒粒子(PDV)的萼液。在寄生初期,包裹在蜂卵表面的纤维层和VLFs首先起到保护蜂卵不被小菜蛾血细胞包囊的被动防御作用。随后,PDV发挥主动的免疫抑制作用。通过假寄生手段,证明了菜蛾盘绒茧蜂PDV(CvBV)具有较持久的克服寄主免疫攻击的能力,是主要的免疫抑制因子。在假寄生后连续8d的观察时间内,菜蛾盘绒茧蜂的蜂卵均未被包囊。结果提示,在菜蛾盘绒茧蜂-小菜蛾寄生体系中,菜蛾盘绒茧蜂采取被动防御和主动攻击两种方式应对寄主小菜蛾的免疫攻击。

Suppression of host immune response after parasitization is essential for the survival of endoparasitoids. In Plutella xylostella （ L. ） （ Lepidoptera ： Plutellidae） larvae parasitized by Cotesia vestalis （Haliday） （ Hymenoptera ： Braeonidae）, how the parasitoid egg avoids encapsulation by the host＇ s immune attack is still not well understood. In this study, the ultrastructure of C. vestalis egg surface was investigated systematically using electron microscopes. The results showed that the egg possesses a fibrous layer, which is coated by an overlying sheath of virus-like filaments. During oviposition, C. vestalis lays an egg into the hemocoel of the host larvae, together with calyx fluid mainly including polydnavirus. Both the outer virus-like filaments and the fibrous layer of parasitoid eggs seem to be the first passive protection barrier to host immunity during the initial stages of parasitism. They may delay encapsulation until a more permanent means of suppressing encapsulation is established. Afterwards, polydnavirus plays a great role in the immune suppression of recognition and encapsulation by host hemocytes. Within eight days post pseudoparasitization by C, vestalis, the encapsulation response of P. xylostella larval hemocytes to the irradiated eggs was strongly suppressed, indicating that the polydnavirus has a permanent immunosuppressive capability. The results suggest that in the system of C. vestalis-P, xylostella, C. vestalis avoids the host immune reaction by a combination of both passive and active mechanisms.