A Study On Developmental And Immunological Interactions Between Parasitoids And Their Nonpermissive Host

After parasitoids lay eggs into their hosts,their progenies will face the challenges by the efficient innate immune system of the hosts.Parasitoids have evolved an amazing array of mechanisms to manipulate the development and immune response of their hosts.Polydnavirus(PDV),venom,ovarian proteins and teratocytes are the universal regulatory factors employed by many endoparasitoids to create an environment that is favorable for the development of the parasitoid larvae.Here we used the silkworm Bombyx mori as a nonpermissive host of Microplitis demolitor and Cotesia vestalis to study the developmental and immunological interactions between parasitoids and their nonpermissive host,especially the mechanism of the effects of PDV on their host.The results and conclusions are summarized as follows:(1) In M.demolitor-B.mori system,B.mori larvae became increasingly resistant to the parasitism as they aged.Parasitism almost had no effect on old third or fourth instar larvae,while most parasitized first and early second instar larvae died after they molted to the next instar.Free first instar parasitoid larvae and teratocytes were found in the dying or dead B.mori larvae,which indicated that the host lost the ability of encapsulation.In non-affected B.mori larvae,the parasitoid larvae or eggs were encapsulated and killed by the hemocytes of the hosts.In addition,parasitism could induce supernumerary instars in some young B.mori larvae.(2) Most M.demolitor eggs could develop and hatch in B.mori larvae,although they developed more slowly than the eggs developed in the permissive host Pseudoplusia includens.After egg hatched,the first instar parasitoid larvae ceased development until they were encapsulated by the hosts or died because of the death of the hosts. These results indicated that the physiological,biochemistrical and nutrient conditions in B.mori larvae could support the development of M.demolitor eggs but not the M. demolitor larvae.(3) The injection of MdBV(M.demolitor bracovirus) could not cause the non-parasitized early second instar B.mori larvae to die,but could increase the death rate of the parasitized second instar B.mori larvae from 60%to 100%.MdBV DNA could persist in B.mori larvae and its genes could be expressed,but the level of expression was suppressed.The expression level of MdBV genes was very high in some hemocytes whose abilities of spreading and phagocytosis were affected;while the expression level was relatively low in other hemocytes that could still spread and phagocytize.These results indicated that MdBV could suppress the cellular immune response but this effect was diminished by the low expression level of MdBV genes.(4) Two new genes,CvBV1 and CVBV2,were isolated in CvBV(C.vestalis bracovirus) cDNA library.CvBV1 was located on segment CvBV-S5 with a size of 790 bp,while CvBV2 was located on segment CvBV-S51 with a size of 459 bp.Both these two genes were predicted to have an N-terminal signal peptide.Gene duplication occurred in both genes,tandem gene duplication for CvBV1 and segmental duplication for CVBV2.Gene transcripts of the two genes were detected in hosts as early as 0.5 h post-parasitization and continued to be detected for six days. These two genes were expressed in High Five cells and the recombinant proteins could be detected in the medium,which confirmed that these two proteins were secretive.(5) Parasitism by C.vestalis could neither affect the development of early second instar B.mori larvae obviously nor cause the larvae to die.No C.vestalis egg hatched and embryogenesis proceeded up to germband extension but thereafter development ceased.The embryos died and were encapsulated by the hemocytes of B.mori larvae. The expression of CvBV gene CvBVl,CvBV2,EP1,EP2,ORF805 and Lectin was examined by RT-PCR.,of whcih CvBV2,ORF805 and EP2 could not be detected, CvBV1 and Lectin could only be detected on 1 day post-parasitization,and only EP1 gene could be detected at all the examined time points.CvBV had no effect on the ability of phagocytosis.These results indicate that the expression of CvBV genes in B. mori larvae was inhibited and the virus had no effect on the cellular immune response of B.mori larvae.