MICROPLITIS-DEMOLITOR POLYDNAVIRUS INFECTS AND EXPRESSES IN SPECIFIC MORPHOTYPES OF PSEUDOPLUSIA-INCLUDENS HEMOCYTES

Microplitis demolitor is a polydnavirus-carrying wasp that parasitizes the larval stage of Pseudoplusia includens. M. demolitor eggs are nev er encapsulated by host haemocytes when coinfected with its associated polydnavirus (MdPDV) whereas eggs are encapsulated within 36 h when i njected into hosts without virus. In this study, infection of specific classes of P. includens haemocytes by MdPDV was examined. Electron mi croscopic studies indicated that MdPDV entered all haemocyte morphotyp es. Northern blot analysis revealed that similar size classes of viral mRNAs were produced in granular cells, plasmatocytes and spherule cel ls. Expression of a 1.6 kb MdPDV mRNA in haemocytes from parasitized h osts was detectable by in situ hybridization at 2 h post-parasitism (p .p.) and continued through until day 6 p.p. By 12 h p.p., viral expres sion was detected in greater than 80% of the haemocytes in circulation but thereafter the percentage of haemocytes exhibiting a hybridizatio n signal declined. Similar patterns were observed in haemocytes from l arvae injected with calyx fluid or MdPDV plus venom. Granular cells an d plasmatocytes from unparasitized larvae were purified on Percoll cus hions and maintained in vitro. Both morphotypes were successfully infe cted with MdPDV and exhibited changes in morphology and adhesiveness v ery similar to cells from parasitized hosts. Cell-free plasma from par asitized larvae had a variable effect on haemocyte adhesion. Haemocyte s cultured in plasma from 1 or 4 day p.p. larvae rapidly spread wherea s cells cultured in 7 day p.p. plasma did not. Reciprocally, adhesion of haemocytes from parasitized larvae could not be rescued by cell-fre e plasma from unparasitized larvae. Together, these data suggest that disruption of the host encapsulation response is mediated primarily by direct infection of granular cells and plasmatocytes by MdPDV.