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Microplitis demolitor polydnavirus infects and expresses in specific morphotypes of Pseudoplusia includens haemocytes
1Department of Entomology, University of Wisconsin-Madison 53706.
Abstract:
Microplitis demolitor is a polydnavirus-carrying wasp that parasitizes the larval stage of Pseudoplusia includens. M. demolitor eggs are never encapsulated by host haemocytes when coinfected with its associated polyndnavirus (MdPDV) whereas eggs are encapsulated within 36 h when injected into hosts without virus. In this study, infection of specific classes of P. includens haemocytes by MdPDV was examined. Electron microscopic studies indicated that MdPDV entered all haemocyte morphotypes. Northern blot analysis revealed that similar size classes of viral mRNAs were produced in granular cells, plasmatocytes and spherule cells. Expression of a 1.6 kb MdPDV mRNA in haemocytes from parasitized hosts 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 expression was detected in greater than 80% of the haemocytes in circulation but thereafter the percentage of haemocytes exhibiting a hybridization signal declined. Similar patterns were observed in haemocytes from larvae injected with calyx fluid or MdPDV plus venom. Granular cells and plasmatocytes from unparasitized larvae were purified on Percoll cushions and maintained in vitro. Both morphotypes were successfully infected with MdPDV and exhibited changes in morphology and adhesiveness very similar to cells from parasitized hosts. Cell-free plasma from parasitized larvae had a variable effect on haemocyte adhesion. Haemocytes cultured in plasma from 1 or 4 day p.p. larvae rapidly spread whereas cells cultured in 7 day p.p. plasma did not. Reciprocally, adhesion of haemocytes from parasitized larvae could not be rescued by cell-free plasma from unparasitized larvae. Together, these data suggest that disruption of the host encapsulation response is medicated primarily by direct infection of granular cells and plasmatocytes by MdPDV.
Insights
Microplitis demolitor polydnavirus (MdPDV) directly infects Pseudoplusia includens haemocytes, including granular cells and plasmatocytes. This viral infection disrupts the host
Area of Science:
- Insect pathology
- Virology
- Immunology
Background:
- Microplitis demolitor parasitizes Pseudoplusia includens larvae.
- Polydnaviruses are essential for polydnavirus-carrying wasps to evade host immune responses.
- MdPDV prevents encapsulation of M. demolitor eggs by P. includens haemocytes.
Purpose of the Study:
- To investigate the infection of specific P. includens haemocyte classes by MdPDV.
- To understand the mechanism by which MdPDV disrupts host encapsulation.
Main Methods:
- Electron microscopy to observe viral entry into haemocytes.
- Northern blot analysis to detect viral mRNA production in different cell types.
- In situ hybridization to track viral gene expression in haemocytes.
- In vitro infection of purified haemocytes.
Main Results:
- MdPDV entered all haemocyte morphotypes, with viral mRNAs produced in granular cells, plasmatocytes, and spherule cells.
- Viral expression was detected in over 80% of circulating haemocytes by 12 hours post-parasitism.
- In vitro infected granular cells and plasmatocytes showed altered morphology and adhesiveness.
- Cell-free plasma from parasitized hosts affected haemocyte adhesion differently depending on the day post-parasitism.
Conclusions:
- MdPDV primarily infects granular cells and plasmatocytes.
- Direct infection of these haemocytes by MdPDV is the main mechanism for disrupting the host encapsulation response.