CYTOPATHOLOGY AND EXPERIMENTAL HOST RANGE OF RHOPALOSIPHUM-PADI VIRUS, A SMALL ISOMETRIC RNA VIRUS INFECTING CEREAL GRAIN APHIDS

The pathogenicity, host specificity, and tissue specificity of Rhopalosiphum padi virus (RhPV) were studied by inoculation of virus-free clones of five cereal grain aphid species with density gradient-purified RhPV. When aphids were allowed a 16-hr acquisition feeding on 10 μg/ml of RhPV in sucrose-parafilm membranes and then examined by immunospecific electron microscopy, virus was consistently detected in R. padi and Schizaphis graminum, but not in Metopolophium dirhodum, R. maidis, or Sitobion avenae. Similar results were obtained when the same clones of virus-free aphids were allowed to acquire RhPV by feeding 48 hr on leaves from plants used to rear RhPV-infected colonies. Ultrastructural examinations of midgut tissues of aphids from the above treatments verified the host species specificity of the virus and indicated tissue specificity. Virus was visualized only in the posterior region of the midgut and in the hindgut of infected aphids. Virus was not observed in the anterior midgut, in accessory or principal salivary gland, or in randomly observed muscle or nerve cells. Early interactions between RhPV and midgut cells suggested virus ingress by endocytosis. Infection resulted in progressive loss of cytoplasmic organelles, especially ribosomes, and in the development of small 200-nm membrane vesicles. Virions accumulated only in the cytoplasm and were subsequently released into the gut lumen and hemolymph. Infection and replication of RhPV in R. padi and S. graminum were verified by double-stranded RNA (ds-RNA) analysis. Two ds-RNAs with molecular weights of approximately 7.6 and 1.7 × 106 were consistently isolated from R. padi or S. graminum which had acquired RhPV, and no difference in ds-RNA profile occurred between the two aphid species. No ds-RNA was isolated from uninoculated RhPV-free aphids. These results confirmed the pathogenicity of RhPV and indicated a high degree of host and tissue specificity for the virus. © 1990.