Morphological properties of the occlusion body of Adoxophyes orana granulovirus

A granulovirus (GV) that produces occlusion bodies (OBs) having an unusual morphology was found in an Adoxophyes sp. (Lepidoptera: Tortricidae) larva in a tea field in Miyazaki Prefecture, Japan. This isolate is considered to be a mutant of Adoxophyes orana granulovirus, designated AdorGV-M, because the nucleotide sequence of its genome is 99.7% identical to that of an English isolate of AdorGV, AdorGV-E. AdorGV-E produces typical ovocylindrical OBs that contain one occlusion-derived virus (ODV) per OB. On the other hand, AdorGV-M produces large cuboidal OBs, but the number of ODVs per OB was unknown. In this study, we quantified viral DNA in OBs of both AdorGV-E and -M, and determined the number of ODVs occluded in an OB of AdorGV-M. The two isolates had the same quantity of viral DNA in each OB, and we thus confirmed that one OB of AdorGVM contains one ODV. To investigate the process of OB formation, fat body tissue of A. honmai larvae inoculated with each isolate was observed in a time course by transmission electron microscopy, and OB sizes were measured from micrographs. The main difference in OB formation was that AdorGV-M required more time to mature than AdorGV-E. In AdorGV-E, ODVs began to be covered from one end with an ovocylindrical OB at 96 h postinoculation (hpi), and most of them were completely occluded at 120 hpi. Occlusion of AdorGV-M ODVs also began at 96 hpi, but the OB shape was cuboidal. Moreover, the OB size of AdorGV-M was similar to that of AdorGV-E at 120 hpi, but continued to grow until 192 hpi. AdorGV-M thus took more time to complete OB formation. Consequently, AdorGV-E has mature OBs with a diameter 0.22 μm and length 0.39 μm, but those of AdorGV-M are 1.34×1.23 μm.