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 AdorGV-M 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 post-inoculation (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.