Intranuclear virus-like particles in a laboratory strain of Phytophthora drechsleri

Abstract

Electron microscopy of ultrathin sections demonstrated virus-like particles (VLPs) within nuclei of Phytophthora drechsleri (strain 6503) during asexual reproduction. Two types of particle occurred in interphase nuclei of zoospores and encysted zoospores, and in interphase and mitotic nuclei of germlings, but particles were not observed within the cytoplasm of these structures. Type 1 VLPs had a shell and a core which was either empty or contained an electron-opaque granule. The mean outer diameter of the shell was 41.6 nm. These particles occurred singly or in aggregates of as many as 50, embedded in a granulofibrillar matrix. Type 2 VLPs looked like type 1 VLPs surrounded by electron-opaque, chromatin-like material from which the core was set off by a concentric clear zone. The diameter including this ‘halo’ was 54 nm. These particles only occurred singly, commonly associated with the nuclear envelope and connected to it by the surrounding dense material. In size and morphology both of these particles resembled mycoviruses and VLPs in other fungi. There was no ultrastructural evidence for cytopathological effects, but if the VLPs proved to be viruses, their presence could explain the slow rate of hyphal extension and somatic segregation of colony morphology in this strain.