Use of nucleotide sequence data to identify a microsporidian pathogen of Pieris rapae (Lepidoptera, Pieridae)

Nucleotide sequence was determined for a portion of genomic DNA which spans the V4 variable region of the small subunit ribosomal RNA gene of an unidentified microsporidium from the cabbage white butterfly, Pieris rapae (174 base pairs). Comparison with equivalent sequence data obtained here for two other microsporidian species, Nosema bombycis (240 base pairs) and Nosema bombi (200 base pairs), and from the GenBank database for 11 other microsporidian species suggests that the unidentified species from P. rapae is most closely related to some Vairimorpha species. Light and electron microscopic observations of the developmental stages of this parasite were in accord with this. Infection experiments conducted at 20 and 26 degrees C demonstrated temperature-dependent dimorphism, with the production of both binucleate free spores (mean dimensions: 3.8 x 1.8 microns; 10-13 polar filament coils) and membrane-bound uninucleate octospores (mean dimensions: 3.1 x 1.9 microns). Macrospores (mean dimensions 8.0 x 2.1 microns) were also observed. Sites of infection were the gut epithelium, the Malpighian tubules, the salivary glands, and the fat body. Infections were found in all insect life stages, including the egg. This microsporidium was found to be indistinguishable from both Nosema mesnili (Paillot) and Microsporidium (Thelohania) mesnili (Paillot) and we propose that these species be combined and transferred to the genus Vairimorpha Pilley.