Intraspecific polymorphism of rDNA among five Nosema bombycis isolates from different geographic regions in China.

The microsporidian Nosema bombycis is the causative agent of pébrine, a highly infectious disease of the silkworm Bombyx mori. Three regions of the multicopy rDNA gene were examined in order to investigate the relationships among five Nosema isolates from various regions of China. Ribosomal DNA alleles are present on each of the 18 chromosomes of N. bombycis and show a high degree of variation. In this study the small subunit (SSU) rDNA, internal transcribed spacer (ITS) and intergenic spacer (IGS) regions for up to 10 different rDNA copies from each N. bombycis isolate were cloned and sequenced. As expected we see greater polymorphism in the ITS region (88 variable sites in 179 nucleotides) and IGS (200 variable sites in 279 nucleotides) than in the SSU rDNA (24 variable sites in 1232 nucleotides). Phylogenetic analysis shows greater differences between alleles within an isolate than between the same alleles from different isolates. The data reveal two very different groups, one from the Sichuan province and the other with a broad distribution including four provinces in southeast China and Japan. The Sichuan isolate does not have any rDNA alleles with sequences identical to those in the other isolates, implying that it is a separate, non-intermixing, population or perhaps a separate species from the other isolates. In light of the polymorphic nature of the rDNA alleles in N. bombycis and their presence on every chromosome, the rDNA gene may be useful for understanding the movement and ultimately the source of pébrine infections.