High-frequency intragenomic heterogeneity of the ribosomal DNA intergenic spacer region in Trichophyton violaceum

The intergenic spacer (IGS) of the rRNA genes was analyzed from the dermatophyte Trichophyton violaceum isolated from cases of tinea capitis in Taiwan and Iran. T. violaceum strains were cultured from different colonies, from single conidial colonies derived by dilution plating, and from micromanipulation of single conidia from clinical samples. A ribosomal DNA probe hybridizing to multiple EcoRI fragments was used to compare restriction fragment length polymorphisms in different T. violaceum isolates. The arthroconidia of T. violaceum that form in vivo during infection were shown to contain a single nucleus by 4',6'-diamidino-2-phenylindole staining. IGS regions from an isolate cultured from a single conidium were amplified, cloned, and sequenced. The results identified that heterogeneity exists between IGS regions within a single T. violaceum genome due to different copy numbers of a 171-bp tandem repeat. This suggests that the IGS of T. violaceum is partially excluded from the concerted evolution of the rRNA gene locus. The heterogeneous character of the IGS regions in T. violaceum contrasts with the closely related dermatophyte Trichophyton rubrum, posing further questions on the phylogeny and the evolution of dermatophyte fungi.