Like a rock? Lack of sequence variation in clinical isolates of the dermatophyte \u3cem\u3eTrichophyton rubrum\u3c/em\u3e

Trichophyton rubrum (Ascomycota) – the cause of athlete’s foot, ringworm and nail infections – is the most common dermatophyte affecting humans. Despite availability of an annotated genome, this microbe’s pathogenicity, epidemiology, and genetics remain poorly understood due to an inability to conduct genetic crosses, lack of adequate sampling, and apparent clonality. The only evidence of sexual mating in T. rubrum is indirect - a single genetic hybrid was formed between T. rubrum and Arthroderma simii, the teleomorph of a related species (T. mentagrophytes). Trichophyton rubrum lacks a known teleomorph, and although its genome contains mating type genes, only a single MAT allele (MAT1-1) is known. This study has three aims: (1) survey sequence variation in our 194 clinical samples using sequence-based and previously-developed variable number tandem repeat (VNTR) markers; (2) use paired-end Illumina sequencing to identify additional polymorphism within and among strains belonging to different VNTR types, and (3) search for evidence of (same-sex) mating using genetic and culture-based methods. First, we sequenced 7.5 kb of mainly non-coding regions located within 20 kb of the ABC1 and CAP59 genes in ten T. rubrum isolates collected over a span of ten years. These sequences revealed six singleton polymorphisms, which distinguished only half of the strains. We are now transitioning to VNTR genotyping and whole genome Illumina sequencing to characterize the diversity more completely. Lastly, in order to test for formation of ascomata (sexual structures), we will co-culture ten of our T. rubrum isolates and a T. tonsurans isolate (MAT1-2) in single, pair-wise and triple combinations on dermatophyte mating medium. Cultures with extensive hyphal coiling (indicative of early sexual development) will be dissected and examined for presence of ascomata or ascospores. Thus far, our results provide no evidence of sexual reproduction and re-confirm low sequence variation in T. rubrum clinical isolates