Unequal distribution of the mating type (MAT) locus idiomorphs in dermatophyte species

The mating type (MAT) locus is the key regulator of sexual reproduction in fungi. In the dermatophytes and other Ascomycetes this genomic region exists in two distinct forms (idiomorphs) and their balanced presence is a precondition for successful mating in heterothallic fungi. But the MAT locus not only drives sexual reproduction, it has also been shown to influence pathogenicity, virulence, and/or morphological changes in pathogenic fungi of the genera Candida, Histoplasma, and Cryptococcus. In order to find out whether there are similar trends in dermatophytes, we investigated the MAT locus of 19 anthropophilic and zoophilic species via Sanger sequencing and primer walking. We identified for the first time the MAT locus idiomorphs of the dermatophyte species Microsporum audouinii (MAT1-2), M. ferrugineum (MAT1-2), Trichophyton schoenleinii (MAT1-2), T. bullosum (MAT1-1), T. quinckeanum (MAT1-1), T. concentricum (MAT1-1), T. eriotrephon (MAT1-1), and T. erinacei (MAT1-2). In addition, we determined the MAT locus sequence for dermatophyte species whose mating type idiomorphs had been described on the basis of results of classical confrontation experiments (e.g. M. canis, MAT1-2) and we confirmed recently published molecular data (e.g. T. rubrum, MAT1-2). Our results corroborate that MAT locus idiomorphs are unequally distributed in the majority of the analyzed species and the ability to mate with a partner of the opposite sex is limited to a few zoophilic species. Clonal spreads are identified that are connected to one of the idiomorphs and a higher virulence and/or a higher transmission rate to humans (T. benhamiae and T. mentagrophytes). For the imbalanced idiomorph distribution pattern we hypothesize that either: (I) one of the mating type idiomorphs may be extinct due to clonal reproduction (e.g., T. rubrum and M. canis), (II) mating partners of one species adapted to different hosts followed by speciation in the new niche (e.g., T. equinum and T. tonsurans) or (III) unisexual reproduction is the next evolutionary stage of propagation in dermatophytes which involves the extinction of one mating idiomorph.