The curious adventures ofTrichophyton equinumin the realm of molecular biology: a modern fairy tale

Abstract

Sequence analysis of the highly variable internal transcribed spacer (ITS) regions 1 and 2 of the ribosomal DNA (rDNA) of the nuclear DNA has been used to explore the phylogeny of the dermatophytes. Results have led some investigators to recommend that a number of dermatophyte species be reduced to synonymy with other established species. One such recommendation is that both varieties of the zoophilic dermatophyte Trichophyton equinum be reduced to synonymy with the anthropophilic species T. tonsurans. The morphologies of both species are reviewed, as are their roles in human infection, their physiological characteristics and their respective ecologies. Close examination of these attributes shows clear differences between the varieties of T. equinum and T. tonsurans. The significance of the homogeneity of the ITS sequences of these two dermatophytes is discussed in the context of more recent appraisals of the results of ITS sequencing in other fungi. It is concluded that the results of ITS analysis for both varieties of T. equinum and T. tonsurans are indicative only of common ancestry. Similar arguments could be put forward for rescinding other changes in dermatophyte nomenclature that have been published in recent years.

Closely related dermatophyte species produce different patterns of secreted proteins

“…The phylogenetic delineation of dermatophytes has been improved using genetic techniques such as rRNA gene PCR-restriction fragment length polymorphism analysis (Mochizuki et al, 2003a, b;Machouart et al, 2006), DNA sequence analysis of the 28S RNA gene and the ITS regions (Summerbell et al, 1999;Gupta et al, 2001;Ninet et al, 2003). Differences in morphology, physiology, ecology and types of human infections caused by these fungi in fact support a clear separation between T. equinum and T. tonsurans (Woodgyer, 2004). One of the most interesting examples of controversy is the disagreement existing in the classification of T. equinum and T. tonsurans, which, based on results of the sequence analysis of the highly variable ITS regions, were described as synonymous (De Hoog et al, 1998;Gräser et al, 1999).…”

Sexual Reproduction in Dermatophytes

“…Because sexual reproduction is observed in almost all phylogenetic branches in dermatophytes, the clonal lineages are thought to have arisen from a sexual ancestor and the other mating type might have been lost or extremely rare or the species might have been derived from one mating type [40]. This process might have been induced by a mutation that confers the ability to adapt to a new ecological niche such as a new host and before or during this process, the species might have lost one of the mating types and become genetically distinct because of the lack of suitable conditions for mating for a long time or due to niche separation [44–45]. …”

“…The two species have very similar ITS regions that might be indicative of recent divergence [43–44]. …”

Multilocus differentiation of the related dermatophytes Microsporum canis, Microsporum ferrugineum and Microsporum audouinii

“…Recently, rapid molecular diagnostics tools have become available for dermatophytes, particularly sequencing of the rDNA internal transcribed spacer (ITS) region (Makimura et al, 1999;Woodgyer, 2004) and PCR-RFLP (De Baere et al, 2010;Jackson et al, 1999). The present paper reports on an 18-month screening study of dermatophytoses in Tehran, Iran, where .700 isolates were identified using ITS-RFLP with MvaI.…”