Objectives
Currently available microsatellite assay for molecular typing of Trichophyton rubrum consists from 8 loci and was published in the year 2016 by Gong et al. We assessed the performance of this assay for uncovering genetic structure of bipartite Russian population of T. rubrum.
Methods
A total of 41 clinical isolates of T. rubrum were isolated in St. Petersburg and Yekaterinburg, Russia in the years 2015-2017. Species identification was performed by DNA sequencing of ITS region. Urease activity was assessed by use of Christensen's urea broth. We performed molecular strain typing by sequencing of A7C99_6411 locus and amplification of eight microsatellite loci according to Gong et al. 2016. Bruvo pairwise genetic distances were calculated on the basis of the lengths of microsatellite markers in polysat package for R and further subjected to PCA analysis and construction of phylogenetic network in SplitsTree program. Simpson’s index of genotype diversity was calculated in the package polysat.
Results
The sequencing of ITS rDNA region revealed one sequence throughout the sample, identical to KT285224. All isolates were urease-negative. The protein-coding locus A7C99_6411 was polymorphic at the position 793, containing alternatively adenine or guanine. Mutation in this locus is known to be correlated with mutation in another protein-coding locus A7C99_6714 and TRS-1 genotype of an isolate, dividing Russian population of T. rubrum in two clearly defined genetic lineages. However, genetic distances that were obtained by microsatellite typing were distributed unimodally (Fig. 1A) and principal component analysis of these distances revealed a single cloud (Fig. 2B). The phylogenetic network did not contain a well-defined clade for either A7C99_6411 genotype (Fig. 1C). These results suggested one uniform population. On the other hand, Simpson’s index of genotype diversity differed significantly for representatives of particular A7C99_6411 genotypes (Fig. 1D). This index measures the probability that two individuals randomly selected from a sample will belong to the same genetic lineage and its lower value for A7C99_6411 793G genotype is indicative of higher genetic diversity.
Conclusion
Thus, the microsatellite assay used had not enough discriminative power to resolve the structure of local T. rubrum population and more loci should be involved in further studies. This task appears to be challenging, since Gong et al. reported only 6 microsatellite loci from 100 studied to be polymorphic.

We need more powerful microsatellite assay for population genetic studies of Trichophyton rubrum