Population Genetic Structure of Clinical and Environmental Isolates of Blastomyces dermatitidis, Based on 27 Polymorphic Microsatellite Markers

Abstract

Blastomyces dermatitidis, a thermally dimorphic fungus, is the etiologic agent of North American blastomycosis. Clinical presentation is varied, ranging from silent infections to fulminant respiratory disease and dissemination to skin and other sites. Exploration of the population genetic structure of B. dermatitidis would improve our knowledge regarding variation in virulence phenotypes, geographic distribution, and difference in host specificity. The objective of this study was to develop and test a panel of microsatellite markers to delineate the population genetic structure within a group of clinical and environmental isolates of B. dermatitidis. We developed 27 microsatellite markers and genotyped B. dermatitidis isolates from various hosts and environmental sources (n ‫؍‬ 112). Assembly of a neighbor-joining tree of allele-sharing distance revealed two genetically distinct groups, separated by a deep node. Bayesian admixture analysis showed that two populations were statistically supported. Principal coordinate analysis also reinforced support for two genetic groups, with the primary axis explaining 61.41% of the genetic variability. Group 1 isolates average 1.8 alleles/locus, whereas group 2 isolates are highly polymorphic, averaging 8.2 alleles/locus. In this data set, alleles at three loci are unshared between the two groups and appear diagnostic. The mating type of individual isolates was determined by PCR. Both mating type-specific genes, the HMG and ␣-box domains, were represented in each of the genetic groups, with slightly more isolates having the HMG allele. One interpretation of this study is that the species currently designated B. dermatitidis includes a cryptic subspecies or perhaps a separate species.

Identification of the Mating-Type (MAT) Locus That Controls Sexual Reproduction of Blastomyces dermatitidis

“…Studies of population genetics further support that these dimorphic fungi reproduce sexually in nature (21,(37)(38)(39)(40). Strain typing results based on 27 microsatellite markers suggest genetic recombination and sexual reproduction of B. dermatitidis in nature (10). The identification of the B. dermatitidis MAT locus that exhibits a highly similar structure with other fungi and the population genetic studies indicate that B. dermatitidis may reproduce sexually in the environment.…”

“…Both mating types were found in the natural population of B. dermatitidis, and they have a similar frequency in nature (10,34). In two other dimorphic fungi, H. capsulatum and Coccidioides, both have a similar ratio of the two mating types in nature (35,36), and a recent study performed by Torres et al reveals an ϳ1:1 distribution of the MAT1-1 and MAT1-2 strains of P. brasiliensis in South America (20).…”

“…B. dermatitidis is generally regarded as a heterothallic species that requires isolates of both mating types to complete the sexual cycle. However, a B. dermatitidis isolate of both mating types has been reported from a patient with blastomycosis (9,10). This suggests that patients might be infected either with two isolates of different mating types or with diploids heterozygous for mating type.…”

Development of a Highly Sensitive and Specific Blastomycosis Antibody Enzyme Immunoassay Using Blastomyces dermatitidis Surface Protein BAD-1

“…Second, the BAD-1 antigen may not contain epitopes recognized by the antibodies produced in some patient samples. This could be associated with the genetic variability of B. dermatitidis, which has recently been described (21)(22)(23). Third, anti-BAD-1 antibodies may be complexed with antigens in the specimen and therefore not free for detection in our assay.…”

Blastomycosis

“…Polymerase chain reaction (PCR)-based assays targeting the promoter region of the BAD-1 virulence gene identified two major genotypic groups A and B, representing 47.2 and 48.1 %, respectively, and three minor groups (C, D, and E) in an analysis of 106 clinical and environmental isolates of B. dermatitidis from Wisconsin, Georgia, and Africa [8]. Recently, the technique of microsatellite analysis has been applied to a number of fungal species as well as B. dermatitidis [9][10][11][12][13]. Microsatellite analysis offers several advantages over both PCRrestriction fragment length polymorphism (RFLP) and random amplified polymorphic DNA [14,15] (RAPD) methods.…”