Uneven distribution of mating types among genotypes of Candida glabrata isolates from clinical samples

Abstract

In order to shed light on its basic biology, we initiated a population genetic analysis of Candida glabrata, an emerging pathogenic yeast with no sexual stage yet recognized. A worldwide collection of clinical strains was subjected to analysis using variable number of tandem repeats (VNTR) at nine loci. The clustering of strains obtained with this method was congruent with that obtained using sequence polymorphism of the NMT1 gene, a locus previously proposed for lineage assignment. Linkage disequilibrium supported the hypothesis of a mainly clonal reproduction. No heterozygous diploid genotype was found. Minimum-spanning tree analysis of VNTR data revealed clonal expansions and associated genotypic diversification. Mating type analysis revealed that 80% of the strains examined are MATa and 20% MATalpha and that the two alleles are not evenly distributed. The MATa genotype dominated within large clonal groups that contained only one or a few MATalpha types. In contrast, two groups were dominated by MATalpha strains. Our data are consistent with rare independent mating type switching events occurring preferentially from type a to alpha, although the alternative possibility of selection favoring type a isolates cannot be excluded.

FIG. 1.

MStree analysis of 198 C. glabrata strains based on allelic profiles at the eight VNTR loci. Each circle corresponds to an RT, the number of which is indicated inside the circle. The lines between circles indicate the similarity between profiles (bold, seven alleles in common; normal, six alleles; dotted, five alleles) Cross-links corresponding to a single allelic mismatch are indicated. Gray zones between some groups of circles delineate the six CCs (see text). In order to illustrate the concordance of VNTR data with NMT1 sequences, pie chart sections of circles were colored according to the NMT1 allele and number of isolates sequenced (white parts of pie charts correspond to the proportion of strains for which NMT1 was not sequenced).

FIG. 2.

Mating type determination for C. glabrata strains. (A) Genomic organization of Mat cassettes in C. glabrata and primers designed to specifically amplify genes at the MAT locus. Terminology for the HML, HMR, and MAT loci was by homology with S. cerevisiae. Note that the HML and MAT loci are located on chromosome B, while the HMR locus is located on chromosome E. (B) Results of amplification targeting loci α1 (upper left), α2 (upper right), a1 (lower left), and a2 (lower right). Type α strains (lanes 5, 7, and 8) are US02Bal12, EF1521Blo1, and EF0510Blo1. Type a strains (lanes 1 to 4, 6, 9, and 10) are EF1515Blo1 ES0085Blo1, US003NY110, US02Bal020, EF1213Blo1, EF1204Blo1, EI4101Blo1, EF1031Blo1, US02Bal003, and USTCC90030.

FIG. 3.

Phylogenetic distribution of mating types. The neighbor-joining tree is based on Jukes-Cantor corrected distances between NMT1 sequences. The mating types of NMT1-sequenced strains are indicated at the tips of the branches: green circles, MATa; red triangles, MATα. The mating types of isolates of the six CCs, each being positioned at the tip of its corresponding branch, are indicated as a color pattern (red, MATα; green, MATa) on the MStree of each CC. Bootstrap values obtained after 1,000 replicates are given at the nodes.