Comparison of two approaches for identification of haplotypes and point mutations in Candida albicans and Saccharomyces cerevisiae

Biological Roles of Protein-Coding Tandem Repeats in the Yeast Candida Albicans

Tandem repeat (TR) DNA mutates faster than other DNA by insertion and deletion of repeats. Large parts of eukaryotic proteomes are encoded by ORFs containing protein-coding TRs (TR-ORFs, pcTRs) with largely unknown biological consequences. We explored these in the yeast Candida albicans, an opportunistic human pathogen. We found that almost half of C. albicans’ proteins are encoded by TR-ORFs. pcTR frequency differed only moderately between different gene (GO) categories. Bioinformatic predictions of genome-wide mutation rates and clade-specific differences in pcTR allele frequencies indicated that pcTRs (i) significantly increase the genome-wide mutation rate; (ii) significantly impact on fitness and (iii) allow the evolution of selectively advantageous clade-specific protein variants. Synonymous mutations reduced the repetitiveness of many amino acid repeat-encoding pcTRs. A survey, in 58 strains, revealed that in some pcTR regions in which repetitiveness was not significantly diminished by synonymous mutations the habitat predicted which alleles were present, suggesting roles of pcTR mutation in short-term adaptation and pathogenesis. In C. albicans pcTR mutation apparently is an important mechanism for mutational advance and possibly also rapid adaptation, with synonymous mutations providing a mechanism for adjusting mutation rates of individual pcTRs. Analyses of Arabidopsis and human pcTRs showed that the latter also occurs in other eukaryotes.

Highly mutable tandem DNA repeats generate a cell wall protein variant more frequent in disease-causing Candida albicans isolates than in commensal isolates

During adaptation to host environments, many microorganisms alter their cell surface. One mechanism for doing so is variation in the number of amino acid repeats in cell surface proteins encoded by hypermutable DNA tandem repeats. In the yeast Candida albicans, an opportunistic human pathogen, the gene SSR1 encodes a GPI-anchored cell wall protein with a structural role. It contains two regions consisting of tandem repeats, almost exclusively encoding the amino acid pair Ser-Ala. As expected, the repeat regions make SSR1 highly mutable. New SSR1 alleles arose with a frequency of 1.11×10⁻⁴ per cell division in serially propagated cells. We also observed a large number (25) of SSR1 alleles with different repeat lengths in a survey of 131 isolates from a global strain collection. C. albicans is diploid, and combinations of these allele generated 41 different SSR1 genotypes. In both repeat regions, nonsynonymous mutations were largely restricted to one particular repeat unit. Two very similar allele combinations were largely restricted to one clade, clade 1. Each combination was present in ~30% of 49 infection-causing clade 1 strains, but one was rare (2%), the other absent in 46 infection-causing strains representing the remainder of the species (P < 0.00018 and 0.00004; Fisher’s exact test). These results indicate that both repeat regions are under selection and that amino acid repeat length polymorphisms generate Ssr1 protein variants most suitable for specific genetic backgrounds. One of these two allele combinations was 5.51 times more frequent, the other 1.75 times less frequent in 49 clade 1 strains that caused disease than in 36 commensal clade 1 strains (P = 0.0105; Chi² test). This indicates that insertion and deletion of repeats not only generates clade-optimized Ssr1p variants, but may also assist in short-term adaptation when C. albicans makes the transition from commensal to pathogen.

Selective Advantages of a Parasexual Cycle for the Yeast Candida albicans

The yeast Candida albicans can mate. However, in the natural environment mating may generate progeny (fusants) fitter than clonal lineages too rarely to render mating biologically significant: C. albicans has never been observed to mate in its natural environment, the human host, and the population structure of the species is largely clonal. It seems incapable of meiosis, and most isolates are diploid and carry both mating-type-like (MTL) locus alleles, preventing mating. Only chromosome loss or localized loss of heterozygosity can generate mating-competent cells, and recombination of parental alleles is limited. To determine if mating is a biologically significant process, we investigated if mating is under selection. The ratio of nonsynonymous to synonymous mutations in mating genes and the frequency of mutations abolishing mating indicated that mating is under selection. The MTL locus is located on chromosome 5, and when we induced chromosome 5 loss in 10 clinical isolates, most of the resulting MTL-homozygotes could mate with each other, producing fusants. In laboratory culture, a novel environment favoring novel genotypes, some fusants grew faster than their parents, in which loss of heterozygosity had reduced growth rates, and also faster than their MTL-heterozygous ancestors-albeit often only after serial propagation. In a small number of experiments in which co-inoculation of an oral colonization model with MTL-homozygotes yielded small numbers of fusants, their numbers declined over time relative to those of the parents. Overall, our results indicate that mating generates genotypes superior to existing MTL-heterozygotes often enough to be under selection.