The Genome of the Human PathogenCandida albicansis Shaped by Mutation and Cryptic Sexual Recombination
Authors: Wang Joshua M. , Bennett Richard J. , Anderson Matthew Z. 
Published: April 2018

The opportunistic fungal pathogenCandida albicanslacks a conventional sexual program and is thought to evolve, at least primarily, through the clonal acquisition of genetic changes. Here, we performed an analysis of heterozygous diploid genomes from 21 clinical isolates to determine the natural evolutionary processes acting on theC. albicansgenome. Consistent with a model of inheritance by descent, most single nucleotide polymorphisms (SNPs) were shared between closely related strains. However, strain-specific SNPs and insertions/deletions (indels) were distributed non-randomly across the genome. For example, base substitution rates were higher in the immediate vicinity of indels, and heterozygous regions of the genome contained significantly more strain-specific polymorphisms than homozygous regions. Loss of heterozygosity (LOH) events also contributed substantially to genotypic variation, with most long-tract LOH events extending to the ends of the chromosomes suggestive of repair via break-induced replication. Importantly, some isolates contained highly mosaic genomes and failed to cluster closely with other isolates within their assigned clades. Mosaicism is consistent with strains having experienced inter-clade recombination during their evolutionary history and a detailed examination of nuclear and mitochondrial genomes revealed striking examples of recombination. Together, our analyses reveal that both (para)sexual recombination and mitotic mutational processes drive evolution of this important pathogen in nature. To further facilitate the study of genome differences we also introduce an online platform, SNPMap, to examine SNP patterns in sequencedC. albicansgenomes.

AUTHOR SUMMARY

Mutations introduce variation into the genome upon which selection can act. Defining the nature of these changes is critical for determining species evolution, as well as for understanding the genetic changes driving important cellular processes such as carcinogenesis. The fungusCandida albicansis a heterozygous diploid species that is both a frequent commensal organism and a prevalent opportunistic pathogen. Prevailing theory is thatC. albicansevolves primarily through the gradual build-up of mutations, and a pressing question is whether sexual or parasexual processes also operate within natural populations. Here, we determine the evolutionary patterns of genetic change that have accompanied species evolution in nature by examining genomic differences between clinical isolates. We establish that theC. albicansgenome evolves by a combination of base-substitution mutations, insertions/deletion events, and both short-tract and long-tract loss of heterozygosity (LOH) events. These mutations are unevenly distributed across the genome, and reveal that non-coding regions and heterozygous regions are evolving more quickly than coding regions and homozygous regions, respectively. Furthermore, we provide evidence that genetic exchange has occurred between isolates, establishing that sexual or parasexual processes have transpired inC. albicanspopulations and contribute to the diversity of both nuclear and mitochondrial genomes.