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Same-sex mating and the origin of the Vancouver Island Cryptococcus gattii outbreak

Abstract

Genealogy can illuminate the evolutionary path of important human pathogens. In some microbes, strict clonal reproduction predominates, as with the worldwide dissemination of Mycobacterium leprae, the cause of leprosy1. In other pathogens, sexual reproduction yields clones with novel attributes, for example, enabling the efficient, oral transmission of the parasite Toxoplasma gondii2. However, the roles of clonal or sexual propagation in the origins of many other microbial pathogen outbreaks remain unknown, like the recent fungal meningoencephalitis outbreak on Vancouver Island, Canada, caused by Cryptococcus gattii3. Here we show that the C. gattii outbreak isolates comprise two distinct genotypes. The majority of isolates are hypervirulent and have an identical genotype that is unique to the Pacific Northwest. A minority of the isolates are significantly less virulent and share an identical genotype with fertile isolates from an Australian recombining population. Genotypic analysis reveals evidence of sexual reproduction, in which the majority genotype is the predicted offspring. However, instead of the classic a–α sexual cycle, the majority outbreak clone appears to have descended from two α mating-type parents. Analysis of nuclear content revealed a diploid environmental isolate homozygous for the major genotype, an intermediate produced during same-sex mating. These studies demonstrate how cryptic same-sex reproduction can enable expansion of a human pathogen to a new geographical niche and contribute to the ongoing production of infectious spores. This has implications for the emergence of other microbial pathogens and inbreeding in host range expansion in the fungal and other kingdoms.

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Figure 1: Congruence of gene phylogeny in C. gattii.
Figure 2: MAT locus inheritance.
Figure 3: The C. gattii major outbreak genotype is hypervirulent and the minor genotype is attenuated.
Figure 4: The origin of an outbreak.

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Acknowledgements

We thank J. Kronstad for the R265 BAC library and strains; A. Litvintseva for MLST information; F. Dromer, D. Carter and C. Paula for strains; A. Idnurm, C. Fraser, X. Lin, K. Nielsen, J. Blankenship, A. Litvintseva, D. Lew and J. Anderson for reading the manuscript; and the Broad Fungal Genome Initiative. This work was supported by an NIAID R01 grant to J.H. Author Contributions J.A.F. and J.H. designed the experiments and wrote the manuscript. J.A.F. conducted or participated in all of the experiments, and S.S.G. and S.G.G-B. assisted with animal experiments. J.R.W. and J.R.P. supervised animal experiments and assisted with data analysis. E.C.W. contributed technical support. S.D. assisted with phylogenetic analysis. A.A. and F.S.D. contributed sequence analyses for MLST and MAT, and J.E.S. conducted computer modelling for genetic drift.

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Correspondence to Joseph Heitman.

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DNA sequences identified in this study have been deposited in GenBank; a full list of accession numbers is available in Supplementary Table 5. Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Supplementary information

Supplementary Figure S1

Congruence of gene phylogeny in C. gattii. (PDF 65 kb)

Supplementary Notes

This file contains Supplementary Methods and Supplementary Discussion. (DOC 58 kb)

Supplementary Table S1

A spreadsheet detailing strain MLST data (XLS 58 kb)

Supplementary Table S2

A spreadsheet detailing strain more MLST data (XLS 21 kb)

Supplementary Table S3

A spreadsheet detailing strain MAT locus fingerprint data (XLS 58 kb)

Supplementary Table S4

A spreadsheet listing the oligonucleotide primers employed in this study (XLS 34 kb)

Supplementary Table S5

A spreadsheet listing the GenBank accession numbers of DNA sequences described in this study (XLS 65 kb)

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Fraser, J., Giles, S., Wenink, E. et al. Same-sex mating and the origin of the Vancouver Island Cryptococcus gattii outbreak. Nature 437, 1360–1364 (2005). https://doi.org/10.1038/nature04220

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