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Chapter 29 : Sex in Natural Populations of

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Abstract:

This chapter reviews what is known about sexual recombination in naturally occurring populations of . As it is readily isolated from its environmental source, allows a unique insight into how ecology and infection interface in a primary fungal pathogen. Cryptococcosis in humans usually begins as a primary pulmonary disease resulting from the inhalation of an infectious propagule. The authors have clear evidence that at least some veterinary and clinical infections have resulted from sexually produced propagules, and it is likely that these are basidiospores. Interestingly, the four VGII isolates obtained from environmental sources to date are significantly less fertile than the majority of the clinical and environmental VGII isolates, suggesting that sexual fecundity is linked with infectivity in this genotype. Ongoing and future studies allows one to assess how moves in the environment and its potential to expand into new regions and to contact new environmental, human, and animal hosts. Such expansions may also allow it to come into contact with sexually compatible fungal genotypes, which, as proposed in the Vancouver Island outbreak, can have important consequences in the emergence of new pathogenic strains.

Citation: Carter D, Saul N, Leona C, Bui T, Krockenberger M. 2007. Sex in Natural Populations of , p 477-488. In Heitman J, Kronstad J, Taylor J, Casselton L (ed), Sex in Fungi. ASM Press, Washington, DC. doi: 10.1128/9781555815837.ch29
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Figures

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Figure 29.1

(A) Diagrammatic representation of the genetic structure of asexually and sexually reproducing populations. Here, genotypes consist of three loci, a, b, and c, with two alleles, 1 or 0, at each locus. In each population, the ancestral genotype (111) mutates over time to give new genotypes 110 and 101. In the asexual population these genotypes are passed intact to the offspring to give two clonal lineages, and recombinant genotypes are absent. The sexual population undergoes genetic reassortment to produce recombinant genotypes 111 and 100 in addition to the parental genotypes. The clonal population can be fitted to a phylogenetic tree with clear branches linking each genotype, whereas in the recombining population all isolates are equally related in a brush-like structure. (B) Complications that can cause a sexual population to appear asexual: clonal expansion of one genotype can overwhelm genetic evidence of recombination. Genetic partitioning can result in the fixation of alleles in each subgroup—here the populations above and below the partition (represented by a dashed line) are fixed at locus b for alleles 1 and 0, respectively. Two or more alleles that become fixed in this way will appear to be linked and will skew the analysis in favor of clonality. Inbreeding of highly related genotypes will also give the appearance of a clonally derived population. (C) Asexual populations can appear to be sexual if there is a particularly high mutation rate, or if experimental artifact results in the random loss of alleles. (Modified from reference with permission from the publisher.)

Citation: Carter D, Saul N, Leona C, Bui T, Krockenberger M. 2007. Sex in Natural Populations of , p 477-488. In Heitman J, Kronstad J, Taylor J, Casselton L (ed), Sex in Fungi. ASM Press, Washington, DC. doi: 10.1128/9781555815837.ch29
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Image of Figure 29.2
Figure 29.2

Nonmetric multidimensional scaling showing the genetic relationships among MAT and MATα isolates derived from two tree hollows. •, tree no. 13, MAT; ○, tree no. 13, MATα ▪, Tree #15, MAT; □, Tree #15, MATα. While isolates from the two trees are distinct, there is no clear separation of the MATα and MAT isolates obtained from either tree.

Citation: Carter D, Saul N, Leona C, Bui T, Krockenberger M. 2007. Sex in Natural Populations of , p 477-488. In Heitman J, Kronstad J, Taylor J, Casselton L (ed), Sex in Fungi. ASM Press, Washington, DC. doi: 10.1128/9781555815837.ch29
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Image of Figure 29.3
Figure 29.3

Unrooted phylogenetic tree derived from AFLP analysis of clinical and veterinary isolates. VGI isolates group together on the top of the phylogram, whereas VGII and VGIII isolates form separate clusters in the bottom left and right corners, respectively. A group of isolates without a clear genotype affiliation lies between the VGI and VGIII clusters. PNG isolates are indicated by plain font, NT isolates are indicated in boldface, and veterinary isolates are italicized. indicates MAT isolates. Limited genetic diversity, shown by the tight clustering of isolates on short branches, is particularly evident in the PNG and veterinary VGI populations. The recombining VGII populations are circled. Stippling depicts the VNT-gene flow subgroup, which contains NT and veterinary isolates that did not appear to be genetically differentiated. (Modified from reference with permission from the publisher.)

Citation: Carter D, Saul N, Leona C, Bui T, Krockenberger M. 2007. Sex in Natural Populations of , p 477-488. In Heitman J, Kronstad J, Taylor J, Casselton L (ed), Sex in Fungi. ASM Press, Washington, DC. doi: 10.1128/9781555815837.ch29
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References

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Tables

Generic image for table
Table 29.1

MLST genotypes for 20 VGI isolates from Renmark

Citation: Carter D, Saul N, Leona C, Bui T, Krockenberger M. 2007. Sex in Natural Populations of , p 477-488. In Heitman J, Kronstad J, Taylor J, Casselton L (ed), Sex in Fungi. ASM Press, Washington, DC. doi: 10.1128/9781555815837.ch29
Generic image for table
Table 29.2

Division of VGII isolates into Vet/NT and NT-only subgroups

Citation: Carter D, Saul N, Leona C, Bui T, Krockenberger M. 2007. Sex in Natural Populations of , p 477-488. In Heitman J, Kronstad J, Taylor J, Casselton L (ed), Sex in Fungi. ASM Press, Washington, DC. doi: 10.1128/9781555815837.ch29
Generic image for table
Table 29.3

Summary of sexual characteristics in natural populations studied

Citation: Carter D, Saul N, Leona C, Bui T, Krockenberger M. 2007. Sex in Natural Populations of , p 477-488. In Heitman J, Kronstad J, Taylor J, Casselton L (ed), Sex in Fungi. ASM Press, Washington, DC. doi: 10.1128/9781555815837.ch29

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