Sexual Reproduction of Cryptococcus gattii: a Population Genetics Perspective

Dee Carter; Leona Campbell; Nathan Saul; Mark Krockenberger

doi:10.1128/9781555816858.ch22

Chapter 22 : Sexual Reproduction of Cryptococcus gattii: a Population Genetics Perspective

Authors: Dee Carter¹, Leona Campbell², Nathan Saul³, Mark Krockenberger⁴

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Affiliations: 1: School of Molecular Bioscience, University of Sydney, Sydney, NSW 2006, Australia; 2: School of Molecular Bioscience, University of Sydney, Sydney, NSW 2006, Australia; 3: Faculty of Veterinary Science, University of Sydney, Sydney, NSW 2006, Australia; 4: Faculty of Veterinary Science, University of Sydney, Sydney, NSW 2006, Australia

Content Type: Monograph

Publication Year: 2011

Category: Clinical Microbiology; Fungi and Fungal Pathogenesis

Book DOI: 10.1128/9781555816858

Chapter DOI: 10.1128/9781555816858.ch22

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Abstract:

The current knowledge of sexual reproduction of Cryptococcus gattii from a population genetics perspective is reviewed in this chapter. In genetic studies, cryptic species are usually detected as exclusive groups of organisms occupying strongly supported branches on phylogenetic trees that have been derived from different, independent loci. Assessing the relative numbers of α and a cells is therefore an important aspect of determining if populations are likely to have undergone sexual recombination. The initial studies of mating type in C. gattii found 84% of clinical isolates to be of the α mating type. Mating type was assessed by coculture with tester strains, and the majority of strains (~90%) were fertile, producing basidia and basidiospores. Amplified fragment length polymorphisms (AFLPs), which are highly discriminatory molecular markers, were therefore selected to establish multilocus genotypes. The population genetics of clinical collections is generally more complicated than the study of environmental populations as humans travel and may acquire an infection far from where they eventually present with clinical disease and an isolate is obtained. The pattern of pairwise compatibility among global VGII isolates presents a striking contrast to that seen in VGI. All VGII populations studied to date are heavily biased for one or the other mating type, and the most probable scenario is that mating occurs between isolates of the same sex.

Citation: Carter D, Campbell L, Saul N, Krockenberger M. 2011. Sexual Reproduction of Cryptococcus gattii: a Population Genetics Perspective, p 299-311. In Heitman J, Kozel T, Kwon-Chung K, Perfect J, Casadevall A (ed), Cryptococcus. ASM Press, Washington, DC. doi: 10.1128/9781555816858.ch22

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Sexual Reproduction of Cryptococcus gattii: a Population Genetics Perspective

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FIGURE 1

Mechanisms of allopatric and sympatric speciation. Geographic (allopatric speciation) or genetic (sympatric speciation) barriers arise within a population that limit genetic exchange. Over time the two populations differentiate and become fixed for different alleles at independent loci. ABC, three independent loci; A’B’C, alternative alleles at each locus.

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FIGURE 1

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FIGURE 2

Phylogenetic relationships of the different C. gattii molecular genotypes that may represent separate cryptic species. Tree based on 459 parsimony informative sites from six independent MLST markers. Adapted from reference 3 .

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FIGURE 2

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FIGURE 3

Frequency of mating types α and a in different C. gattii populations. Most populations show an extreme bias of one mating type over the other. Renmark, Australia, is the only region found to date with close to the 50:50 ratio of α:a expected for sexual reproduction. (Note: only populations with ≥10 isolates have been included in this survey.) References: 1 ( 21 ), 2 ( 9 ), 3 ( 13 ), 4 ( 43 ), 5 ( 33 ), 6 ( 24 ), 7 ( 14 ), 8 ( 37 ), 9 ( 36 ).

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FIGURE 3

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FIGURE 4

Multilocus molecular genotypes can indicate whether a population is clonal or recombining. (A) Hallmarks of clonal and recombining populations: clonal populations inherit their genotypes intact from their parents so that there are few genotypes present, there are no recombinant genotypes, loci appear to be genetically linked, and the population fits into a well-resolved phylogenetic tree as the individual taxa effectively behave like different species. In contrast, a sexual population can contain all possible combinations of genotypes, loci are not linked, and the population will not fit into a well-resolved phylogenetic tree, instead forming a bush-like structure. Complications: (B) Recombining populations can appear clonal if (i) they contain an overwhelming number of asexually derived isolates, as with a clonal bloom; (ii) there is inbreeding, which prevents genetic reassortment from being detected; or (iii) they are genetically subdivided, as loci fixed on either side of the division will appear linked. (C) Clonal populations can appear recombining if molecular markers have a high mutation rate leading to homoplasy or if there is a random loss of alleles.

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FIGURE 4

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FIGURE 5

Recombination in a C. gattii population derived from a single hollow on tree #15 in Renmark. When the entire population derived from one tree hollow was analyzed (population #1), both TL and IA were outside the range of the derived recombining populations, and the population appeared clonal, despite containing closely related and interspersed α and a isolates. By focusing on particular groups identified on the phylogram, it was possible to identify four recombining subpopulations (#2 to 5). Figure adapted from reference 40 with permission.

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FIGURE 5

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FIGURE 6

Pairwise compatibility of MLST alleles for a subset of C. gattii isolates from Renmark. The Renmark population is very restricted in MLST diversity, with only two polymorphic loci identified: SXI1 α/SXI2 a and GPD1. These loci segregate independently to give four possible combinations, as would be expected in a recombining population.

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FIGURE 6

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FIGURE 7

Phylogram of relatedness of clinical C. gattii isolates derived from AFLP data. Isolates were obtained from (i) infected people living in Papua New Guinea (prefixed by PNG); (ii) infected people living in the Northern Territory, Australia (underlined); and (iii) infected animals living in the Sydney region (in italics). Isolates are largely divided into the VG groups, and VGI isolates are grouped according to their origins. Two recombining subpopulations were identified in the VGII group. Mating tests found fertility to segregate with molecular genotype: almost all VGI isolates were infertile in laboratory crosses (indicated by an x), whereas the majority of VGII isolates were fertile (indicated by a check mark), with many robustly fertile (indicated by a large check mark). Figure adapted from reference 8 with permission.

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FIGURE 7

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FIGURE 8a

Pairwise compatibility of MLST alleles for global C. gattii isolates belonging to molecular genotypes VGI to VGIV. (A) For VGI, only locus pair SXI1 α/SXI2 a and GPD1 possessed four possible combinations of alleles, two of which were present in isolates from Renmark only, suggesting that recombination is largely confined to Renmark. (B) In contrast, in the VGII population there are six incompatible locus pairs, and most global isolates appear to participate in recombination. Two groups are exceptions: one group of isolates that have the genotype of the Vancouver Island outbreak strain (VGIIa) and the small group of distinct isolates identified as a recombining population in the Northern Territory. (C) Recombination at two or more loci is apparent in most isolates in the global VGIII population. (D) The VGIV population consists of a recombining group from Botswana, South Africa, and Malawi. A large number of genetically identical isolates from Botswana and Malawi are outside the recombining group. CLIN, clinical isolate; ENV, environmental isolate; VET, veterinary isolate. Only one representative isolate name is given for groups of two or more isolates; for details of isolates see reference 17 .

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FIGURE 8a

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FIGURE 8b

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FIGURE 8b

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