Serotype AD strains of Cryptococcus neoformans are diploid or aneuploid and are heterozygous at the mating-type locus

Chromosomal polymorphism, syntenic relationships, and ploidy in the pathogenic fungus Paracoccidioides brasiliensis

Pulsed field gel electrophoresis (PFGE) and DNA hybridization were used to establish and compare the electrophoretic karyotypes of 12 clinical and environmental Paracoccidioides brasiliensis isolates from different geographic areas. Gene mapping allowed the identification of synteny groups and the use of isolated whole chromosomal bands to probe chromoblots indicated the existence of repetitive sequences, contributing to a better understanding of the structure and organization of the fungus genome. This represents the first comparative mapping study among different isolates. The results are indicative of the existence of genetic differences among natural isolates. DNA content of DAPI-stained nuclei of each isolate was estimated by confocal microscopy. Comparison of the genome sizes estimated by PFGE with those calculated by microfluorometry indicated the possible existence of haploid and diploid (or aneuploid) isolates of the fungus.

Mitochondria are inherited from the MATa parent in crosses of the basidiomycete fungus Cryptococcus neoformans

Previous studies demonstrated that mitochondrial DNA (mtDNA) was uniparentally transmitted in laboratory crosses of the pathogenic yeast Cryptococcus neoformans. To begin understanding the mechanisms, this study examined the potential role of the mating-type locus on mtDNA inheritance in C. neoformans. Using existing isogenic strains (JEC20 and JEC21) that differed only at the mating-type locus and a clinical strain (CDC46) that possessed a mitochondrial genotype different from JEC20 and JEC21, we constructed strains that differed only in mating type and mitochondrial genotype. These strains were then crossed to produce hyphae and sexual spores. Among the 206 single spores analyzed from six crosses, all but one inherited mtDNA from the MATa parents. Analyses of mating-type alleles and mtDNA genotypes of natural hybrids from clinical and natural samples were consistent with the hypothesis that mtDNA is inherited from the MATa parent in C. neoformans. To distinguish two potential mechanisms, we obtained a pair of isogenic strains with different mating-type alleles, mtDNA types, and auxotrophic markers. Diploid cells from mating between these two strains were selected and 29 independent colonies were genotyped. These cells did not go through the hyphal stage or the meiotic process. All 29 colonies contained mtDNA from the MATa parent. Because no filamentation, meiosis, or spore formation was involved in generating these diploid cells, our results suggest a selective elimination of mtDNA from the MATalpha parent soon after mating. To our knowledge, this is the first demonstration that mating type controls mtDNA inheritance in fungi.

PCR-restriction fragment length polymorphism analysis of the phospholipase B (PLB1) gene for subtyping of Cryptococcus neoformans isolates

Cryptococcus neoformans is a pathogenic yeast that is currently divided into three varieties, five serotypes, and eight molecular types. The following report describes the use of PCR-restriction fragment length polymorphism (RFLP) analysis of the phospholipase B gene (PLB1) as a simple tool to differentiate between C. neoformans subgroups. A PLB1 fragment, 1,970 bp, was amplified and digested with either AvaI or HindIII. Both sets of profiles grouped the isolates into their respective varieties, but only the AvaI profiles allowed for the identification of the eight molecular types via the corresponding RFLP profiles A1 to A8. Digestion of the same fragments with HindIII resulted in RFLP profiles H1 to H5, which distinguished only between serotype A, AD, D, and B/C. Neither enzyme distinguished serotype B from serotype C. The serotype AD profile was a composite of the serotype A and D profiles. Further investigation showed that the serotype AD isolates used in this study are heterozygous, with one allele of PLB1 originating from a serotype A parent and the other from a serotype D parent.

Serotype, mating type and ploidy of Cryptococcus neoformans strains isolated from patients in Brazil

Serotype, mating type and ploidy of 84 strains of Cryptococcus neoformans isolated from 61 AIDS and 23 non-AIDS patients admitted in a tertiary teaching hospital in São Paulo, Brazil were examined. Among 61 strains isolated from AIDS patients, 60 strains were var. grubii (serotype A). Only one strain was var. gattii (serotype B). No var. neoformans (serotype D) was found. Among 23 strains isolated from non-AIDS patients, 15 were var. grubii (serotype A) and the remaining 8 were var. gattii, all of which were serotype B. Seventy-three of the 75 serotype A strains were the heterothallic alpha type (MATalpha) and the remaining 2 were untypable (asexual). Most of the MATalpha strains (69/73) were haploid and the remaining 4 strains were diploid. Similarly, both of the 2 asexual strains among the 75 serotype A strains were haploid. There were no alpha-mating type (MATalpha) strains among the 84 isolates. All of the 8 var. gattii strains were serotype B and haploid. Among a total of 84 strains tested, neither serotype AD nor serotype D were found. Neither triploid nor tetraploid were found. These results suggest that the serological, sexual and ploidy characteristics in C. neoformans strains isolated from AIDS patients in São Paulo were rather simple, whereas strains isolated from non-AIDS patients presented serotype A and B with predominance of serotype A.

The role of mating type and morphology in Cryptococcus neoformans pathogenesis

Cryptococcus neoformans is a major fungal pathogen of both humans and animals. The fungus can be divided into two varieties, with each variety being composed of two serotypes. A sexual phase has been identified, which classifies C. neoformans as a bipolar heterothallic fungus with two mating types, MATa and MATalpha. The analysis of mating and mating type in this organism is important for a number of reasons. Both clinical and environmental isolates display a severe bias of the MATalpha mating type over MATa. MATalpha cells are also more virulent than MATalpha cells. Molecular and genetic analyses of the genes that make up the mating pathway have revealed that some of these genes are required for virulence. Finally, although it is well known that infection begins in the lungs after inhalation of infectious particles, it still remains unclear what constitutes the infectious particle. This review will discuss current information about what is known about the role that mating type and morphology play in virulence.

Molecular genetic analyses of mating pheromones reveal intervariety mating or hybridization in Cryptococcus neoformans

The sexual mating of the pathogenic yeast Cryptococcus neoformans is important for pathogenesis studies because the fungal virulence is linked to the alpha mating type (MAT(alpha)). We characterized C. neoformans mating pheromones (MF(alpha) 1 and MFa1) from 122 strains to understand intervariety hybridization or mating and intervariety virulence. MF(alpha) 1 in three C. neoformans varieties showed (a) specific nucleotide polymorphisms, (b) different copy numbers and chromosomal localizations, and (c) unique deduced amino acids in two geographic populations of C. neoformans var. gattii. MF(alpha) 1 of different varieties cross-hybridized in Southern hybridizations. Their phylogenetic analyses showed purifying selection (neutral evolution). These observations suggested that MAT(alpha) strains from any of the three C. neoformans varieties could mate or hybridize in nature with MATa strains of C. neoformans var. neoformans. A few serotype A/D diploid strains provided evidence for mating or hybridization, while a majority of A/D strains tested positive for haploid MF(alpha) 1 identical to that of C. neoformans var. grubii. MF(alpha) 1 sequence and copy numbers in diploids were identical to those of C. neoformans var. grubii, while their MFa1 sequences were identical to those of C. neoformans var. neoformans; thus, these strains were hybrids. The mice survival curves and histological lesions revealed A/D diploids to be highly pathogenic, with pathogenicity levels similar to that of the C. neoformans var. grubii type strain and unlike the low pathogenicity levels of C. neoformans var. neoformans strains. In contrast to MF(alpha) 1 in three varieties, MFa1 amplicons and hybridization signals could be obtained only from two C. neoformans var. neoformans reference strains and eight A/D diploids. This suggested that a yet undiscovered MFa pheromone(s) in C. neoformans var. gattii and C. neoformans var. grubii is unrelated to, highly divergent from, or rarer than that in C. neoformans var. neoformans. These observations could form the basis for future studies on the role of intervariety mating in C. neoformans biology and virulence.

Geographic distribution of mating type alleles of Cryptococcus neoformans in four areas of the United States

To better understand the epidemiology and population structure of Cryptococcus neoformans, we determined mating types for 358 C. neoformans strains isolated through the active surveillance program from 1992 to 1994 in four geographic areas in the United States: San Francisco, California; Georgia; Texas; and Alabama. Two assays were used to determine mating types: (i) crossing with standard laboratory tester strains JEC20 and JEC21 on V8 agar medium; and (ii) PCR with the mating type alpha allele-specific primer of the STE12 gene and with serotype (A and D)- and mating type (a and alpha)-specific primers of the STE20 gene. Using these two methods, we found that this sample consisted of the following: (i) 324 serotype A, mating type (MAT) alpha (Aalpha) strains; (ii) 12 serotype D, alpha (Dalpha) strains; (iii) 14 serotype AD strains with mating type alleles Aa and Dalpha (AaDalpha); (iv) 2 serotype AD strains with mating type alleles Aalpha and Da (AalphaDa); (v) 3 serotype B, alpha (Balpha) strains; and (vi) 3 serotype AD strains but with only one mating type allele. No strain with MATa was found within serotype A, B, or D in this collection. Interestingly, 14 of the 19 serotype AD strains contained the Aa allele at the STE20 locus; 13 of these 14 were from San Francisco. Our results suggest that the environment in San Francisco might contain Aa strains capable of mating with Dalpha strains. In addition, our result demonstrate that the sample from San Francisco had a significantly higher proportion of self-fertile strains than those from the other three areas.

Multiple origins of hybrid strains of Cryptococcus neoformans with serotype AD

Cryptococcus neoformans is a major pathogen of humans throughout the world. Using commercial mAbs to capsular epitopes, strains of C. neoformans manifest five distinct serotypes--A, B, C, D and AD. Previous studies demonstrated significant divergence among serotypes A, B, C and D, which are thought to be haploid. In this study the origins and evolution of strains of serotype AD were investigated. A portion (537 bp) of the laccase gene was cloned and sequenced from 14 strains of serotype AD. Each strain contained two different alleles and sequences for both alleles were obtained. These sequences were compared to those from serotypes A, B, C and D. This analysis indicated that each of the 14 serotype AD strains contained two phylogenetically distinct haplotypes: one haplotype was highly similar to the serotype A group and the other to the serotype D group. To explain the origins of these serotype AD strains, genealogical analysis is consistent with at least three recent and independent hybridization events. The results demonstrate that the evolution of C. neoformans is continuing and dynamic.

Origin of Cryptococcus neoformans var. neoformans diploid strains

The basidiomycetous yeast Cryptococcus neoformans is an important human fungal pathogen. Two varieties, C. neoformans var. neoformans and C. neoformans var. gattii, have been identified. Both are heterothallic with two mating types, MATa and MATalpha. Some rare isolates are self-fertile and are considered occasional diploid or aneuploid strains. In the present study, 133 isolates, mostly from Italian patients, were investigated to detect the presence of diploid strains in the Igiene Università Milano culture collection. All of the diploid isolates were further investigated by different methods to elucidate their origins. Forty-nine diploid strains were identified by flow cytometry. PCR fingerprinting using the (GACA)(4) primer showed that the diploid state was associated with two specific genotypes identified as VN3 and VN4. Determination of mating type on V8 juice medium confirmed that the majority of the strains were sterile. PCR and dot blotting using the two pheromone genes (MFa and MFalpha) as probes identified 36 of the 49 diploid isolates as MATa/alpha. The results of pheromone gene sequencing showed that two allelic MFalpha genes exist and are distinct for serotypes A and D. In contrast, the MFa gene sequence was conserved in both serotype alleles. Amplification of serotype-specific STE20 alleles demonstrated that the diploid strains contained one mating locus inherited from a serotype A parent and one inherited from a serotype D parent. The present results suggest that diploid isolates may be common among the C. neoformans population and that in Italy and other European countries serotype A and D populations are not genetically isolated but are able to recombine by sexual reproduction.

Isolation of a Cryptococcus neoformans serotype A MATa strain from the Italian environment

Cryptococcus neoformans is a heterothallic basidiomycete which possesses a bipolar mating system based on two mating type alleles, MATa and MATalpha. In the type variety, C. neoformans var. neoformans, both mating types have been found among strains of one serotype, serotype D, whereas only MATalpha was identified after extensive survey of serotype A strains. Serotype A MA Ta appeared to be extinct or to exist only in a vestigial, non-functional form. We report the isolation of a C. n. var. neoformans serotype A MATa strain from the Italian environment. The strain was serotyped by slide agglutination test, genotyped by polymerase chain reaction (PCR) fingerprinting using the (GACA)4 primer, and its haploid state was determined by flow cytometry. The mating type was identified by PCR amplification of the pheromone a gene. In addition, the amplification of the four STE20 alleles, specific for the mating type of serotypes A and D, showed that the strain contains only the MATa locus. By crossing experiments the strain was found to be fertile. The interest in the finding of this fertile isolate is related to the possibility to construct a congenic pair of serotype A MATa/MATalpha strains to be used in genetic and pathogenesis studies.

Identification of the MATa mating-type locus of Cryptococcus neoformans reveals a serotype A MATa strain thought to have been extinct

Cryptococcus neoformans is an opportunistic fungal pathogen with a defined sexual cycle involving mating between haploid MATa and MATalpha cells. Here we describe the isolation of part of the MATa mating-type locus encoding a Ste20 kinase homolog, Ste20a. We show that the STE20a gene cosegregates with the MATa mating type in genetic crosses, maps within the mating-type locus on a 1.8-Mb chromosome, and is allelic with the MATalpha locus. We identify the first MATa isolate of the most common pathogenic variety of C. neoformans (serotype A, variety grubii) which had been thought to be extinct. This serotype A MATa strain is sterile, fails to produce mating pheromone, and is less virulent than a serotype A MATalpha strain in an animal model. Our studies illustrate an association of mating type with virulence and suggest that, like Candida albicans, pathogenic isolates of C. neoformans may be largely asexual.