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.     

Many globally isolated AD hybrid strains of Cryptococcus neoformans originated in Africa

Interspecific and intervarietal hybridization may contribute to the biological diversity of fungal populations. Cryptococcus neoformans is a pathogenic yeast and the most common fungal cause of meningitis in patients with AIDS. Most patients are infected with either of the two varieties of C. neoformans, designated as serotype A (C. neoformans var. grubii) or serotype D (C. neoformans var. neoformans). In addition, serotype AD strains, which are hybrids of these two varieties, are commonly isolated from clinical and environmental samples. While most isolates of serotype A and serotype D are haploid, AD strains are diploid or aneuploid, and contain two sets of chromosomes and two mating type alleles, MATa and MATalpha, one from each of the serotypes. The global population of serotype A is dominated by isolates with the MATalpha mating type (Aalpha); however, about half of the globally analyzed AD strains possess the extremely rare serotype A MATa allele (Aa). We previously described an unusual population of serotype A in Botswana, in which 25% of the strains contain the rare MATa allele. Here we utilized two methods, phylogenetic analysis of three genes and genotyping by scoring amplified fragment length polymorphisms, and discovered that AD hybrid strains possessing the rare serotype A MATa allele (genotype AaDalpha) cluster with isolates of serotype A from Botswana, whereas AD hybrids that possess the MATalpha serotype A allele (AalphaDa and AalphaDalpha) cluster with cosmopolitan isolates of serotype A. We also determined that AD hybrid strains are more resistant to UV irradiation than haploid serotype A strains from Botswana. These findings support two hypotheses: (i) AaDalpha strains originated in sub-Saharan Africa from a cross between strains of serotypes A and D; and (ii) this fusion produced hybrid strains with increased fitness, enabling the Botswanan serotype A MATa genome, which is otherwise geographically restricted, to survive, emigrate, and propagate throughout the world.     

A homolog of Ste6, the a-factor transporter in Saccharomyces cerevisiae, is required for mating but not for monokaryotic fruiting in Cryptococcus neoformans

Fungal pheromones function during the initial recognition stage of the mating process. One type of peptide pheromone identified in ascomycetes and basidiomycetes terminates in a conserved CAAX motif and requires extensive posttranslational modifications to become mature and active. A well-studied representative is the a-factor of Saccharomyces cerevisiae. Unlike the typical secretory pathway utilized by most peptides, an alternative mechanism involving the ATP-binding cassette transporter Ste6 is used for the export of mature a-factor. Cryptococcus neoformans, a bipolar human pathogenic basidiomycete, produces CAAX motif-containing lipopeptide pheromones in both MATa and MATalpha cells. Virulence studies with a congenic pair of C. neoformans serotype D strains have shown that MATalpha cells are more virulent than MATa cells. Characterization of the MATalpha pheromones indicated that an autocrine signaling loop may contribute to the differentiation and virulence of MATalpha cells. To further address the role of pheromones in the signaling loop, we identified a STE6 homolog in the C. neoformans genome and determined its function by gene disruption. The ste6 mutants in either mating-type background showed partially impaired mating functions, and mating was completely abolished in a bilateral mutant cross. Surprisingly, the MATalpha ste6 mutant does not exhibit a defect in monokaryotic fruiting, suggesting that the activation of the autocrine signaling loop by the pheromone is via a Ste6-independent mechanism. MFalpha pheromone itself is essential for this process and could induce the signaling response intracellularly in MATalpha cells. Our data demonstrate that Ste6 is evolutionarily conserved for mating and is not required for monokaryotic fruiting in C. neoformans.     

Isolation and characterization of the Cryptococcus neoformans MATa pheromone gene

Cryptococcus neoformans is a heterothallic basidiomycete with two mating types, MATa and MATalpha. The mating pathway of this fungus has a number of conserved genes, including a MATalpha-specific pheromone (MFalpha1). A modified differential display strategy was used to identify a gene encoding the MATa pheromone. The gene, designated MFa1, is 42 amino acids in length and contains a conserved farnesylation motif. MFa1 is present in three linked copies that span a 20-kb fragment of MATa-specific DNA and maps to the MAT-containing chromosome. Transformation studies showed that MFa1 induced filament formation only in MATalpha cells, demonstrating that MFa1 is functionally conserved. Sequence analysis of the predicted Mfa1 and Mfalpha1 proteins revealed that, in contrast to other fungi such as Saccharomyces cerevisiae, the C. neoformans pheromone genes are structurally and functionally conserved. However, unlike the MFalpha1 gene, which is found in MATalpha strains of both varieties of C. neoformans, MFa1 is specific for the neoformans variety of C. neoformans.     

Blue light negatively regulates the sexual filamentation via the Cwc1 and Cwc2 proteins in Cryptococcus neoformans

Cryptococcus neoformans is a heterothallic basidiomycetous yeast that primarily infects immunocompromised individuals. Dikaryotic hyphae resulting from the fusion of the MATa and MATalpha mating type strains represent the filamentous stage in the sexual life cycle of C. neoformans. In this study we demonstrate that the production of dikaryotic filaments is inhibited by blue light. To study blue light photoresponse in C. neoformans, we have identified and characterized two genes, CWC1 and CWC2, which are homologous to Neurospora crassa wc-1 and wc-2 genes. Conserved domain analyses indicate that the functions of Cwc1 and Cwc2 proteins may be evolutionally conserved. To dissect their roles in the light response, the CWC1 gene deletion mutants are created in both mating type strains. Mating filamentation in the bilateral cross of cwc1 MATa and MATalpha strains is not sensitive to light. The results indicate that Cwc1 may be an essential regulator of light responses in C. neoformans. Furthermore, overexpression of the CWC1 or CWC2 gene requires light activation to inhibit sexual filamentation, suggesting both genes may function together in the early step of blue light signalling. Taken together, our findings illustrate blue light negatively regulates the sexual filamentation via the Cwc1 and Cwc2 proteins in C. neoformans.     

Intimate Relationship between mtDNA, Plasmids, and the Fusion of Mitochondria

Physarum polycephalum. The conformation of Physarum mtDNA is currently thought to be circular. The inheritance of its mtDNA depends on the multiallelic mating type loci, matA. In a cross with ordinary matA combinations, the strain that has the higher matA status transmits its mtDNA to the progeny (uniparental inheritance). The mF plasmid promotes the fusion of mitochondria in the zygote and during sporulation. When it exists in a strain with a lower status matA, the mF plasmid overcomes the force of uniparental inheritance and is preferentially transmitted to the progeny via mitochondrial fusion. Moreover, the conformation of mtDNA is changed from circular to linear by recombination with the mF plasmid. Since biparental inheritance usually occurs in a cross involving a combination of matA1 and matA15, two types of inheritance of Physarum mtDNA exist. Considering the existence of the mF plasmid, there are four patterns of cytoplasmic inheritance in P. polycephalum: 1) uniparental inheritance of mtDNA, 2) uniparental inheritance of mtDNA and preferential transmission of the mF plasmid, 3) biparental inheritance of mtDNA, and 4) biparental inheritance of mtDNA and the mF plasmid. This article describes the events involved in each pattern. Finally, we discuss a hypothetical mechanism for mitochondrial fusion. The essential protein may be the ORF640 protein encoded in the mF plasmid. Received 8 March 2000/ Accepted in revised form 23 March 2000     

Characterization of the MFalpha pheromone of the human fungal pathogen cryptococcus neoformans

Cryptococcus neoformans is an important human pathogenic fungus with a defined sexual cycle and well-developed molecular and genetic approaches. C. neoformans is predominantly haploid and has two mating types, MATa and MATalpha. Mating is known to be regulated by nutritional limitation and thought also to be regulated by pheromones. Previously, a portion of the MATalpha locus was cloned, and a presumptive pheromone gene, MFalpha1, was identified by its ability to induce conjugation tube-like filaments when introduced by transformation into MATa cells. Here, the ability of the MFalpha1 gene to induce these morphological changes in MATa cells was used as a phenotypic assay to perform a structure-function analysis of the gene. We show that the MFalpha1 open reading frame is required for the morphological response of MATa cells. We also find that the cysteine residue of the C-terminal CAAX motif is required for activity of the MFalpha1 pheromone. In addition, we use a reporter system to measure the expression levels of the MFalpha1 pheromone gene and find that two signals, nutrient starvation and the presence of factors secreted by mating partner cells, impinge on this promoter and regulate MFalpha1 expression. We identify a second pheromone gene, MFalpha2, and show phenotypically that this gene is also expressed. Finally, we have synthesized the MFalpha1 pheromone and show that only the predicted mature modified form of the alpha-factor peptide triggers morphological responses in MATa cells.     

Signal transduction cascades regulating mating, filamentation, and virulence in Cryptococcus neoformans.

Cryptococcus neoformans is a basidiomycetous fungal pathogen that infects the central nervous system. The organism has a defined sexual cycle involving mating between haploid MATalpha and MATa cells. Recent studies have revealed signaling cascades that coordinately regulate differentiation and virulence of C. neoformans. One signaling cascade involves a conserved G-protein alpha subunit and cAMP, and senses nutrients during mating and virulence. The second is a conserved mitogen activated protein (MAP) kinase cascade that senses pheromone during mating, and also regulates haploid fruiting and virulence. Interestingly, some of the MAP kinase components are encoded by the MAT locus itself, which may explain the unique association of the MATalpha locus with physiology and virulence.     

Diploid strains of the pathogenic basidiomycete Cryptococcus neoformans are thermally dimorphic

Cryptococcus neoformans is an opportunistic human pathogenic fungus with a defined sexual cycle. Clinical and environmental isolates of C. neoformans are haploid, and the diploid stage of the lifecycle is thought to be transient and unstable. In contrast, we find that diploid strains are readily obtained following genetic crosses of congenic MATalpha and MATa strains. At 37 degrees C, the diploid strains grow as yeast cells with a single nucleus that is larger than a haploid nucleus, contains a 2n content of DNA by FACS analysis, and is heterozygous for the MATalpha and MATa loci. At 24 degrees C, these diploid self-fertile strains filament and sporulate, producing recombinant haploid progeny in which meiotic segregation has occurred. In contrast to dikaryotic filament cells that are typically linked by fused clamp connections during mating, self-fertile diploid strains produce monokaryotic filament cells with unfused clamp connections. We also show that these diploid strains can be transformed and sporulated and that an integrated selectable marker segregates in a mendelian fashion. The diploid state could play novel roles in the lifecycle and virulence of the organism and can be exploited for the analysis of essential genes. Finally, the observation that dimorphism is thermally regulated suggests similarities between the lifecycle of C. neoformans and other thermally dimorphic human pathogenic fungi, including Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis, Paracoccidioides brasiliensis, and Sporothrix schenkii.     

On the origins of congenic MATalpha and MATa strains of the pathogenic yeast Cryptococcus neoformans.

The basidiomycetous yeast Cryptococcus neoformans infects humans and causes a meningoencephalitis that is uniformly fatal if untreated. The organism has a defined sexual cycle involving mating of haploid MATa and MATalpha strains, gene disruption by transformation and homologous recombination is now readily accomplished, and robust animal models for infection have been well established. In addition, a pair of congenic MATalpha and MATa haploid strains have been constructed that permit detailed studies on physiology and virulence by classical genetic approaches. These strains represent a valuable resource for further studies in this organism, and the genomic sequence of one of these strains, JEC21 (=B-4500), was recently chosen to be sequenced by an international consortium. Because of the importance of these strains for genetic studies in C. neoformans and the fact that the genomic sequence of one of these strains is in progress, we review here how these congenic strains were originally constructed.     

Haploid fruiting in Cryptococcus neoformans is not mating type alpha-specific

Under appropriate conditions, haploid Cryptococcus neoformans cells can undergo a morphological switch from a budding yeast form to develop hyphae and viable basidiospores, which resemble those produced by mating. This process, known as haploid fruiting, was previously thought to occur only in MATalpha strains. We identified two new strains of C. neoformans var. neoformans serotype D that are MATa type and are able to haploid fruit. Further, a MATa reference strain, B-3502, also produced hyphae and fruited after prolonged incubation on filament agar. Over-expression of STE12a dramatically enhanced the ability of all MATa strains tested to filament. Segregation analysis of haploid fruiting ability confirmed that haploid fruiting is not MATalpha-specific. Our results indicate that MATa cells are intrinsically able to haploid fruit and previous observations that they do not were probably biased by the examination of a small number of genetically related isolates that have been maintained in the laboratory for many years.     

The G-protein beta subunit GPB1 is required for mating and haploid fruiting in Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen with a defined sexual cycle. The gene encoding a heterotrimeric G-protein beta subunit, GPB1, was cloned and disrupted. gpb1 mutant strains are sterile, indicating a role for this gene in mating. GPB1 plays an active role in mediating responses to pheromones in early mating steps (conjugation tube formation and cell fusion) and signals via a mitogen-activated protein (MAP) kinase cascade in both MATalpha and MATa cells. The functions of GPB1 are distinct from those of the Galpha protein GPA1, which functions in a nutrient-sensing cyclic AMP (cAMP) pathway required for mating, virulence factor induction, and virulence. gpb1 mutant strains are also defective in monokaryotic fruiting in response to nitrogen starvation. We show that MATa cells stimulate monokaryotic fruiting of MATalpha cells, possibly in response to mating pheromone, which may serve to disperse cells and spores to locate mating partners. In summary, the Gbeta subunit GPB1 and the Galpha subunit GPA1 function in distinct signaling pathways: one (GPB1) senses pheromones and regulates mating and haploid fruiting via a MAP kinase cascade, and the other (GPA1) senses nutrients and regulates mating, virulence factors, and pathogenicity via a cAMP cascade.     

Inheritance of organelle DNA sequences in a citrus-poncirus intergeneric cross

Many land plants deviate from the maternal pattern of organelle inheritance. In this study, heterologous mitochondrial and chloroplast probes were used to investigate the inheritance of organelle genomes in the progeny of an intergeneric cross. The seed parent was LB 1-18 (a hybrid of Citrus reticulata Blanco cv. Clementine x C. paradisi Macf. cv. Duncan) and the pollen parent was the cross-compatible species Poncirus trifoliata (L.) Raf. All 26 progeny examined exhibited maternal inheritance of plastid petA and petD loci. However, 17 of the 26 progeny exhibited an apparent biparental inheritance of mitochondrial atpA, cob, coxII, and coxIII restriction fragment length polymorphisms (RFLPs) and maternal inheritance of mitochondrial rrn26 and coxI RFLPs. The remaining nine progeny inherited only maternal mitochondrial DNA (mtDNA) configurations. Investigations of plant mitochondrial genome inheritance are complicated by the multipartite structure of this genome, nuclear gene control over mitochondrial genome organization, and transfer of mitochondrial sequences to the nucleus. In this study, paternal mtDNA configurations were not detected in purified mtDNA of progeny plants, but were present in progeny DNA preparations enriched for nuclear genome sequences. MtDNA sequences in the nuclear genome therefore produced an inheritance pattern that mimics biparental inheritance of mtDNA.     

Calcineurin is required for hyphal elongation during mating and haploid fruiting in Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen that causes meningitis in immunocompromised patients. Its growth is sensitive to the immunosuppressants FK506 and cyclosporin, which inhibit the Ca2+- calmodulin-activated protein phosphatase calcineurin. Calcineurin is required for growth at 37 degrees C and virulence of C.neoformans. We found that calcineurin is also required for mating. FK506 blocks mating of C.neoformans via FKBP12-dependent inhibition of calcineurin, and mutants lacking calcineurin are bilaterally sterile. Calcineurin is not essential for the initial fusion event, but is required for hyphal elongation and survival of the heterokaryon produced by cell fusion. It is also required for hyphal elongation in diploid strains and during asexual haploid fruiting of MATalpha cells in response to nitrogen limitation. Because mating and haploid fruiting produce infectious basidiospores, our studies suggest a second link between calcineurin and virulence of C.neoformans. Calcine urin regulates filamentation and 37 degrees C growth via distinct pathways. Together with studies revealing that calcineurin mediates neurite extension and neutrophil migration in mammals, our findings indicate that calcineurin plays a conserved role in the control of cell morphology.     

Paternal inheritance of mitochondria in Chlamydomonas

To analyze mitochondrial DNA (mtDNA) inheritance, differences in mtDNA between Chlamydomonas reinhardtii and Chlamydomonas smithii, respiration deficiency and antibiotic resistance were used to distinguish mtDNA origins. The analyses indicated paternal inheritance. However, these experiments raised questions regarding whether paternal inheritance occurred normally. Mitochondrial nucleoids were observed in living zygotes from mating until 3 days after mating and then until progeny formation. However, selective disappearance of nucleoids was not observed. Subsequently, experimental serial backcrosses between the two strains demonstrated strict paternal inheritance. The fate of mt+ and mt− mtDNA was followed using the differences in mtDNA between the two strains. The slow elimination of mt+ mtDNA through zygote maturation in darkness was observed, and later the disappearance of mt+ mtDNA was observed at the beginning of meiosis. To explain the different fates of mtDNA, methylation status was investigated; however, no methylation was detected. Variously constructed diploid cells showed biparental inheritance. Thus, when the mating process occurs normally, paternal inheritance occurs. Mutations disrupting mtDNA inheritance have not yet been isolated. Mutations that disrupt maternal inheritance of chloroplast DNA (cpDNA) do not disrupt inheritance of mtDNA. The genes responsible for mtDNA inheritance are different from those of chloroplasts.     

A CIS-Acting Mutation within the MATa Locus of SACCHAROMYCES CEREVISIAE That Prevents Efficient Homothallic Mating-Type Switching

Homothallic strains of Saccharomyces cerevisiae are able to switch from one mating-type to the other as frequently as every cell division. We have identified a cis-dominant mutation of the MATa locus, designated MATa-inc, that can be converted to MATalpha at only about 5% of the normal efficiency. In homothallic MATa-inc/mata* diploids, the MATa-inc locus switched to MATalpha in only one of 30 cases, while the mata* locus switched to MATalpha in all 30 cases. The MATa-inc mutation can be "healed" by a series of switches, first to MATalpha and then to a normal allele of MATa. These data are consistent with the "cassette" model of Hicks, Strathern and Herskowitz (1977), in which mating conversions involve the transposition of wild-type copies of a or alpha information from silent genes elsewhere in the genome. The MATa-inc mutation appears to alter a DNA sequence necessary for the replacement of MATa by MATalpha. The MATa-inc mutation has no other effect on MATa functions. In beterothallic backgrounds, the mutation has no effect on the sensitivity to alpha-factor, synthesis of a-factor, expression of barrier phenotype or ability to mate or sporulate.--The MATa-inc allele does, however, exhibit one pleiotropic effect. About 1% of homothallic MATa-inc cells become completely unable to switch mating type because of mutations at HMa, the locus proposed to carry the silent copy of alpha information.--In addition, we have isolated a less efficient allele of the HO gene.