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Eukaryot Cell. Apr 2011; 10(4): 578–587.
PMCID: PMC3127640

Evolution of Mating within the Candida parapsilosis Species Group[down-pointing small open triangle]


Candida orthopsilosis and Candida metapsilosis are closely related to Candida parapsilosis, a major cause of infection in premature neonates. Mating has not been observed in these species. We show that ~190 isolates of C. parapsilosis contain only an MTLa idiomorph at the mating-type-like locus. Here, we describe the isolation and characterization of the MTL loci from C. orthopsilosis and C. metapsilosis. Among 16 C. orthopsilosis isolates, 9 were homozygous for MTLa, 5 were homozygous for MTLα, and 2 were MTLa/α heterozygotes. The C. orthopsilosis isolates belonged to two divergent groups, as characterized by restriction patterns at MTL, which probably represent subspecies. We sequenced both idiomorphs from each group and showed that they are 95% identical and that the regulatory genes are intact. In contrast, 18 isolates of C. metapsilosis contain only MTLα idiomorphs. Our results suggest that the role of MTL in determining cell type is being eroded in the C. parapsilosis species complex. The population structure of C. orthopsilosis indicates that mating may occur. However, expression of genes in the mating signal transduction pathway does not respond to exposure to alpha factor. C. parapsilosis is also nonresponsive, even when the GTPase-activating protein gene SST2 is deleted. In addition, splicing of introns in MTLa1 and MTLa2 is defective in C. orthopsilosis. Mating is not detected. The alpha factor peptide, which is the same sequence in C. parapsilosis, C. orthopsilosis, and C. metapsilosis, can induce a mating response in Candida albicans. It is therefore likely either that mating of C. orthopsilosis takes place under certain unidentified conditions or that the mating pathway has been adapted for other functions, such as cross-species communication.


Candida parapsilosis is a nosocomial fungal pathogen, and infection is most common in neonates, transplant patients, and patients receiving parenteral nutrition (56). Candida parapsilosis is closely related to Candida albicans and is a member of the CTG clade (species in which CTG is translated as serine rather than leucine) (12, 22, 51). C. parapsilosis isolates historically have been categorized as group I, II, or III (24, 34, 39). However, molecular fingerprinting and mitochondrial genome architectures have shown that these groups correspond to three species now known as C. parapsilosis (formerly group I), C. orthopsilosis (group II), and C. metapsilosis (group III) (61, 69) (Fig. 1A).

Fig. 1.
Organization of MTL in C. orthopsilosis. (A) Phylogenetic relationship of the C. parapsilosis species group. The phylogenetic tree was inferred from an alignment of ITS sequences and was drawn using the PhyML package in SeaView (25). Sequences were obtained ...

The CTG clade contains a number of fully sexual haploid species, such as Candida lusitaniae and Candidia guilliermondii, and a number of diploid species (e.g., Candida tropicalis, Candida dubliniensis) in which a full sexual cycle has never been observed (6, 9, 10, 12, 37). In C. albicans, the discovery of a mating-type-like (MTL) locus that resembled the Saccharomyces cerevisiae MAT locus was the first indication that a sexual cycle may be present (29). There are two MTL idiomorphs in C. albicans. The MTLa idiomorph encodes a homeodomain protein, a1, and an HMG domain protein, a2 (11, 73). The MTLα idiomorph encodes α1 and α2 proteins; α1 is required for expression of α-specific mating genes (73). In a heterozygous diploid a/α cell, the a1/α2 heterodimeric complex represses expression of mating genes (73). Both idiomorphs carry alleles of a poly(A) polymerase gene (PAP), an oxysterol binding protein gene (OBP), and a phosphatidylinositol kinase gene (PIK), which were acquired early in the evolution of the CTG clade but have no known function in mating (12).

Discovery of the MTL loci was shortly followed by experimental evidence for mating between diploid cells of opposite mating types, although initially observed to occur at a very low frequency (30, 50). Subsequent studies showed that the mating frequency is greatly increased when cells undergo a phenotypic switch from white to opaque cells (52). The switch to opaque cells was first observed in the clinical isolate WO-1 by Slutsky et al. (67). Opaque cells are elongated and have “pimples” present on the cell surface. They also absorb the dye phloxine B, resulting in pink colonies, whereas white cells cannot take up the dye (4). The white/opaque switch is regulated by WOR1, whose expression is repressed by a1/α2 (28, 68, 78). WOR1 regulates it own expression and also expression of EFG1, WOR2, and CZF1 (79). This circuit results in high levels of Efg1 in white cells and of Wor1 in opaque cells (79). The white/opaque switch is also regulated by many environmental stimuli, such as temperature (47), oxygen and carbon dioxide concentrations (27, 58), and genotoxic and oxidative stress (1). Opaque cells are more stable at low temperatures, which favors mating on the surface of the skin (21, 35). At body temperature, opaque-phase cells are stabilized under low oxygen and high carbon dioxide conditions (27).

Many of the genes required for mating in S. cerevisiae are conserved in the genomes of C. albicans and other species in the CTG clade (12, 14, 49). These include the diffusible mating pheromones, a-factor and α-factor. In C. albicans, α-factor pheromone is produced by MTLα cells and is sensed by the receptor protein Ste2 on MTLa cells (7, 42, 55). Conversely, a-factor is produced by MTLa cells, and MTLα cells respond via the receptor Ste3 (18). Exposure of opaque cells to mating pheromones results in shmoo formation and the expression of genes required for mating (7, 42, 55). Although white-phase cells do not generate shmoos, they do have a well-characterized response to α-factor. Daniels et al. (17) showed that α-factor induces adhesiveness and biofilm formation in white cells. Biofilms containing white-phase cells promoted chemotropism between the rarer opaque a/a and α/α cells and therefore enhanced mating. The white cell response to α-factor is mediated via the same pathway as the opaque response except for the final downstream targets (62, 76, 77). However, white/opaque switching is confined to the very closely related species C. albicans and C. dubliniensis and has no role in mating in other species in the CTG clade (62).

The mating signal transduction pathway is generally conserved in C. parapsilosis (12). However, MTLa1 is a pseudogene (46), and we show here that the majority (and possibly all) isolates contain only MTLa idiomorphs. We now describe the organization of MTL in the sister species C. orthopsilosis and C. metapsilosis. We show that the pseudogenization of MTLa1 is a recent event that is specific to C. parapsilosis but that the role of MTL appears to be degenerating in all the species. We also show that Candida species may communicate via α-factor and the mating signal transduction pathway. However, we did not observe mating in any of the C. parapsilosis group.


Species and strains.

The isolates used are listed in Table S1 of the supplemental material. The majority of isolates of C. parapsilosis were obtained from Portugal (A. Rodrigues [64]) and Hungary (A. Gacser). Some isolates originated from Italy and Germany. C. orthopsilosis (68) and C. metapsilosis (26) isolates originated from Italy (S. Senesi), Belgium, the United Kingdom, the United States, and Hong Kong.

Media and reagents.

Alpha pheromones KPHWTTYGYYEPQ (C. parapsilosis) and GFRLTNFGYFEPG (C. albicans) were synthesized by CS Bio Co. Strains were grown in YPD medium (1% yeast extract, 2% peptone, 2% dextrose; Formedium) prior to DNA extraction. For colony selection, 2% agar was used. Transformants were selected on YPD agar containing nourseothricin (Werner Bioagents, Jena, Germany). Mating experiments were carried out on various media (described in Table S3 of the supplemental material). Mating products were isolated on the appropriate dropout agar (0.19% yeast nitrogen broth without amino acids and ammonium sulfate, plus 2% dextrose, 2% agar, 0.5% ammonium sulfate, and 0.75% amino acid mix). Oligonucleotide primers used for PCR and other applications are listed in Table S2 of the supplemental material.

Identification of MTLa and MTLα idiomorphs from C. orthopsilosis and C. metapsilosis.

A fosmid library was generated from C. orthopsilosis 90-125 (type 2) and C. metapsilosis ATCC 96143 by Agowa (Germany), and the ends of 2,000 clones were sequenced. One fosmid insert from each was sequenced in full, and these were predicted to contain the MTL based on conserved synteny with C. parapsilosis and other Candida species. MTLa was isolated from C. orthopsilosis, and MTLα was from C. metapsilosis. A second MTLa idiomorph was amplified by PCR from C. orthopsilosis J981224 (type 1) by using oligonucleotide primers BUT282 (from within an ortholog of orf19.3202) and BUT284 (3′ to GAP1) and a long-template PCR kit from Roche. MTLα idiomorphs were amplified from C. orthopsilosis CP289 (type 1) and CP185 (type 2) using primers BUT282 and DL04 (within GAP1).

Sixteen isolates of C. orthopsilosis were categorized by digestion with EcoRI. The MTL idiomorphs were amplified using primers BUT282 and DL04. Mating type was confirmed using primers specific for each subgroup [MTLalp1(T1)FP and MTLalp1(T1)RP within MTLα1 type 1, CO3 and CO6 within MTLa1 type 1, MTLalp1(T2)FP and MTLalp1(T2)RP within MTLα1 type 2, and MTLa1(T2)FP(a) and MTLa1(T2)RP within MTLa1 type 2].

Screening of MTL in C. parapsilosis isolates.

Degenerate PCR was used to screen for the presence of MTLα idiomorphs in C. parapsilosis. Primers were designed from an alignment of sequences from C. albicans, C. tropicalis, C. orthopsilosis, and C. metapsilosis by using iCODEHOP v1.0 (8) and Bioedit v7.0 (http://www.mbio.ncsu.edu/bioedit/bioedit.html). Two sets of primers were designed. DMTLalpha1F1B and DMTLalpha1R3 amplify a 500-bp fragment from MTLα1 from three of the test species (excluding C. tropicalis). DMTLalpha1F3B (within PAPα) and DMTLalpha1R4 (within MTLα1) amplify a 750-bp fragment from all the test species. MTX3 and MTX6 (which amplify a fragment from within MTLa1 in C. parapsilosis [46]) were used as a positive control. A total of 120 clinic isolates of C. parapsilosis from Portugal, 63 isolates from three sites in Hungary, and 6 isolates from Germany, Italy, and Puerto Rico were screened (see Table S1 of the supplemental material). All isolates in which a putative MTLα-specific product was amplified were checked by sequencing the equivalent internal transcribed spacer (ITS) region (oligos ITS4 and ITS5; [see Table S2 of the supplemental material]). All were derived from species incorrectly identified as C. parapsilosis (see Table S1).

Screening of MTL in C. metapsilosis isolates.

Degenerate PCR primers were designed from an alignment of MTLa2 sequences. Primers MTLa2D2F and MTLa2D2R amplify a fragment of approximately 260 bp from the test species. Eighteen isolates of C. metapsilosis were screened. Any amplified fragments were cloned in vector pCR4-TOPO (Invitrogen Life Technologies) and sequenced. None corresponded to MTLa sequences (data not shown).

Construction of gene deletions in C. parapsilosis and C. orthopsilosis.

To delete SST2 (cpar2910), a 502-bp upstream region was amplified by PCR from C. parapsilosis CLIB214 genomic DNA using GoTaq DNA polymerase (Promega) and the oligonucleotide primers CpSST2KO1 and CpSST2KO2, which contain recognition sites for KpnI and ApaI, respectively (see Table S2 in the supplemental material). A 501-bp region downstream from SST2 was also amplified using the primers CpSST2KO3 and CpSST2KO4, which contain recognition sites for SacII and SacI. The PCR products were purified using a Qiagen PCR purification kit and ligated at either end of a SAT1 flipper cassette in pCD8 (20), generating plasmid pCpSST2. The entire cassette plus flanking regions was excised by digestion with KpnI and SacI, gel purified, and transformed into C. parapsilosis CLIB214. Integration of the cassette at SST2 was confirmed by PCR using a primer 5′ to SST2 (CpSST2KO5) and a primer from inside the cassette (BUT237) (1.7 kb). Intact SST2 alleles were identified by PCR with CpSST2KO5 and CpSST2KO6 (1.2 kb). Primers CpSST2KO1 and CpSST2KO4 were used to verify recycling of the cassette and deletion of the SST2 gene (1.0 kb). The same cassette was used to delete both SST2 alleles. The deletion was also confirmed by Southern blotting, using approximately 20 μg of RNase-treated genomic DNA digested with ScaI. A probe was amplified from C. parapsilosis CLIB214 genomic DNA by using primers SST2For and SST2Rev, which amplify a region upstream of SST2. The probe was labeled, and Southern blotting was performed using the DIG High Prime labeling and DIG detection starter kit II (Roche).

A similar method was used to delete HIS1 and LEU2 from the C. orthopsilosis strains CP47 (MTLa) and CP289 (MTLα), respectively. Primers CoHIS1KO1/CoLEU2KO1 and CoHIS1KO2/CoLEU2KO7 were used to amplify an upstream region of either HIS1 or LEU2 (approximately 500 bp). Primers CoHIS1KO3/CoLEU2KO3 and CoHIS1KO4/CoLEU2KO4 were used to amplify a region downstream of either HIS1 or LEU2 (approximately 500 bp). Integration of the cassette at the HIS1 or LEU2 allele was confirmed by PCR using a primer 5′ to the gene (CoHIS1KO5/CoLEU2KO9) and a primer from inside the cassette (BUT237) (approximately 1.6 kb). To confirm the presence of intact HIS1 or LEU2 alleles, the primer pairs CoHIS1KO5/CoLEU2KO9 and CoHIS1KO6/CoLEU2KO6 were used (approximately 1.2 kb). Primers CoHIS1KO1/CoLEU2KO1 and CoHISKO4/CoLEUKO4 were used to verify recycling of the cassette and deletion of the HIS1 and LEU2 genes (approximately 1 kb).

Transformation of C. parapsilosis and C. orthopsilosis and recycling of the SAT1 flipper cassette.

Strains were transformed by electroporation as described previously with some modifications (20). After electroporation, 950 μl of fresh YPD was added immediately and the mixture was incubated at 30°C for 3 to 4 h. Following incubation cells were pelleted, washed once in 1 ml of water, and resuspended in 300 μl of water. A 100-μl aliquot was plated onto YPD plates supplemented with nourseothricin at a concentration of 200 μg ml−1. Transformants were obtained following 48 h of incubation at 30°C.

The cassette was recycled from positive transformants by growth overnight in YPM (1% yeast extract, 2% peptone, and 2% maltose). One hundred cells were plated onto YPD plates containing 10 μg ml−1 of nourseothricin and incubated overnight at 30°C. Following incubation, a mixture of large and small colonies was visible on the plate. Small colonies were restreaked onto fresh YPD agar plates and checked for nourseothricin sensitivity. The second allele was deleted using the same protocol.

Shmoo formation.

Opaque-phase cells of C. albicans CA12 (sst2 deletion) and RBY1220 (bar1 deletion) were grown overnight at 25°C. Cultures were then resuspended to an A600 of 0.2 in fresh SpiderM medium supplemented with 10% dimethyl sulfoxide (DMSO), C. albicans α-factor (10 μg ml−1), or C. parapsilosis α-factor (10 μg ml−1) and incubated for 6 h at 25°C with shaking. From each culture 500 μl was centrifuged, washed in 1 ml of water, resuspended in 100 μl of calcofluor white (1 mg ml−1 in 10 mM NaOH), and incubated at room temperature for 10 min. Images were obtained using a ColorView II camera (Soft Imaging Systems) mounted on an Olympus BX40 fluorescence microscope with analySIS software.

Mating assays.

Control mating experiments were carried out using both white- and opaque-phase cells of C. albicans RBY1118 (MTLa/a his1/his1 leu2/leu2) and RBY1180 (MTLα/α arg4/arg4) (65). Strains were cross-streaked on SpiderM agar plates and incubated for 3 days at room temperature. The cultures were replica plated to minimal medium lacking histidine, leucine, and arginine and incubated for several days at room temperature to observe mating products. Similar experiments were carried out using C. parapsilosis CDH4 (MTLa/a his1/his1) and C. parapsilosis CDU1 (MTLa/a ura3/ura3). C. orthopsilosis LHH1 (MTLa/a his1/his1) and C. orthopsilosis LHL1 (MTLα/α leu2/leu2) and other combinations were also crossed (see Table S3 in the supplemental material). Cells of opposite mating type of C. orthopsilosis (2 × 107) were also mixed and incubated under various conditions before plating on selective media. No mating products were detected. Mating experiments were carried out at various temperatures and in multiple media (see Table S3). Cells were incubated from 4 to 12 days prior to plating on selective agar.

Analysis and extraction of RNA.

C. parapsilosis CLIB214, C. parapsilosis LHS1 (sst2Δ), and C. orthopsilosis CP47 were grown in SpiderM overnight at 30°C with shaking. Opaque cells of C. albicans CA12 (sst2Δ) and RBY1220 (bar1Δ) were incubated in the same medium overnight at 25°C. All cultures were diluted to an A600 of 0.2 in fresh SpiderM medium prior to incubation at the appropriate temperature. C. albicans strains were incubated at 25°C (three biological replicates). C. parapsilosis and C. orthopsilosis strains were incubated at 25°C (two biological replicates), 30°C (three biological replicates), and 37°C (two biological replicates). Cultures were grown for 2 h followed by addition of synthetic alpha factor from C. parapsilosis and C. albicans at a final concentration of 10 μg ml−1 or DMSO and incubated for a further 4 h at the appropriate temperature. Following incubation, cells were pelleted, resuspended in 100 μl of RNAlater (Ambion), and stored at −80°C. RNA was extracted using a RiboPure yeast kit from Ambion. A supplementary treatment with DNase I was carried out to ensure that there was no DNA contamination. To generate cDNA, 2 μg of DNase I-treated RNA was incubated at 70°C for 10 min with 0.1 μg of oligo(dT) in a final volume of 5 μl and then chilled on ice. A 15-μl cocktail containing 4 μl of 5× reverse transcription (RT) reaction buffer (Promega), 1 μl 10 mM deoxynucleoside triphosphates, 1 μl RNasin (40 U μl−1; Promega), and 1 μl of Moloney murine leukemia virus reverse transcriptase (200 U μl−1; Promega) was then added. The mixture was incubated at 37°C for 1 h followed by 95°C for 2 min. Quantitative RT-PCR was carried out on an Agilent Technologies Stratagene Mx2005p system using Brilliant II SYBR green QPCR Low Rox master mix (600830) as per the manufacturer's instructions. Two technical replicates were used for each sample. Cycling conditions consisted of 1 cycle at 95°C for 10 min followed by 40 cycles of 95°C for 30 s, 55°C for 30 s, and 72°C for 30 s. A final cycle of 95°C for 1 min was followed by melting curve analysis performed at 55°C to 95°C (temperature transition, 0.2°C/s) with stepwise fluorescence detection. Primers used for analysis are listed in Table S2 of the supplemental material. Relative expression changes were identified using the ΔCT method, in relation to the expression of ACT1.

Analysis of RNA splicing.

C. parapsilosis CLIB214, C. orthopsilosis type 1 strains (5 MTLa, 1 MTLa/α, and 3 MTLα/α), C. orthopsilosis type 2 strains (4 MTLa, 1 MTLa/α, and 2 MTLα/α), and C. metapsilosis (strains CP5, CP86, CP92, CP286, ATTC 96143, and ATCC 96144) were grown overnight in SpiderM at 30°C. The cultures were diluted to an A600 of 0.2 in fresh SpiderM medium and incubated at 25°C or 30°C with shaking until an A600 of 1.0 was reached. RNA was extracted as described above. Both cDNA and genomic DNA were used as templates for PCR amplification, using GoTaq (Promega). The primers used are listed in Table S2 of the supplemental material.


Identification of MTL idiomorphs from C. orthopsilosis and C. metapsilosis.

MTL idiomorphs were first identified by constructing fosmid libraries using genomic DNA from C. orthopsilosis 90-125 and C. metapsilosis ATCC 96143. The fosmids were screened by sequencing the ends of the inserts and searching for similarities to the MTL region of other Candida species. An MTLa idiomorph was identified from C. orthopsilosis (Fig. 1) and an MTLα idiomorph from C. metapsilosis (Fig. 2).

Fig. 2.
(A) Amplification of the MTL locus from C. metapsilosis. A 13.2-kb fragment spanning the MTL locus was amplified from 18 isolates of C. metapsilosis by using oligos BUT287 and BUT289 from within the GAP1 and orf19.3202 genes. The PCR fragments were digested ...

To help identify isolates containing the opposite mating type, we designed oligonucleotides from unique regions outside the idiomorphs and amplified a region of approximately 10 kb for 16 C. orthopsilosis isolates. Restriction analysis of this region revealed several different patterns from individual isolates (Fig. 1C). The isolates fell into two groups, labeled type 1 and type 2 (Fig. 1C). We sequenced entire MTLa and MTLα idiomorphs from a representative of each type.

Unlike C. parapsilosis, where the MTLa1 region is a pseudogene and no MTLα idiomorphs have been identified (46), the MTLa and MTLα idiomorphs from C. orthopsilosis are all intact and resemble the structures in C. albicans and C. tropicalis (12). Each idiomorph encodes two regulatory genes (a1 and a2 in MTLa and α1 and α2 in MTLα) and idiomorph-specific alleles of PIK, PAP, and OBP, which have no known role in mating. The MTLa idiomorph in the type 2 isolate is 9,078 bp long, and the MTLα idiomorph is slightly longer, at 9,492 bp. The type 1 idiomorphs are similar in size; however, we could not determine the exact size, because we did not sequence across the left junction of the type 1 MTLa idiomorph. Interestingly, the left-hand junction of MTLα lies directly between the stop codons of MTLα2 and of the adjacent gene, GAP1, in both type 1 and type 2 isolates.

The two MTLa idiomorphs and the two MTLα idiomorphs from C. orthopsilosis are 95% identical in DNA sequence (data not shown). However, at the protein level the regulatory proteins range in similarity from 80% (Mtla1) (Fig. 1D) to 93% (Mtlα1) (Fig. 2B), whereas the Pik, Pap, and Obp proteins are much more similar (97 to 99%) (data not shown).

The differences between the idiomorphs of the same mating type suggest that isolates characterized as C. orthopsilosis may belong to more than one species. Sequence analysis of the ITS confirmed that these isolates were correctly identified as C. orthopsilosis but that there are at least two divergent groups within these species (see Fig. S1 in the supplemental material), equivalent to C. orthopsilosis type 1 and C. orthopsilosis type 2 (Fig. 1). Allele-specific oligonucleotides designed to amplify parts of MTLa1 and MTLα1 showed that each type includes isolates with only MTLa idiomorphs, only MTLα idiomorphs, or both MTLa and MTLα idiomorphs (Fig. 1C).

In contrast to C. orthopsilosis, amplification of the MTL region from all 18 isolates of C. metapsilosis yielded fragments with identical restriction patterns (Fig. 2). We could not detect any evidence of the presence of MTLa genes elsewhere in the genome, based on degenerate PCR (data not shown). We therefore assume that all the isolates tested contain only MTLα idiomorphs, similar to the sequenced isolate. The organization of MTLα in C. metapsilosis is identical to that of C. orthopsilosis, except for the presence of a repeated motif in Mtlα1 in C. metapsilosis that is not well conserved in the other species (Fig. 2B). In contrast, we detected only MTLa idiomorphs in 189 isolates of C. parapsilosis, based on degenerate PCR designed to amplify the region between PAPα and MTLα1 (see Fig. S2 and Table S1 in the supplemental material).

C. orthopsilosis isolates do not mate.

We found approximately equal numbers of MTLa and MTLα isolates of C. orthopsilosis (for both types), with one heterozygous (MTLa/MTLα) isolate of each type (Fig. 1). This population structure suggests that individual isolates may be mating and undergoing some form of sexual reproduction. We therefore attempted to find evidence of mating products, by using a his1 knockout generated in a type 1 MTLa isolate (CP47) and a leu2 knockout generated in an MTLα isolate (CP289) of the same type (Fig. 3A). Attempted crosses were performed under several conditions (see Table S3 in the supplemental material); however, no His+/Leu+ prototrophs were identified. Mating between MTLa and MTLα isolates of C. albicans was clearly detectable, in both opaque- and white-phase cells (Fig. 3B). Because it has been reported that identical idiomorphs of C. albicans can mate (same-sex or homothallic mating) (3) and that C. albicans can mate with C. dubliniensis (57), we also looked for mating products between C. orthopsilosis or C. parapsilosis and opaque cells of C. albicans, between two MATa strains of C. parapsilosis, and between C. orthopsilosis and C. albicans. However, none was identified (see Table S3).

Fig. 3.
(A) Construction of his1 (LHH1) and leu2 (LHL1) knockouts in C. orthopsilosis. (i) HIS1 and LEU2 alleles were disrupted using a modified disruption cassette adapted from the SAT1 flipper system (20) (see descriptions in Materials and Methods). (ii) PCR ...

Analysis of the mating signal transduction pathway in C. orthopsilosis.

In an attempt to identify the molecular basis of the defect in mating in C. orthopsilosis, we investigated the response of the mating signal transduction pathway. In both S. cerevisiae and C. albicans, exposure of cells to mating pheromone from cells of the opposite mating type results in induction of expression of the pheromone receptor and of other genes in the mitogen-activated protein kinase pathway (7, 32, 45, 55). We identified the α-factor pheromone gene from C. parapsilosis and from genome surveys of C. orthopsilosis 90-125 and C. metapsilosis ATCC 96143 (Fig. 4). After processing, the C. parapsilosis gene is predicted to encode two copies of a 13-amino-acid peptide and one copy of a 14-amino-acid peptide. C. orthopsilosis encodes one copy each of a 13- and a 14-amino-acid peptide. The gene sequence from C. metapsilosis is not complete, but it encodes at least three copies of a 13-amino-acid peptide and one copy of a 14-amino-acid peptide. The predicted mature peptide sequences are identical in the three species (Fig. 4). Based on similarities with S. cerevisiae and C. albicans, the 13-amino-acid peptide (KPHWTTYGYYEPQ) is likely to be biologically active (7, 55), and this one was chemically synthesized. However, the addition of this factor exogenously did not induce expression of STE2 (the predicted pheromone receptor), STE4 (a component of the heterotrimeric G protein required for signaling), or CPH1 (a transcription factor at the end of the mating pathway) in either C. orthopsilosis or C. parapsilosis (Fig. 5A). Deleting SST2, a GTPase-activating protein, renders both S. cerevisiae and C. albicans more sensitive to the effects of α-factor (13, 19). We therefore knocked out SST2 in an MTLa isolate of C. parapsilosis, generating strain LHS1 (see Fig. S3 in the supplemental material). However, once again, adding exogenous α-factor does not result in induction of expression of STE2 (Fig. 5A).

Fig. 4.
Identification of alpha factor pheromone from the C. parapsilosis species group. The pre-pro-alpha factor sequences were aligned using T-Coffee (54) and manually edited using SeaView (25). The arrows indicate the putative sites (KR) cleaved by the Kex2 ...
Fig. 5.
Transcriptional response to alpha factor treatment. (A and B) C. parapsilosis CLIB214 (wild type) and its isogenic sst2 knockout C. parapsilosis LHS1, C. orthopsilosis CP47 (A) and opaque C. albicans CA12 (sst2Δ) and C. albicans RBY1220 (bar1 ...

To test if the synthetic α-factor from the C. parapsilosis group was functional, we tested its effects on C. albicans (Fig. 5B and C). We used C. albicans with a knockout of either sst2 (18) or bar1 (65) to increase sensitivity. The C. parapsilosis α-factor induced expression of the pheromone receptor STE2 and the transcription factor CPH1 in opaque cells of both sst2 and bar1 knockouts of C. albicans (Fig. 5B). Opaque cells also formed mating projections (or shmoos) when exposed to α-factor from either C. albicans or C. parapsilosis/C. orthopsilosis/C. metapsilosis (Fig. 5C). Our observations are supported by very recent reports that α-factor from C. parapsilosis (and other Candida species) induces same-sex mating and adherence in C. albicans (2).

Defective splicing in MTL in C. orthopsilosis.

Three of the regulatory genes (a1, a2, and α2) at the MTL in Candida species contain introns. Intron position and structure are generally conserved. We investigated splicing of the regulatory genes in C. orthopsilosis and C. metapsilosis. MTLα2 is efficiently spliced in C. orthopsilosis, both in α/α homozygotes and in a/α heterozygotes, and also in C. metapsilosis α/α homozygotes (Fig. 6B and C). However, MTLa2 was not spliced at all in 11 isolates of C. orthopsilosis tested (from both type 1 and type 2), even when exogenous alpha factor was added (Fig. 6B shows results for one isolate). MTLa1, which contains two introns, is inefficiently spliced at intron 2. In contrast, the introns in MTLa1 are efficiently spliced in C. parapsilosis (Fig. 6A) (46). C. parapsilosis does not have an intact MTLa2 gene (46).

Fig. 6.
Splicing of MTLa2, MTLa1, and MTLα2 in C. parapsilosis CLIB214 (A), C. orthopsilosis 981224 (type 1) (B), and C. metapsilosis ATCC 96143, ATCC 96144 and CP5 (C). The oligo combinations were used in a PCR amplification using genomic DNA (D lanes) ...


Species within the Candida (CTG) clade fall into two related groups. One consists of predominantly haploid species (Candida guilliermondii, Candida lusitaniae, Debaryomyces hansenii, and Pichia stipitis), in which mating and meiosis is intact, or relatively intact (9, 10, 37). The sister group contains species such as C. albicans, C. dubliniensis, C. tropicalis, C. parapsilosis, and Lodderomyces elongisporus, for which all described isolates are diploid and, for most, sexual structures have not been observed. Potential ascospores of L. elongisporus have been described (44, 59, 75), but these have not been studied in detail. C. albicans and C. dubliniensis undergo a parasexual cycle; mating between diploid cells with opposite mating types leads to the formation of a tetraploid cell (30, 50). Chromosomes are gradually lost in a random process (5), and recombination between homologous chromosomes occurs in a Spo11-dependent manner (23). Mating only occurs in cells that have undergone a developmental switch from white to opaque forms (reviewed in reference 48). This raises the question of whether mating and meiosis were lost in the entire diploid Candida clade and regained in some format in an ancestor of C. albicans and C. dubliniensis, or whether all (or most) of the species retain the ability to undergo sexual reproduction under very specific conditions.

An analysis of gene loss in Candida species suggests that most have retained the core genes required for mating and meiosis, although there have been some species-specific and lineage-specific losses (12). For example, C. guilliermondii and C. lusitaniae have lost the Dmc1-dependent recombination pathway, as well as some proteins associated with synaptonemal complex formation and crossover inference (12). However, C. lusitaniae (and probably C. guilliermondii also) has retained the ability to undergo meiosis (60).

The first description of the C. parapsilosis MTL locus revealed that MTLa2 is a pseudogene (46), and this was supported by analysis of seven other isolates and by whole-genome sequencing of a clinical isolate (12). More interestingly, all the isolates analyzed contained only MTLa idiomorphs. Including the present study, more than 200 isolates have now been tested (predominantly originating from European hospitals, with a small number from other locations). No MTLα idiomorphs have been found, although it is of course possible that MTLα isolates are present in environmental niches that we have not sampled. We note that >1,000 isolates of the fungal pathogen Cryptococcus neoformans var. grubii were screened before a idiomorphs were eventually found, particularly in African populations (38, 40, 41). However, C. parapsilosis isolates are predominately clonal and are extremely difficult to differentiate using standard techniques, such as randomly amplified polymorphic DNA (RAPD) and amplified fragment length polymorphism analysis (33, 36, 71). The level of single-nucleotide polymorphisms (SNPs) between chromosomes in the sequenced isolate was also 25 to 70 times lower than in other Candida genomes (12). Polymorphisms are more frequent in C. orthopsilosis and C. metapsilosis (26, 70, 71). This suggests that C. parapsilosis may have undergone a population bottleneck and that all global isolates are closely related. C. parapsilosis is much more commonly isolated from clinical samples than either C. metapsilosis or C. orthopsilosis (15, 43, 66). Our analysis shows that the degeneration of MTLa1 is a recent event in C. parapsilosis, because the gene is intact in C. orthopsilosis. It is therefore tempting to speculate that the loss of a mating partner (MTLα) and the degeneration of the MTLa idiomorph may have led to the global spread of a clone with increased virulence.

The population structure of the C. orthopsilosis isolates (approximately equal proportions of MATa and MATα idiomorphs) suggests that mating is occurring. However, we cannot observe this in the laboratory. It is possible that we have not yet identified the ideal conditions; for example, it took several years to correlate mating efficiency in C. albicans with the formation of opaque-phase cells (52). However, even white-phase cells of C. albicans mate at a low frequency that is easily detectable (Fig. 3B). We therefore believe that if mating were occurring in C. orthopsilosis, even at very low frequencies, we would be able to detect it by using crosses of auxotrophic strains (Fig. 3B). We have tested only one pair of C. orthopsilosis isolates in direct mating assays. However, we saw no mating response (shmoo formation, growth arrest, or transcriptional induction) in any of the isolates examined.

It is possible that loss of mating in C. orthopsilosis is also a recent event and is correlated with deficiencies in splicing of the regulatory genes at the MTL idiomorph (Fig. 6). Inefficient splicing of MTLa1 and MTLa2 was observed in all available isolates of both subtypes of C. orthopsilosis, but not of C. parapsilosis or C. albicans. Similar defects in splicing of MTLa1 were identified in Candida glabrata, which is also apparently incapable of mating (53). Both mating types are commonly found in the C. glabrata population, and MATa cells are insensitive to treatment with alpha factor (53). It is possible that splicing in both C. orthopsilosis and C. glabrata is regulated under conditions (not yet known), which allow mating. Intriguingly, L. elongisporus, once thought to represent the sexual form of C. parapsilosis, has completely lost the MTL locus (12). Same-sex (homothallic) mating may be occurring in this species, but it remains to be characterized. Our analyses suggest that the mating idiomorphs are degenerating in the C. parapsilosis group, either through complete loss of the locus (L. elongisporus), pseudogenization (C. parapsilosis), loss of mating partners (C. parapsilosis and possibly C. metapsilosis), and errors in splicing (C. orthopsilosis). Regulation of cell identity may be under the control of other undefined loci.

The mating process is distinct from the means of regulating mating type. However, we have been unable to detect mating in any of the C. parapsilosis group, which raises the question as to why the signal transduction pathways have been conserved intact. One possibility is that they are also required for other biological functions. For example, in C. albicans, the mating pathway is involved in biofilm formation, and mating pheromone induces cohesiveness of white cells (17, 63, 64). However, this white cell response is mediated via a region of the alpha-pheromone receptor that is found only in C. albicans and C. dubliniensis (9, 77). We also do not observe any induction of biofilm formation in the C. parapsilosis group following treatment with pheromone (data not shown). It is, however, very interesting that C. albicans responds to synthetic alpha factor derived from C. parapsilosis/C. orthopsilosis/C. metapsilosis. This raises the possibility that Candida species have adapted the mating pathway as a mechanism of signaling between species. To date, however, the only response we can identify is from C. albicans. We also note that the predicted α-factor sequences are identical in C. parapsilosis, C. orthopsilosis, and C. metapsilosis (Fig. 4). This is somewhat surprising, considering that the equivalent peptides from C. albicans and C. dubliniensis (which are much more closely related than C. parapsilosis and C. orthopsilosis) differ by 1 amino acid. The species in the C. parapsilosis group may therefore be communicating in ways that we cannot yet measure.

Our analysis demonstrated that the C. parapsilosis species group is more complex than initially realized. Isolates identified as C. orthopsilosis fall into two types, distinguished by the analysis of the structure of the MTL idiomorph. Sequencing of the ITS region of the ribosomal DNA (see Fig. S1 in the supplemental material) also places most isolates into two groups (with >99% identity), although at least one isolate (C. orthopsilosis 90-125) differs from both. Tay et al. (72) used RAPD and ITS sequencing to place C. orthopsilosis isolates into two groups (P2 and P3), again with one isolate that was different to both. The P3 grouping is identical in ITS sequence to our type 2 isolates (data not shown). However, there are differences between the P2 group of Tay et al. (72) and our type 1 isolates. Other studies have also identified heterogeneity among C. orthopsilosis isolates (70, 74). We suggest that C. orthopsilosis isolates can be divided into at least two subspecies, represented by type 1 and type 2 in this study. The C. orthopsilosis type strain (ATCC 96139) belongs to the type 1 group (see Fig. S1 in the supplemental material). However, the variation in ITS sequences (for example, of C. orthopsilosis 90-125, and as reported by Tay et al. [72]) suggests that the number of subspecies may be even greater. This should be taken into account when analyzing phenotypic diversity, such as that for drug sensitivity.

Supplementary Material

[Supplemental material]


We are very grateful to Sonia Senesi (University of Pisa), Acácio Rodrigues and Isabel Miranda (University of Porto), and Attila Gacser (Szeged University) for providing clinical isolates. We also recognize the help of Richard Bennett (Brown University) in developing the mating assays and for providing C. albicans strains.

This work was supported by Science Foundation Ireland and the Irish Research Council for Science, Engineering and Technology.


Supplemental material for this article may be found at http://ec.asm.org/.

[down-pointing small open triangle]Published ahead of print on 18 February 2011.


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