Evolution of mating within the Candida parapsilosis species group

Abstract

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.

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 from GenBank (accession numbers AY391845.1, AJ635316.1, AJ698048.1, and AJ698049.1). (B) Organization of the MTLa idiomorphs (top) and MTLα idiomorphs (bottom). Genes are drawn approximately to scale. Sizes of the idiomorphs in type 2 isolates are shown. (C, top) The entire MTL idiomorphs were amplified from type 1 and type 2 isolates of C. orthopsilosis by using primers from within GAP1 and orf19.3202. The PCR products were digested with EcoRI. (Bottom) Amplification of MTLa-specific products (a lanes) and MTLα-specific products (b lanes). The order of the isolates in both panels are as follows. Type 1: lane 1, CP25; lane 2, CP47; lane 3, CP288; lane 4, CP334; lane 5, 942211K; lane 6, CP85; lane 7, CP124; lane 8, CP289; lane 9, J981224. Type 2: lane 10, CP269; lane 11, CP287; lane 12, CP296; lane 13, 90-125; lane 14, CP185; lane 15, CP331; lane 16, CP125; lane M1, 1-kb Plus ladder from Invitrogen. (D) Alignment of Mtla1 from type 1 and type 2 isolates of C. orthopsilosis, in comparison with the ortholog from C. albicans. The sequences were aligned using T-Coffee (54) and visualized with Boxshade.

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 with EcoRV (two strains are not shown). The restriction patterns are almost identical, with only minor differences, suggesting that all isolates contain only one MTL idiomorph (MTLα). Lane M, 1-kb Plus ladder (Invitrogen); lane 2, CP61; lane 3, CP86; lane 4, CP87; lane 5, CP88; lane 6, CP92; lane 7, CP178; lane 8, CP231; lane 9, CP261, lane 10, ATCC95143; lane 11, ATCC95144; lane 12, CP5; lane 13, CP271; lane 14, CP286; lane 15, CP376; lane 16, CP397. (B) Alignment of Mtlα1 sequences, obtained using the methods described in the legend for Fig. 1. An additional repeated motif in the C. metapsilosis sequence is indicated by the dark line above the sequence.

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 verification of HIS1 deletion. Lane 1, C. orthopsilosis CP47 wild type (MTLa); lane 2, integration of the SAT1 flipper at the first HIS1 allele; lane 3, recycling of the SAT1 flipper; lane 4, integration of the SAT1 flipper at the second HIS1 allele; lane 5, recycling of the second SAT1 flipper. The primer pair But237/HIS1KO5 was used to confirm the presence of the SAT1 flipper cassette at HIS1, HIS1KO5/HIS1KO6 were used to detect the intact HIS1 gene, and HIS1KO1/HIS1KO4 were used to confirm disruption of the HIS1 alleles. (iii) PCR verification of LEU2 deletion. Lane 1, C. orthopsilosis CP289 wild type (MTLα); lane 2, integration of the SAT1 flipper at the first LEU2 allele; lane 3, recycling of the SAT1 flipper; lane 4, integration of the SAT1 flipper at the second LEU2 allele; lane 5, recycling of the second SAT1 flipper. Primer pair But237/LEU2KO9 was used to confirm the presence of the SAT1 flipper cassette at LEU2, LEU2KO9/LEU2KO6 were used to detect the intact LEU2 gene, and LEU2KO1/LEUKO4 were used to confirm disruption of the LEU2 alleles. (B) Opaque and white cells of C. albicans RBY1118 (MTLa/a leu2 his1) and C. albicans RBY1180 (MTLα/α arg4) were grown on SpiderM medium (at 25°C), and mating products were selected on SC-his-leu-arg. C. orthopsilosis LHH1 (MTLa/a his1) and LHL1 (MTLα/α leu2) isolates were also grown on SpiderM medium at 25°C. No mating products were detected on selection medium (SC-his-leu).

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 orthologs (16). The adjacent repeating X-Ala sequences are predicted to be removed by orthologs of Ste13 (31), leaving the mature peptide (indicated with a solid black line). The peptide from C. tropicalis is likely to be longer than those from the other species (indicated with a broken line). The C. metapsilosis sequence is not complete and was obtained from a sequence survey. C. par, C. parapsilosis; C. ortho, C. orthopsilosis; C. meta, C. metapsilosis; C. alb, C. albicans; C. dub, C. dubliniensis; C. trop, C. tropicalis.

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 (bar1Δ) (B) were grown in SpiderM medium at 25°C in the presence or absence of synthetic alpha factor from C. parapsilosis (CpAlpha) or C. albicans (CaAlpha) (10 μg ml⁻¹) or in the presence of DMSO. Expression levels of orthologous STE2, STE4, and CPH1 genes were determined using quantitative RT-PCR. Expression is shown relative to that of the species-specific ACT1 gene. The graphs show the expression levels relative to ACT1 for two biological replicates (C. parapsilosis and C. orthopsilosis) or three biological replicates (C. albicans) plus the standard deviations. (C) Shmoo formation is induced in opaque C. albicans CA12 (sst2Δ) and RBY1220 (bar1Δ) cells grown the presence of α-factor (10 μg ml⁻¹) from C. albicans or C. parapsilosis, but not by DMSO. Cultures were grown in SpiderM medium for 6 h with shaking at 25°C. Bar, 2 μm.

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) and cDNA (C lanes) as templates. Controls without reverse transcriptase were also used (N). The sizes of the predicted spliced and unspliced products are indicated below the gel. Lane M, 1-kb ladder (Promega).