Pondering Mating: Pneumocystis jirovecii, the Human Lung Pathogen, Selfs without Mating Type Switching, in Contrast to Its Close Relative Schizosaccharomyces pombe

Almeida et al. (1) have interrogated the genomes of two Pneumocystis species, Pneumocystis jirovecii and its sister Pneumocystis carinii, for genes known to be involved in sexual reproduction in the widely studied fission yeast Schizosaccharomyces pombe, with the hope that defining genetic pathways governing sexual reproduction in Pneumocystis will inform disease prevention strategies. Pneumocystis spp. cause host-specific lung infections in mammals, and sexual reproductive propagules appear to be the infectious stage of the life cycle (2). P. jirovecii, a genetically intractable obligate human pathogen, causes pneumonia in immunosuppressed individuals, with an estimated 400,000 life-threatening infections reported annually worldwide and a mortality rate of up to 80% (3). P. carinii inhabits the lungs of rats (4). P. carinii and relatives were long thought to be protozoan parasites until molecular phylogenetic analysis (1988) clearly placed them within the ascomycetes (5, 6), together with baker’s yeast (Saccharomyces cerevisiae), the human pathogen Candida albicans (in the subphylum Saccharomycotina), the human pathogen Coccidioides immitis, pricey European truffles (Tuber spp.) and morels (Morchella spp.), and familiar contemporary genetic models, such as the saprobes Neurospora spp. and the destructive cereal pathogens Cochliobolus heterostrophus and Fusarium graminearum (all Pezizomycotina). Although in the same phylum, Pneumocystis is only distantly related to these other fungi. In fact, it is associated with a diverse group of ancient lineages at the base of the ascomycete phylogenetic tree collectively known as the Taphrinomycotina (7, 8). The Taphrinomycotina include, in addition to the Pneumocystis mammalian pathogens, Taphrina deformans, a dimorphic plant pathogen that causes leaf curl disease of peach, and S. pombe, used in the fermentation of millet beer and a genetic model second only to S. cerevisiae (9, 10). Molecular requirements for S. pombe sexual reproduction were elucidated more than 25 years ago (11). Unlike S. pombe and T. deformans, Pneumocystis species are obligate pathogens and thus cannot be cultured. This element complicates the study of Pneumocystis biology, including its possible sexual cycle, and is challenging from a clinical perspective, because sex is thought to play a crucial role in the survival of Pneumocystis. Only the cysts, which are considered to be asci containing the sexual spores, are infectious and able to spread to new hosts (2). Despite the crucial potential importance of sex to the epidemiology of Pneumocystis pneumonia, little is known about molecular mechanisms associated with this developmental pathway in Pneumocystis. Earlier studies hinting at a sexual lifestyle include a report on the possible observation of synaptonemal complexes (12), a report identifying conserved mating and meiotic genes that are functional when heterologously expressed in S. pombe mutants (13), and evidence that the meiotic recombinase Dmc1 is expressed in cysts (14). The study by Almeida et al. (1) offers significant insight into the mechanism by which sexual reproduction might occur in Pneumocystis. Almeida et al. (1) queried genome sequences of P. jirovecii, P. carinii, and their relative T. deformans with genes known to be involved in sexual reproduction in S. pombe and identified candidate homologs. Mating in S. pombe is controlled by the single mating type locus mat1 and is successful when strains of opposite mating type, designated P and M, pair. P and M cells differ in gene content at mat1 (15, 16). Furthermore, as with the budding yeast, S. cerevisiae (17, 18), S. pombe has, in addition to the active mat1 mating type locus, two linked but silent mating type loci, one containing the P and the other the M gene content. By programmed interconversion, one of the silent copies can change places with the active copy at the mat1 locus, leading to “switching” of cell type. Thus, homothallism in both yeasts refers to a change in mating type in some of the cells within a culture of a formerly uniform mating type, followed by mating of “switched” cells with “unswitched” cells within the culture, culminating in the production of sexual spores. This type of homothallism with mating type switching has not been described in Pezizomycotina to date. Given that Pneumocystis is related to S. pombe, one might expect these fungi to have similar mating systems, but this is not what Almeida et al. (1) found. Instead, they detected a single mating type locus in the two Pneumocystis species, one or two loci in T. deformans (short contig sequences make linkage uncertain), and no silent loci (Fig. 1). This configuration indicates that these fungi are unable to switch mating type using an S. pombe-type mechanism. Also, the Pneumocystis and Taphrina mating type loci contain both P and M mating type genes, an arrangement, denoted as primary homothallism, known to enable selfing in Pezizomycotina. Where it has been examined carefully in Pezizomycotina, all instances of primary homothallism arose from a genetic recombination event (and loss in some cases) between heterothallic relatives. Evidence for primary homothallism is new to the Taphrinomycotina, but homothallism was inferred previously in population genetics studies which demonstrated widespread clonality in P. jirovecii (19, 20). As noted above, primary homothallism has been