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

Almeida et al. (1) have interrogated the genomes of two Pneu-mocystis species, Pneumocystis jirovecii and its sister Pneumo-cystis carinii, for genes known to be involved in sexual reproduc-tion in thewidely studied fission yeast Schizosaccharomyces pombe, with the hope that defining genetic pathways governing sexual reproduction in Pneumocystis will inform disease prevention strate-gies. Pneumocystis spp. cause host-specific lung infections in mam-mals, and sexual reproductive propagules appear to be the infectious stage of the life cycle (2). P. jirovecii, a genetically intractable obligate humanpathogen, causes pneumonia in immunosuppressed individ-uals, 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. cariniiandrelativeswere long thought tobeprotozoanparasites until molecular phylogenetic analysis (1988) clearly placed them within the ascomycetes (5, 6), together with baker’s yeast (Saccharo