Molecular and phenotypic description of Pneumocystis wakefieldiae sp. nov., a new species in rats

Organisms in the genus Pneumocystis are fungi that reside in the lungs of mammals that can cause a lethal pneumonia once the hosts lose immune function. The genus Pneumocystis contains many members, but only two species have been described formally to date, P. carinii, the type species found in rats, and P. jirovecii, resident in human beings. Rats have been shown to harbor another organism in addition to P. carinii, Pneumocystis wakefieldiae sp. nov., formerly known as Pneumocystis carinii f. sp. ratti, which is described here. Although often found together and morphologically similar, P. carinii and P. wakefieldiae are phenotypically and genetically divergent. We used the phylogenetic species recognition approach to distinguish these organisms as two distinct species and estimated the evolutionary time of their separation. Nucleotide sequence comparisons of seven homologous genes showed 4-7% divergence between the P. wakefieldiae and P. carinii sequences, which was in contrast to the 0-0.8% divergence observed within P. carinii species. Even greater divergence (30%) occurred in sequences located between genes. The MSG (major surface glycoprotein) gene families of P. carinii and P. wakefieldiae are 35% divergent from one another and differ with respect to sequence elements associated with regulation of their transcription. Differences in reactivity of monoclonal antibodies and polyclonal antisera reflected these genetically distinct surface antigens. Karyotypic analysis of P. wakefieldiae produced a single profile that was distinct from all 12 profiles known for P. carinii. Eight homologous genes were localized to chromosomes of different sizes in the two species. The cumulative genotypic and phenotypic data support a species distinction between these two organisms.