Ultrastructural morphology of intermediate forms and forms suggestive of conjugation in the life cycle of Pneumocystis carinii

The intrapulmonary life cycle of Pneumocystis carinii has been proposed to include asexual reproduction of haploid trophic forms and sexual reproduction to form cysts or ascospores containing 8 intracystic bodies or spores. This study presents the ultrastructural morphology of trophic forms and developing ascospores. Nuclear-associated organelles (NAO) were observed in these various forms and in a binucleated trophozoite. The unique configuration of the nuclei and NAO in this binucleated trophic form were similar to ultrastructural features observed in conjugation of Saccharomyces cerevisiae. NAOs in developing ascospores were present in pairs and were associated with multiple nuclei. In more mature ascospores, NAO were associated with individual spores. The intermediate forms described in this study support an asexual and sexual reproductive process in P. carinii.