Nuclear events during conjugation in the poorly studied model ciliate Paramecium jenningsi

Ciliated protists are highly differentiated unicellular eukaryotes that possess special sexual processes (conjugation and autogamy) that rely on their unique nuclear dimorphism, i.e., the presence of both a germline micronucleus (MIC) and a somatic macronucleus (MAC) in a single cell. The sexual processes show a high diversity in different ciliates. To better understand the differentiation and evolution of sexual processes in closely related species, we investigated the nuclear events during conjugation in Paramecium jenningsi strain GZ, a poorly studied close relative of the well-known P. aurelia complex. The main results include: (1) the conjugation process takes about 48 ​h, including three prezygotic divisions (meiosis I, II, and mitosis) and three postzygotic divisions; (2) the MICs are dominated by the “parachute” phase at the prophase of meiosis I; (3) after meiosis II, a variable number of nuclei undergo the third prezygotic division, and the two products near the paroral cone become the genetically identical migratory and stationary pronuclei; (4) the synkaryon divides three times to form the MIC and the MAC anlagen, but only one nuclear product of the first postzygotic division completes the next two divisions; and (5) an extra cell division is required to complete the last step of conjugation, during which two MIC anlagen undergo mitotic division while two MAC anlagen are distributed between the daughter cells without division. The comparison of the nuclear events during conjugation in representatives of the class Oligohymenophorea reveals that usually there are three prezygotic divisions and a variable number of postzygotic divisions. However, the number of selected and differentiated nuclei after each division differs among species. This study provided a new model ciliate for further investigations of nuclear selection and differentiation as well as nuclear morphology during meiosis.