The somatic function of the micronucleus in sexual reproduction of Paramecium tetraurelia: initiation of oral membranelle assembly

Abstract

In a previous study, cell lines possessing defective micronuclei, generated by laser-microbeam irradiation, gave rise to cells lacking both oral apparatus and micronuclear derivatives after autogamy. It was concluded that astomy arose as a result of degeneration of all of the meiotic products of the micronuclei after meiotic telophase II, instead of leaving one product for subsequent nuclear reorganization. The present study consolidates this conclusion by employing 15 micronucleus-defective cell lines; these were generated by laser-irradiation of the micronucleus, treatment of the cells with cis-dichlorodiamineplatinum (II), and conjugation between diploids and amicronucleates to produce haploids. A good correlation between the presence of pregametic, gametic and zygotic nuclei and the initiation of oral membranelle assembly in stomatogenesis was demonstrated in 17 cases of autogamy. Therefore, postmeiotic micronuclear activities up to the zygotic nucleus stage, in particular in the gametic stage, are crucial for the initiation of oral membranelle assembly, while premeiotic micronuclear activities are insufficient. Micronuclear genic factors are also likely to be involved in the determination of the fate of the meiotic products.