* Author for correspondence Summary

The present study further analyses the import-ance of postmeiotic divisional derivatives of the micronucleus in the development of the oral apparatus of Paramecium during sexual repro-duction. Cell lines possessing defective micronu-clei generated by laser microbeam irradiation of the micronucleus were employed. They exhibited anomalies in nuclear reorganization and sto-matogenesis in the sexual cycle. During autogamy, in some cells the micronuclear cycle terminated shortly after meiosis, resulting in the loss of all postmeiotic micronuclear derivatives. Stomatogenesis became arrested at an early stage of assembly of the oral membranelles, but the old oral apparatus was resorbed as usual, leading to the production of astomatous cells at the end of the sexual cycle. Conjugation of these cell lines with normal micronucleates rescued both nucleo-genesis and stomatogenesis in the defective-micronucleate conjugant, primarily as a result of transfer of the male gametic nucleus from the normal conjugant to the defective-micronucleate mate. These observations demonstrate the sto-matogenic significance, in particular in the in-itiation of oral membranelle assembly, of the gametic nuclei during sexual reproduction. The present study also suggests the possibility of micronuclear activities in the early part of the sexual cycle affecting postzygotic nucleogenesis