Nuclear behavior in triple-conjugant fusion complexes of the ciliate Stylonychia pustulata: Inhibition of meiosis and retention of the macronucleus

Abstract

The relationship between temporary conjugation and the conjugant fusion of the hypotrich ciliate Stylonychia pustulata was examined by use of singlet cells of stocks HH1 and TK1, and back-to-back doublet cells of stock NM2 with two attachment sites. The TK1 cells caused conjugant fusion in cell pairing. Triple-conjugant fusion (TCF) complexes composed of an HH1 cell, an NM2 doublet and a TK1 cell were obtained by mixing cells from three stocks. Multiple-conjugant fusion complexes composed of a TK1 cell and three or four NM2 doublets were also found. Initiation of meiosis in TCF complexes was not disturbed by the union of a TK1 cell and a component of the doublet member, but meiosis was blocked at the parachute stage. Thereafter, many micronuclei underwent mitosis. These results suggest that a meiosis blocking factor is present in the cytoplasm of the TK1 cell and migrates to both the doublet and the HH1 members. The macronuclei in doublet and HH1 members changed from elongated and fragmented shapes to spheres. The HH1 and doublet members shifted from conjugation to conjugant fusion. The doublet and HH1 members split from TCF complexes within an hour of the onset of pairing underwent either autogamy or cell division.