Nuclear control of cortical reorganization during sexual reproduction of Stylonychia mytilus: A study with...

During conjugation, Stylonychia mytilus undergoes repetitive somatic development by reorganizing the cortical ciliature consecutively for three times. The control of these reorganizations by the nucleus was investigated employing amicronucleates generated by microsurgery. When amicronucleates conjugate, they underwent only the first round of cortical reorganization; the exconjugants perished before the second cortical reorganization commenced. This shows that the micronucleus is dispensable for the first cortical reorganization, but that the micronucleus, or its divisional derivatives in sexual nucleogenesis, are essential for immediate postconjugational survival. Conjugation of amicronucleates with micronucleates in most cases resulted in the arrest of macronuclear anlage development at the post-polyteny DNA-poor stage; the second cortical reorganization could not be initiated in such cells. In others where the macronuclear anlage developed beyond the post-polyteny DNA-poor stage, the cell underwent the second, and then the third round of cortical reorganization. This correlation between macronuclear anlage development and the second cortical reorganization argues for a causal relationship between the two. Very likely, the macronuclear anlage up to the post-polyteny DNA-poor stage provides the crucial morphogenetic signal(s) for the second cortical reorganization. The nature of the first and second rounds of cortical reorganization are thus fundamentally different from each other, as their nuclear controls are radically dissimilar.