Analysis of the effects of the cdaK1 mutation of Tetrahymena thermophila on the morphogenesis of the fission line

Abstract

Summary During divisional morphogenesis of a polarized ciliate cell, a single pattern of distribution of cortical structures along the main body axis is gradually transformed into a metameric chain made up of two sets of cortical structures belonging to the prospective anterior and posterior daughter cells. In an advanced stage of division in wild type Tetrahymena thermophila and in the cdaK1 mutant strain grown at the permissive temperature, a fission line boundary appeared in the form of equatorial gaps in the ciliary rows. An epitope of one of the major components of the membrane skeleton (epiplasm) transiently diminished posterior to this fission line and in the vicinities of the anterior and new posterior oral apparatuses. The changes in the distribution of this epitope depended upon the cell cycle. At a restrictive temperature (37°C), cdaK1 mutant cells displayed abnormal phenotypes in a late stage of cell division in cells delayed in cytokinesis. A localized decrease of immunostaining of epiplasm first appeared on the ventral surface of dividing cell along with fission line gaps at the normal central position, with a lengthening of the ventral cortex between oral apparatuses, but was delayed at some other neighbouring longitudes. Fission line gaps appeared at remaining lateral and dorsal longitudes but on anterior latitudes. During cytokinesis, the ventral cortex bulged and formed a ventral protrusion of the prospective anterior daughter cell. This resulted in an anterior translation of the final position of the fission line. An inhibitor of the serine/threonine kinase activity, 6-DMAP, applied to cdaK1 cells at a permissive temperature, yielded some dividing cells exhibiting cdaK1 phenotypes, with similar patterns of decrease of epiplasmic epitope, localized budding off a part of the cortex, and anterior displacement of the fission line. This suggests, a 6-DMAP and temperature-sensitive cell cycle-dependent coupling of patterned growth and subdivision of cell cortex with spatial distribution of the epiplasmic epitope of Tetrahymena. In cdaK1 cells, the post-telophase migration of micronuclei adjusted to the modified anterior position of the defective fission line. This process was arrested by nocodazole but was only indirectly affected by cytochalasin B.