Cortical control of macronuclear positioning in the ciliate Stentor

Abstract

AbstractIn the ciliate Stentor, the chain macronucleus has a fixed position within the cell; it is located just beneath the region of narrow cortical striping to the right of the gullet (nuclear site). Stentor therefore lends itself well to experiments on control of nuclear positioning. I have studied this problem by microsurgically producing various abnormalities of nuclear positioning and determining how these are corrected. Breaks in the nuclear chain were repaired by segregation of broken parts into the two daughter cells at division. When a second nucleus was transplanted into a stentor, the abnormality was eventually corrected because the two parallel chains tended to move during oral development (regeneration or division) so that one was anterior to the other. The two nuclei were then segregated into the anterior and posterior daughter cells at division. When a doublet stentor, which has two nuclear sites and two macronuclei, was deprived of one nucleus, it acquired a second one because the remaining nucleus was attracted to both nuclear sites when it elongated during oral development. This phenomenon first produced doublets with a single nuclear chain sharing the two nuclear sites at opposite ends of the cell, a break in the chain then presumably created two nuclei. These latter observations indicate that one mechanism by which the nucleus is positioned involves its attraction toward the nuclear site, a differentiated region of cell cortex, during oral development. Other experiments have shown that the nuclear site also holds the nucleus firmly in place.