Colchicine induces nuclear death in <i>Tetrahymena thermophila</i>

We have induced the resorption of the micronucleus in living conjugants of Tetrahymena. After pairing, but prior to meiosis, the micronucleus undergoes an extraordinary elongation, expanding from a sphere of about 1‐μm diameter to a ribbon‐shaped structure whose length is greater than that of the cell. Previous EM studies showed an organized basket of microtubules subjacent to the nuclear envelope in the elongating micronucleus. We exposed early pairs to colchicines (1–5 mM), a microtubule inhibitor, and found that micronuclear elongation was prevented. Further, colchicine caused the collapse of already elongated micronuclei. Unexpectedly, with increased time of exposure to colchicine, micronuclei gradually disappeared from view; by 5 h ∼60% of cells in pairs lost their micronuclei. Direct observation ruled out ejection from the cell. We present evidence that microcnuclei are degraded by the lysosmally mediated process of autophagy. Degrading micronuclei in living pairs exposed to both Hoescht 33342 and Acridine Orange stain yellow—blue (for DNA) and orange (for acidification)—as expected for an autophagic vacuole. Further, 3‐methyladenine, an inhibitor of autophagy, blocked acidif