Elastic mitotic tethers remain functional after microtubules are inhibited

Abstract

During normal anaphase in animal cells, elastic tethers connect partner telomeres of segregating chromosomes and exert backward (anti-poleward) forces on those chromosomes. The experiments reported herein test whether microtubules need to be present in order for tethers to produce backwards forces. We disassembled spindle microtubules by treating anaphase crane-fly primary spermatocytes separately with nocodazole, colcemid, or podophyllotoxin. The drug treatments caused anaphase chromosomes to stop moving poleward; almost immediately thereafter they moved backward. The characteristics of the backward movements of the half-bivalents match those of the backwards movements of arm fragments formed by cutting chromosome arms during anaphase - for example the occurrence and lengths of backward movements were a function of tether length. The only difference from movement of arm fragments is that the chromosomes in the treated cells moved backwards slower than arm fragments did. Immunofluorescence of spindle tubulin after the drug treatments indicated that acetylated kinetochore microtubules were not depolymerized by the drugs, though the non-kinetochore spindle microtubules were depolymerized. Our data indicate that tethers move anaphase chromosomes backwards in the absence of functioning spindle microtubules. We suggest that the backward movements that take place when poleward forces are absent are due to tethers, and that the backward movements are slowed by the presence of acetylated kinetochore microtubules.