EVIDENCE THAT KINETOCHORE MICROTUBULES IN CRANE-FLY SPERMATOCYTES DISASSEMBLE DURING ANAPHASE PRIMARILY AT THE POLEWARD END

Anaphase chromosome motion involves the disassembly of kinetochore mic rotubules. We wished to determine the site of kinetochore microtubule disassembly during anaphase in crane-fly spermatocytes. In crane-fly s permatocyte spindles, monoclonal antibody 6-11B-1 to acetylated alpha- tubulin labels kinetochore microtubules almost exclusively, with an ar ea immediately adjacent to the kinetochore being weakly or not labelle d. This 'gap' in acetylation at the kinetochore serves as a natural ma rker of kinetochore microtubules in the kinetochore fibre, We measured the length of the gap on kinetochore fibres in metaphase and anaphase in order to deduce the fate of the gap during anaphase; we used this information to determine where kinetochore microtubules disassemble in anaphase. Gap lengths were measured from confocal microscope images o f fixed spermatocytes dual labelled with 6-11B-1 to acetylated alpha-t ubulin and YL1/2 to tyrosinated alpha-tubulin, the latter being used t o determine the positions of kinetochores. In metaphase the average ga p length was 1.7 mu m. In anaphase, the gap appeared to decrease in le ngth abruptly by about 0.4 mu m, after which it decreased in length by about 0.2 mu m for every 1 mu m that the chromosome moved poleward, P acMan models of chromosome movement predict that this 'gap' in stainin g should disappear in anaphase at a rate equal to that of chromosome m ovement. Thus, our results do not support theories of chromosome motio n that require disassembly solely at the kinetochore; rather, in crane -by spermatocytes kinetochore microtubule disassembly in anaphase seem s to take place primarily at the poles.