contribution of noncentrosomal microtubules to spindle assembly in drosophila spermatocytes的贡献noncentrosomal微管在果蝇精母细胞纺锤体组装.pdf

PLoS BIOLOGY Contribution of Noncentrosomal Microtubules to Spindle Assembly in Drosophila Spermatocytes ´ * Elena Rebollo, Salud Llamazares, Jose Reina, Cayetano Gonzalez Cell Biology and Biophysics Programme, European Molecular Biology Laboratory, Heidelberg, Germany Previous data suggested that anastral spindles, morphologically similar to those found in oocytes, can assemble in a centrosome-independent manner in cells that contain centrosomes. It is assumed that the microtubules that build these acentrosomal spindles originate over the chromatin. However, the actual processes of centrosome-independent microtubule nucleation, polymerisation, and sorting have not been documented in centrosome-containing cells. We have identified two experimental conditions in which centrosomes are kept close to the plasma membrane, away from the nuclear region, throughout meiosis I in Drosophila spermatocytes. Time-lapse confocal microscopy of these cells labelled with fluorescent chimeras reveals centrosome-independent microtubule nucleation, growth, and sorting into a bipolar spindle array over the nuclear region, away from the asters. The onset of noncentrosomal microtubule nucleation is significantly delayed with respect to nuclear envelope breakdown and coincides with the end of chromosome condensation. It takes place in foci that are close to the membranes that ensheath the nuclear region, not over the condensed chromosomes. Metaphase plates are formed in these spindles, and, in a fraction of them, some degree of polewards chromosome segregation takes place. In these cells that contain both membrane-bound asters and an anastral spindle, the orientation