The spindle assembly checkpoint (SAC) is a universal mechanism which arrests mitosis if a bipolar spindle cannot be formed. Although higher eukaryotic cells that are unable to satisfy the SAC eventually die, such a mechanism would not confer an obvious advantage to filamentous fungi encountering environmental conditions that interfere with spindle formation. Following the status of nuclear pore complex and SAC proteins we show that Aspergillus nidulans cells treated with the spindle poison benomyl activate the SAC and arrest in mitosis. However after a defined period of time, the SAC is actively turned off and cells exit mitosis without chromosomal segregation. Most remarkably we find that cells which have undergone one such failed mitosis surprisingly transit interphase and enter a second mitosis in which the SAC is re-activated before again being inactivated. This cyclic activation then inactivation of the SAC can occur for at least three cell cycles in which the nucleus completes all aspects of mitosis except those depending on spindle function. Further, following one or more cell cycles without spindle function, if cells are allowed to reform microtubules they assemble spindles upon mitotic entry and can undergo successful mitosis. Therefore, we propose that inactivation of the SAC allows filamentous fungi to continue growth under environmental conditions which prevent spindle formation and then periodically test the environment for conditions which are compatible with mitosis. We conclude that the SAC can be silenced in a cyclic regulated manner independent of spindle formation.
Full conference title:
10th EUROPEAN CONFERENCE ON FUNGAL GENETICS
- ECFG 10th (2010)