Cytoplasmic dynein is required to oppose the force that moves nuclei towards the hyphal tip in the filamentous ascomycete Ashbya gossypii

We have followed the migration of GFP-labelled nuclei in multinucleate hyph ae of Ashbya gossypii. For the first time we could demonstrate that the mod e of long range nuclear migration consists of oscillatory movements of nucl ei with, on average, higher amplitudes in the direction of the growing tip. We could also show that mitotic division proceeds at a constant rate of 0. 64 mum/minute which differs from the biphasic kinetics described for the ye ast Saccharomyces cerevisiae, Furthermore we were able to identify the micr otubule-based motor dynein as a key element in the control of long range nu clear migration. For other filamentous fungi it had already been demonstrat ed that inactivating mutations in dynein led to severe problems in nuclear migration, i.e. generation of long nuclei-free hyphal tips and clusters of nuclei throughout the hyphae, This phenotype supported the view that dynein is important for the movement of nuclei towards the tip. In A. gossypii th e opposite seems to be the case. A complete deletion of the dynein heavy ch ain gene leads to nuclear clusters exclusively at the hyphal tips and to an essentially nucleus-free network of hyphal tubes and branches, Anucleate h yphae and branches in the vicinity of nuclear clusters show actin cables an d polarized actin patches, as well as microtubules. The slow growth of this dynein null mutant could be completely reverted to wild-type-like growth i n the presence of benomyl, which can be explained by the observed redistrib ution of nuclei in the hyphal network.