Interactions between mgr, asp, and polo: asp function modulated by polo and needed to maintain the poles of monopolar and bipolar spindles.

We describe genetic interactions between mutations in mgr, asp, and polo, genes required for the correct behaviour of the spindle poles in Drosophila. The phenotype of a polo1 mgr double mutant is more similar to mgr than polo1, but the frequency of circular monopolar figures (CMFs) seen with either mutant alone is additive, suggesting that the two gene products are required for independent functions in the formation of bipolar spindles. The aspE3 mgr double mutant arrests much earlier in development than either mutant alone, indicative of a strong block to cell proliferation. We discuss whether the lack of microtubular structures in these cells reflects an extended mitotic arrest, or if it is a more direct consequence of the double mutant combination. A polo1 aspE3 double mutant shows a dramatic synergistic increase in mitotic frequency. The loss of CMFs normally associated with the polo1phenotype suggests that the Asp microtubule-associated protein is required to maintain the structure of spindle poles. We speculate that Asp protein might be a substrate for the serine-threonine protein kinase encoded by polo.