DROSOPHILA POLO KINASE IS REQUIRED FOR CYTOKINESIS

A number of lines of evidence point to a predominance of cytokinesis d efects in spermatogenesis in hypomorphic alleles of the Drosophila pol o gene. In the pre-meiotic mitoses, cytokinesis defects result in cyst s of primary spermatocytes with reduced numbers of cells that can cont ain multiple centrosomes. These are connected by a correspondingly red uced number of ring canals, structures formed by the stabilization of the cleavage furrow. The earliest defects during the meiotic divisions are a failure to form the correct mid-zone and mid-body structures at telophase. This is accompanied by a failure to correctly localize the Pavarotti kinesin-like protein that functions in cytokinesis, and of the septin Peanut and of actin to be incorporated into a contractile r ing. In spite of these defects, cyclin B is degraded and the cells exi t M phase. The resulting spermatids are frequently binuclear or tetran uclear, in which case they develop either two or four axonemes, respec tively. A significant proportion of spermatids in which cytokinesis ha s failed may also show the segregation defects previously ascribed to polo(1) mutants. We discuss these findings in respect to conserved fun ctions for the Polo-like kinases in regulating progression through M p hase, including the earliest events of cytokinesis.