Mutations in Drosophila Greatwall/Scant Reveal Its Roles in Mitosis and Meiosis and Interdependence with Polo Kinase

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Abstract
Polo is a conserved kinase that coordinates many events of mitosis and meiosis, but how it is regulated remains unclear. Drosophila females having only one wild-type allele of the polo kinase gene and the dominant Scant mutation produce embryos in which one of the centrosomes detaches from the nuclear envelope in late prophase. We show that Scant creates a hyperactive form of Greatwall (Gwl) with altered specificity in vitro, another protein kinase recently implicated in mitotic entry in Drosophila and Xenopus. Excess Gwl activity in embryos causes developmental failure that can be rescued by increasing maternal Polo dosage, indicating that coordination between the two mitotic kinases is crucial for mitotic progression. Revertant alleles of Scant that restore fertility to polo–Scant heterozygous females are recessive alleles or deficiencies of gwl; they show chromatin condensation defects and anaphase bridges in larval neuroblasts. One recessive mutant allele specifically disrupts a Gwl isoform strongly expressed during vitellogenesis. Females hemizygous for this allele are sterile, and their oocytes fail to arrest in metaphase I of meiosis; both homologues and sister chromatids separate on elongated meiotic spindles with little or no segregation. This allelic series of gwl mutants highlights the multiple roles of Gwl in both mitotic and meiotic progression. Our results indicate that Gwl activity antagonizes Polo and thus identify an important regulatory interaction of the cell cycle. Coordination of cell division in development requires a complex interplay between protein kinases, which catalyze the transfer of phosphates to specific substrate proteins to modify their activities. One of these kinases is the conserved Polo, which is the target of anticancer drugs. Using genetics in Drosophila (the fruit fly), we have identified Greatwall, another conserved protein kinase, as an antagonist of Polo. Studies of Scant, a dominant mutation of the greatwall gene, lead us to examine the effects of overexpressing wild-type Greatwall. Too much Greatwall activity relative to Polo leads to developmental defects in early syncytial embryos, which are initiated by the detachment of a single centrosome from the nuclear envelope in prophase. Loss-of-function mutants of greatwall reveal that the kinase is required for proper chromosome structure and segregation in mitosis and meiosis. One of these mutations results in the loss of Greatwall specifically during vitellogenesis (building up the egg's contents) and leads to a failure of meiosis I characterized by the premature loss of sister chromatid cohesion. This study shows that the Greatwall kinase fulfils multiple crucial functions in the different cell cycles of a developing animal and will be the foundation for further investigations.