The role of the chromosomal passenger complex and condensin complex in meiotic chromosome cohesion and segregation

Successful sexual reproduction requires not only the production of haploid gametes, but also the fusion of these cells and reentry of the diploid zygote into the mitotic cell cycle. Coordination of the completion of meiosis and restart of mitosis necessitates modification of the cell cycle in ways that are poorly understood. In many organisms, the early mitotic divisions are also regulated to achieve particular developmental goals. We conducted a screen to identify mutations that disrupt these specialized cell cycles. Using the model system Drosophila melanogaster, we took advantage of the powerful genetic and cytological tools that allow investigation of developmentally regulated cellular events. We identified nineteen mutants that disrupt the processes of completion of meiosis and early embryonic mitosis. These mutations were organized into five classes including mutants that fail in fertilization, pronuclear fusion, embryonic mitosis, or both DNA synthesis and mitosis. One mutation was mapped and molecularly characterized as an allele of an important mitotic regulator, providing strong support for the potential value of mutations characterized through this screen. INTRODUCTION The formation of an embryo from a sperm and an egg relies on a series of carefully regulated cell cycle transitions and non-canonical cell cycles, each with a critical developmental role (for a review see (Foe 1993)). Study of these cell cycle variants not only helps us to understand the mechanisms of embryogenesis, but also provides insight into the ways the cell cycle can be adapted, and therefore reveals much about the fundamental requirements of cell division. Remarkably, in Drosophila many distinct cell cycle programs are coordinated by the single cytoplasm of the egg, supplied with mRNAs and proteins by the female during oogenesis (Zalokar 1976; Edgar and Schubiger 1986). Before ovulation and fertilization, the mature oocyte is arrested in metaphase I of meiosis (first described in Huettner 1924, reviewed in Foe 1993). Upon ovulation, re-entry into meiosis is triggered, homologs separate in anapase I, and then chromosomes proceed through meiosis II without further delay (Mahowald et al. 1983). These meiotic divisions are coordinated by a spindle that is organized by the chromosomes themselves, as the oocyte lacks centrosomes (Theurkauf and Hawley 1992). In addition, Drosophila female meiosis proceeds in the absence of cytokinesis, with all four haploid products remaining in a common cytoplasm. One of them then fuses with the male pronucleus and contributes to the developing embryo, while simultaneously the other three transition through an interphase state and then condense their chromosomes and come together into a characteristic rosette structure. These unused polar bodies are held inactive in this configuration but remain in the common cytoplasm of the embryo through approximately the first ten rounds of mitotic cell cycling, until they are displaced from their position at the cortex and eventually degraded. Very little is understood about the way in which these polar bodies are held inactive, but maintenance of this state is critical so that the meiotic products do not interfere with ongoing embryogenesis. When an egg is fertilized, the sperm enters the oocyte cytoplasm entirely and here it undergoes reprogramming to re-enter the mitotic cell cycle (Foe 1993). The DNA in the sperm head is packaged with sperm-specific protamines, and upon entry into the egg it discards these and replaces them with histones and other chromatin components (Loppin et al. 2005). The product of female meiosis that will become the female pronucleus is brought to the male pronucleus by a large microtubule array, coordinated by the centrosome brought in with the sperm (Huettner 1924). Together, the pronuclei enter the first zygotic mitosis on a spindle organized by a paternally-contributed centrosome. Male and female pronuclei come together to a common metaphase plate, but because the pronuclear envelopes have not completely broken down, they remain as separate masses until telophase, when pronuclear envelopes fuse and chromosomes finally commingle (Stafstrom and Staehelin 1984). The first thirteen mitotic divisions also take place in a common cytoplasm, called a syncytium, and they proceed extremely rapidly, oscillating between DNA synthesis (S) and mitosis (M) without gap phases in between, to allow the embryo to quickly increase its nuclear number (Foe and Alberts 1983). These earliest embryonic nuclear divisions are each completed in approximately nine minutes and therefore must be extremely efficient at transitioning between