Chromosome dynamics during the G2/M transition in meiosis

During the production of sperm and eggs, cells have to half the number of chromosomes, otherwise the resulting foetus will contain too many. This requires a specialized cell division where chromosomes first find their partner and then split from them- a marriage followed by divorce. How do cells ensure that each egg or sperm contains exactly one full set of chromosomes, 23 in the case of humans? Chromosomes pair on the basis of sequence identity and form specialized connections called chiasmata that look like the Greek letter chi under the microscope. Pairing and the formation of chiasmata is facilitated by a structure called the synaptonemal complex; when chiasmata form the synaptonemal complex is broken down and cells get ready to split up the chromosome pairs. We are investigating how the breakdown of the synaptonemal complex is facilitated and how is it coordinated with formation of chiasmata. To this end, we use baker‘s yeast as a model organism. Baker‘s yeast has provided valuable insight into how chiasmata and the synaptonemal complex are formed and we expect that some of the basic principles that govern its breakdown will also be relevant in humans.

The synaptonemal complex is a tripartite proteinaceous structure that connects homologous chromosomes lengthwise during midprophase/pachytene. Its disassembly is the first, dramatic hallmark of the progression of cells to the first meiotic nuclear division. At this time, crossovers are observed in molecular assays and chiasmata become cytologically distinguishable. Monopolin loading has also taken place, facilitating that sister kinetochores co-orient. All of these events promote homologous chromosomes to segregate to opposite poles during the first meiotic division, thereby avoiding aneuploidy formation in the gametes. We propose to determine how these events coordinated with cell cycle progression. We have recently demonstrated that two highly conserved kinases, Ipl1/Aurora kinase and Cdc5/Polo-like kinase, promote SC disassembly. In ipl1Aurora mutants, SCs are retained in 80% of cells that have transited to diplotene or metaphase I. However, crossover completion and cell cycle progression occur normally, suggesting that Ipl1Aurora links SC disassembly to other events occurring upon pachytene exit. The SC disassembly defect is different in cdc5Polo mutants: numerous, distinct, SC foci are retained at post-pachytene stages. These observations suggest that SCs may not be homogenous structures and that at least two mechanisms promote disassembly. In this programme of work, we propose to study SC disassembly and the integration with crossover formation in the model organism budding yeast. Budding yeast has proved a very good model for understanding crossing over and the SC in higher organisms, including mammals. We have designed cytological, molecular, and proteomic experiments to assess how the SC disassembles, where in the genome Zip1 is retained in the ipl1Aurora and cdc5Polo mutants and how Ipl1Aurora might be regulated. Furthermore, we have developed protein kinase assays that should allow us to identify Ipl1Aurora and Cdc5Polo-specific phosphorylation events, with a view to understand the mechanistic regulation of SC disassembly and crossover formation. We have set up relevant collaborations for the various techniques with world leaders in their fields in order to maximize the probability of success. We envision that our studies will provide valuable insights into a basic biological phenomenon in addition to generating a good model system for understanding meiotic regulation in humans.