We use Drosophila oogenesis as a model to explore the developmental regulation of the cell cycle. In Drosophila, the oocyte develops within the context of a 16-cell germline cyst. Individual cells within the cyst are referred to as cystocytes and are connected by actin-rich ring canals. While all 16 cystocytes enter premeiotic S phase, only a single cell remains in the meiotic cycle and becomes the oocyte. The other 15 cells enter the endocycle and develop as highly polyploid nurse cells. Currently, we are working to understand how cells within ovarian cyst enter and maintain either the meiotic cycle or the endocycle. In addition, we are examining how this cell cycle choice drives the development of the mature egg.? ? The endocycle is a commonly observed variant cell cycle in which cells undergo repeated rounds of DNA replication with no intervening mitosis. Cells that are highly metabolically active, such as the giant trophoblast of the mammalian placenta and the Drosophila ovarian nurse cells, often grow via endoreplication. How the cell cycle machinery is modified to transform a mitotic cycle into endocycle has long been a question of interest. In both plants and animals, the transition from the mitotic cycle to the endocycle requires Fzr/Cdh1, a positive regulator of the Anaphase-Promoting Complex/Cyclosome (APC/C). However, because many of its targets are transcriptionally downregulated upon entry into the endocycle, it remained unclear whether the APC/C functioned beyond the mitotic/endocycle boundary. We have shown that APC/CFzr/Cdh1 activity is required to promote the G/S oscillation of the Drosophila endocycle. We find that compromising APC/C activity, after cells have entered the endocycle, inhibits DNA replication and results in the accumulation of multiple APC/C targets including the mitotic Cyclins and Geminin. Notably our data suggest that the activity of APC/CFzr/Cdh1 during the endocycle is not continuous but cyclic, as demonstrated by the APC/C-dependent oscillation of the pre-replication complex component ORC1. Taken together our data suggest a model in which the cyclic activity of APC/CFzr/Cdh1 during the Drosophila endocycle is driven by the periodic inhibition of Fzr/Cdh1 by Cyclin E/Cdk2. Thus, we propose that as is observed in mitotic cycles, during endocycles APC/CFzr/Cdh1 functions to reduce the levels of the mitotic Cyclins and Geminin in order to facilitate the relicensing of DNA replication origins and cell cycle progression. These studies will allow new models on the minimum cell cycle inputs necessary to construct a G/S oscillator to be formulated and tested.? ? In Drosophila, the oscillations of Cyclin E/Cdk2 activity drive the endocycle. How the periodicity of Cyclin E/Cdk2 activity is achieved during endocycles is poorly understood. We have determined that the p21cip1/p27kip1/p57kip2-like Cyclin-dependent kinase inhibitor (CKI) Dacapo (Dap) promotes replication licensing during Drosophila endocycles by reinforcing low Cdk activity during the endocycle Gap phase. In dap mutants, cells in the endocycle exhibit reduced levels of the licensing factor Double-Parked/Cdt1 (Dup/Cdt1) as well as decreased levels of chromatin-bound MCM2-7 complex. In addition, mutations in dup/cdt1 dominantly enhance the dap phenotype in several polyploid cell types. Consistent with their reduced ability to complete genomic replication, dap mutants accumulate increased levels of DNA damage during the endocycle S phase. Our data suggest a model in which Dap inhibits Cyclin E/Cdk2 activity during the Gap phase and thus promotes the efficient licensing of DNA replication origins. Intriguingly, we find that Dap is absolutely required for DNA replication licensing and genomic stability during premeiotic S phase of the oocyte.? ? The pathways that control entry into the meiotic cycle and early meiotic progression are poorly understood in metazoans. We previously identified a gene, missing oocyte (mio) that regulates nuclear architecture and meiotic progression in early ovarian cysts. In mio mutants, the oocyte enters the meiotic cycle and progresses to pachytene. However, this meiotic state is not maintained and ultimately the oocyte withdrawals from meiosis, enters the endocycle and becomes polyploid. mio mutants display some of the earliest meiotic defects reported in Drosophila. Moreover, the Mio protein accumulates in the oocyte nucleus in early prophase of meiosis I. Therefore, mio provides an excellent entry point to explore how the unique cell biology of the early ovarian cyst and the establishment of the meiotic program, influence the downstream events of oocyte differentiation and meiotic progression. To better understand the role of mio in oogenesis, we have initiated a series of experiments to identify additionally proteins that function in the Mio pathway. From these studies we have found that Mio physically and genetically interacts with the nucleoporin Nup44A. Moreover, mutations in Nup44A disrupt meiotic progression and female sterility. These observations provide the framework for future studies on how nuclear pore components influence meiotic progression and the maintenance of the oocyte identity.
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