Our lab is interested in the process of chromosome segregation and how defects in this process can affect the development of a multicellular organism. Over the past few years we have focused on the meiotic divisions that produce haploid gametes. We have been studying a class of temperature-sensitive (ts) embryonic lethal mutants from C. elegans that arrest in metaphase of meiosis I. In wildtype animals, oocytes in prophase of meiosis I are fertilized by sperm. Following fertilization, the oocyte chromosomes undergo two meiotic divisions, discarding the extra chromosomes in the polar bodies. These first meiotic divisions are important as any errors in chromosome segregation at this stage can lead to embryos with an abnormal number of chromosomes, which would likely lead to lethality. In our mutants, the oocyte chromosomes arrest in metaphase of meiosis I and never separate their chromosome homologs and never extrude polar bodies. In order to molecularly identify the genes required for the first meiotic division, we have mapped our mutants and sequenced candidate genes. Five of the six genes have now been identified and they encode subunits of the Anaphase Promoting Complex or Cyclosome (APC/C). This complex serves as an E3 ubiquitin ligase that targets proteins for destruction (by the 26S proteosome) during the metaphase to anaphase transition of the cell cycle. We have named our mutants """"""""mat"""""""" for their defects in the metaphase to anaphase transition during meiosis I. These ts mutants also display defects in spermatocyte meiosis; primary spermatocytes arrest in metaphase of meiosis I with a normal meiotic spindle, yet fail to separate chromosome homologs. Thus, these mutants disrupt meiosis in both oocytes and spermatocytes. To address the role of the mat genes in mitosis, we have performed shift-up experiments during embryogenesis and larval development. Temperature shift-up experiments during embryogenesis do not result in embryonic phenotypes, however, somatic defects in the gonad, vulva, and male tail are apparent in adults. This observation suggests that mitotic divisions in the soma are affected by the mat mutants. For many of the alleles, these shift-up experiments also result in sterility, suggesting mitotic defects in germline proliferation. We have constructed double mutants of many of these APC/C mutants and have found that many combinations are synthetic lethal at the permissive temperature. We are currently using some of the double mutants that are still viable at the permissive temperature to re-examine the role of APC/C during the mitotic cell cycles during embryogenesis and larval development. Specifically, we hope to observe more severe mitotic defects during the development of the germline and various tissues. To further understand the role of these mat genes during development, we are characterizing their expression patterns using antibody staining and GFP transgenic lines. We have also completed a genetic suppression screen to identify extragenic regulators or substrates of these APC/C subunits. The majority of our 29 suppressor mutations are dominant. We anticipate finding novel molecules that shed light on how APC/C functions and is regulated in different tissues and at different times during the development of a multicellular organism. We also want to examine the composition of the APC/C in C. elegans. For these studies, we will generate transgenic lines expressing epitope-tagged APC/C subunits and then purify the complex with epitope-specific antibodies. These proteins that we purify will be subjected to mass spectrometry to identify the components of the APC/C. We will then examine whether this complex varies during development. In a separate study, we are examining the function of the C. elegans Myt1 ortholog. Myt1 belongs to the Wee1 family of kinases and is thought to down regulate Cdk1 during the cell cycle. RNAi studies with the Myt1 ortholog, wee-1.3, result in sterility. Mothers injected with dsRNA quickly become sterile; the oocyte chromosomes are no longer paused in diakinesis of meiosis I. We propose that WEE-1.3 normally functions to keep maternal CDK-1 inactive during oogenesis, and that upon fertilization, CDK-1 becomes activated to allow for the meiotic and mitotic divisions of the embryo. We are further characterizing this phenotype and plan to use RNAi screens to identify other components of this pathway.
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