Edinburgh Napier University

  Meiosis is a specialised cell division that results in the formation of four genetically unique haploid daughter cells from one diploid parent cell. This is achieved by one round of DNA replication followed by two rounds of nuclear division. This is in contrast to mitosis, which produces genetically identical diploid daughter cells.
Errors during meiosis can result in aneupolidy. In humans, aneuploidy can cause miscarriage and disease such as Patau, Edwards and Down syndromes (trisomies 13, 18 and 21, respectively). Therefore, understanding how meiosis is regulated is of great importance in understanding the causes of aneuploidy and disease in humans.
In this thesis, I have used the model organism budding yeast to study how meiosis is regulated in yeast cells, with a view to understanding how meiosis is regulated in human cells. PP2ACdc55 is a highly conserved phosphatase and its role in meiosis was not addressed in any organism before my work. I constructed a meiotic null allele of CDC55 (cdc55-mn) by replacing its promoter with the mitosis-specific PCLB2. By carefully characterising the phenotype of cdc55-mn strains, I showed PP2ACdc55 is crucial for timing the activation of the FEAR network. I have demonstrated that premature activation of FEAR during meiosis (caused by a lack of PP2ACdc55 activity) blocks spindle assembly and nuclear divisions. In cdc55 meiotic null (cdc55-mn) cells, the Cdk-counteracting phosphatase Cdc14 is prematurely released from the nucleolus concomitant with hyperphosphorylation of Net1. I have found that a mutant form of Net1 that lacks 6 of the Cdk phosphorylation sites rescues the meiotic null defect of cdc55-mn cells. Therefore, I have shown that phosphoregulation of Net1 by PP2ACdc55 is essential in order to prevent precocious exit from meiosis I.

In my work described in Chapter 4, I isolated mutant alleles of cdc55 that suppressed the spo12Δ dyad phenotype confirming the opposing roles of Net1-phosphorylation by Cdc55 and Spo12 in the FEAR pathway. I also isolated alleles of CDC55 that suppressed the spo11Δ spo12Δ spore lethality. These alleles affected reductional segregation during meiosis I in achiasmate cells but had no effect in wild type cells. Investigating these alleles further might shed insights into mechanisms that work with chiasmata in ensuring efficient monopolar attachment during meiosis I.