Faithful cell division is crucial for all eukaryotic cells to ensure that daughter cells receive thecorrect number of chromosomes. Errors in chromosome segregation during meiosis in germ cells can result in aneuploid gametes, potentially leading to infertility, genetic disorders like Down Syndrome, and miscarriages. Similarly, chromosome segregation errors during mitosis can produce aneuploid somatic cells, which are often associated with cancer. Although the regulation of these two types of cell division is vital, the mechanisms that ensure their accuracy are still actively being studied. Among the proteins identified as key regulators of both mitotic and meiotic fidelity in various species, including Caenorhabditis elegans (C. elegans), is PCH-2. PCH-2 is a conserved AAA+ ATPase required for timely coordination of the meiotic prophase events pairing, synapsis and recombination, but the mechanism for how it accomplishes this coordination is currently unknown. Additionally, PCH-2 has been shown to promote crossover formation and affect their distribution. Here I use C. elegans as a model system to investigate the conserved meiotic role of PCH-2.

In Chapter 1, I begin my investigation of PCH-2’s conserved role by observing it’s localizationto meiotic chromosomes in the mnT12 background, where two chromosomes, X and IV, are fused. In order to fully understand how defects in chromosomal rearrangements can affect PCH-2’s localization, I also studied its localization in heterozygous inversions, mIn1/+ and hIn1/+. I find that PCH-2 localization is extended in the chromosomal fusion background mnT12, but not in mIn1 heterozygotes and hIn1 heterozygotes. I also find that the extension of PCH-2 in mnT12 worms results in the promotion of crossover formation, providing another example of how PCH-2 can affect crossover formation.

In Chapter 2, I demonstrate that PCH-2 regulates the number and distribution of crossoversby opposing their formation. This antagonistic action has distinct effects at various stages of meiotic prophase. I show that in the early stages, PCH-2 restricts the conversion of double strand breaks (DSBs) into crossovers, thereby limiting crossover events at the initial sites of DSB formation and homolog interactions. And then later in meiotic prophase, PCH-2 reduces the number of crossover-eligible intermediates, aiding in the reinforcement and designation of these intermediates, and ultimately ensuring crossover assurance. Additionally, I show that PCH-2 exerts this control through the meiotic HORMAD protein, HIM-3. The data I present strongly support a model where PCH-2's conserved function is to remodel meiotic HORMADs throughout meiotic prophase, destabilizing crossover-eligible precursors, coordinating meiotic recombination with synapsis, and contributing to the progressive execution of meiotic recombination to ensure precise crossover regulation.

Finally in Chapter 3, I test PCH-2’s role on modulating synapsis. Specifically by observinghow the loading and unloading of a central region component of the synaptonemal complex (SC) is affected in the absence of PCH-2. I show that loss of PCH-2 does not affect how this protein – SYP-3, is loading onto chromosomes during meiotic prophase. Added together, my work establishes a crucial framework for understanding the role of PCH-2 in regulating the number and distribution of crossovers (Chapter 1 and Chapter 2), a function I propose is conserved across species. While specific mechanisms may differ among systems (Chapter 3), I suggest that PCH-2 plays a key role in remodeling meiotic HORMADs throughout meiotic prophase to destabilize crossover-eligible precursors. This process coordinates meiotic recombination with synapsis, contributing to the stepwise progression of recombination and ensuring crossover assurance, interference, and homeostasis.