Dynamics of Meiotic Chromosomes and Structures

Sexually-reproducing organisms generate haploid sex cells by a specialized cell division, meiosis, in order to reduce the chromosome number in the gametes. Accurate reduction of the chromosome number relies on a series of dynamic chromosomal events in early meiosis I that culminate in the alignment of paired homologous chromosomes so that accurate segregation into the daughter cells can occur. In budding yeast and many other organisms, accurate pairing is dependent on
the initiation of homologous recombination, the intentional infliction of double-strand breaks onto the chromosomes and their resolution as recombination events. Accurate homology matches are reinforced by the assembly of the synaptonemal complex (SC), which forms between paired homologs and maintains their tight association.

In this thesis, I aimed to characterize the early prophase events that facilitate homolog pairing and pairing reinforcement. I investigated two nonhomologous pairing processes that occur in yeast for their potential role in homolog pairing. One of these processes, bouquet formation, is defined by a clustering of chromosomes,
tethered by their telomeres, to a subregion of the nuclear envelope. The other, centromere coupling, is characterized by rapid, homology-independent associations between chromosomes at their centromeres. I created a strain in which both of these processes were abolished and observed the effect on homolog pairing. In this strain,
homolog pairing is still achieved, albeit at a delayed rate compared to a wild-type strain. We conclude that these processes are not required for homolog pairing, but may aid in the efficiency of homology recognition.

I also aimed to characterize the steady-state dynamics of the SC. The SC, composed in budding yeast of the transverse filament protein Zip1 and other proteins, assembles along the length of paired homologs in a recombination-dependent manner and is maintained at its full-length state until late meiosis I. Previous observations in
our lab demonstrated that this structure has some dynamic character, with subunits continually incorporated into the full-length, steady-state structure. I constructed strains bearing fluorescent Zip1 and/or an inducible wild-type Zip1 expression system to determine whether subunits are removed from this structure at the steady state. My
observations indicate that subunits are not significantly removed from the steady-state
SC, and that furthermore, this structure appears to increase in Zip1 content throughout the duration of the steady state with no apparent size constraint. Preliminary data also suggests that subunit removal does not occur locally at sites of crossover
recombination. Furthermore, the rate of SC assembly appears to be dependent on Zip1 concentration, with nuclei overexpressing ZIP1 constructing full-length SCs more rapidly.

Throughout my thesis work, I also worked to characterize a collection of Zip1 alleles mutated at residues that may be sites of Zip1 phosphorylation. I identified mutants with meiotic phenotypes, indicating that these mutants may be interesting for further characterization.