Investigating the Role of the Meiotic Chromosome Axes in Mediating Crossover Designation in Wheat and Barley
Meiotic crossovers are skewed towards the chromosome ends in wheat and barley. As a result, ~30% of genes rarely recombine in the 'cold regions', resulting in linkage-drag, which can be problematic for plant breeders as beneficial traits co-segregate with undesirable traits. An increase in the number of COs, or a shift in their positions, may disrupt linkage blocks and consequently benefit breeders. Pachytene Checkpoint Protein 2 (PCH2) is a conserved AAA+ ATPase, required to remodel the
... remodel the chromosome axes during meiosis and ensure accurate crossover formation. Previous work in Arabidopsis thaliana pch2 mutants has shown that crossovers are more likely to cluster together on short regions of the synaptonemal complex, as well as lose the obligate crossover, in the absence of PCH2. By using different methods of gene interference (CRISPR/Cas9, TILLING and VIGS), pch2 knockouts have been generated in diploid barley and tetraploid wheat and hypomorphs have been produced in hexaploid wheat. A cytological analysis has shown that PCH2 is involved in the timing of meiosis in all three ploidies and is required for the formation of the obligate CO in tetraploid wheat, but not in diploid barley. Full synapsis is achieved in barley pch2 mutants, but not tetraploid wheat pch2 mutants, where only short ZYP1 stretches are observed, localising with class I crossover marker HEI10. In addition, ectopic recombination is observed at meiotic metaphase I in tetraploid wheat pch2 mutants, indicating loss of crossover control and possible homoeologous recombination. Immunolocalisation studies indicate that PCH2 is involved in maintaining positive interference in wheat and barley, as a shift from positive to negative interference is observed in the absence of PCH2. This thesis offers a substantial contribution of knowledge to the meiosis community, providing further understanding of the role of PCH2 in crops and, importantly, in PCH2-mediated interference in plants.