Abstract
Meiotic recombination generates genetic variation and provides physical links between homologous chromosomes (crossovers) essential for accurate segregation. In cereals the distribution of crossovers, cytologically evident as chiasmata, is biased toward the distal regions of chromosomes. This creates a bottleneck for plant breeders in the development of varieties with improved agronomic traits, as genes situated in the interstitial and centromere proximal regions of chromosomes rarely recombine. Recent advances in wheat genomics and genome engineering combined with well-developed wheat cytogenetics offer new opportunities to manipulate recombination and unlock genetic variation. As a basis for these investigations we have carried out a detailed analysis of meiotic progression in hexaploid wheat (Triticum aestivum) using immunolocalization of chromosome axis, synaptonemal complex and recombination proteins. 5-Bromo-2′-deoxyuridine (BrdU) labeling was used to determine the chronology of key events in relation to DNA replication. Axis morphogenesis, synapsis and recombination initiation were found to be spatio-temporally coordinated, beginning in the gene-dense distal chromosomal regions and later occurring in the interstitial/proximal regions. Moreover, meiotic progression in the distal regions was coordinated with the conserved chromatin cycles that are a feature of meiosis. This mirroring of the chiasma bias was also evident in the distribution of the gene-associated histone marks, H3K4me3 and H3K27me3; the repeat-associated mark, H3K27me1; and H3K9me3. We believe that this study provides a cytogenetic framework for functional studies and ongoing initiatives to manipulate recombination in the wheat genome.
Highlights
Utilizing the genetic variation that arises from meiotic recombination plays a pivotal role in crop improvement programs
Slides were incubated for 30 min at 37◦C, washed 3 times with PBST and an Arabidopsis telomeric-repeat fluorescence in situ hybridization (FISH) probe labeled with digoxigenin (Armstrong et al, 2001) was applied as described in Armstrong (2013)
A cytological analysis of chiasma frequency and distribution was carried out in pollen mother cells (PMCs) of Cadenza, a UK spring wheat variety which forms the background for an EMS-induced TILLING mutant population (Rakszegi et al, 2010; Krasileva et al, 2017)
Summary
Utilizing the genetic variation that arises from meiotic recombination plays a pivotal role in crop improvement programs. In many species CO distribution exhibits a tendency to localize in particular chromosomes regions, often favoring distal regions (Jones, 1984) This is evident in cereals with large genomes, for example wheat and barley, where a strong distal CO bias limits their formation in interstitial and proximal chromosome regions amounting to 50–70% of the overall genome (Choulet et al, 2014; Higgins et al, 2014). The hyper-rec mutants exhibit some evidence of reduced fertility and meiotic defects, which may prove more problematic in species with larger genomes (Fernandes et al, 2018) It appears that recombination-cold proximal/pericentromeric regions of chromosomes are relatively insensitive to the effects of hyper-rec mutants and HEI10 overexpression (Serra et al, 2018). We believe this study will provide a reference framework for CO modification initiatives and functional studies of meiotic recombination for the benefit of crop improvement
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