Identifying crossover-rich regions and their effect on meiotic homologous interactions by partitioning chromosome arms of wheat and rye.

Chiasmata are usually formed in the distal half of cereal chromosomes. Previous studies showed that the crossover-rich region displays a more active role in homologous recognition at early meiosis than crossover-poor regions in the long arm of rye chromosome 1R, but not in the long arm of chromosome 5R. In order to determine what happens in other chromosomes of rye and wheat, we have partitioned, by wheat-rye translocations of variable-size, the distal fourth part of chromosome arms 1BS and 2BL of wheat and 1RS and 2RL of rye. Synapsis and chiasma formation in chromosome pairs with homologous (wheat-wheat or rye-rye) and homoeologous (wheat-rye) stretches, positioned distally and proximally, respectively, or vice versa, have been studied by rye chromatin labelling using fluorescence in situ hybridisation. Chromosome arm partitioning showed that the distal 12 % of 1BS form one crossover in 50 % of the cells, while the distal 6.7 % of 2RL and the distal 10.5 % of 2BL account for 94 % and 81 % of chiasmata formed in these arms. Distal homoeologous segments reduce the frequency of chiasmata and the possibility of interaction between the intercalary/proximal homologous segments. Such a reduction is related to the size of the homoeologous (translocated) segment. The effect on synapsis and chiasma formation was much lower in chromosome constructions with distal homology and proximal homoeology. All of these data support that among wheat and rye chromosomes, recombining regions are more often involved in homologous recognition and pairing than crossover-poor regions.