MSH2 is required for obligate crossover formation in wheat

In wheat, meiotic crossovers (COs) are skewed towards the ends of the chromosomes. Consequently ~30% of genes rarely or never recombine, which is problematic for wheat breeders as it results in linkage-drag, where beneficial traits co-segregate with undesirable traits. An increase in the number or position of COs in ‘cold’ recombination regions would allow for the separation of gene blocks and reduce linkage-drag.

MutS homolog 2 (MSH2) is a DNA mismatch repair gene that suppresses recombination between divergent sequences during meiosis. Previous work in Arabidopsis F1 hybrids has shown a 40% increase in COs in the absence of msh2 compared to wild type, showing promise for increasing recombination in crop species. To assess the role of MSH2 in wheat we have generated knockout mutant lines using the tetraploid Kronos TILLING population, and reduced its expression in hexaploid wheat through virus-induced gene silencing (VIGS).

In the tetraploid TILLING lines, msh2 was lethal in 52% of cases and those that grew to maturity were dwarfed plants with abnormal floral morphology. At meiotic metaphase I, the mean number of chiasmata per nucleus decreased by around 40%, with an increase in univalents. This meiotic phenotype was partially reproduced in hexaploid wheat by VIGS with an 8% reduction in the mean number of chiasmata per nucleus. MSH2 therefore appears to have a role in the class I pathway and the formation of the obligate CO. An antibody raised against TaMSH2 has been generated and immunolocalisation experiments are ongoing.