ZIP4 is required for normal progression of synapsis and for over 95% of crossovers in wheat meiosis

Tetraploid (AABB) and hexaploid (AABBDD) wheat have multiple sets of similar chromosomes, with successful meiosis and preservation of fertility relying on synapsis and crossover (CO) formation only taking place between homologous chromosomes. In hexaploid wheat, the major meiotic gene ( ) on chromosome 5B, promotes CO formation between homologous chromosomes, whilst suppressing COs between homeologous (related) chromosomes. In other species, mutations eliminate approximately 85% of COs, consistent with loss of the class I CO pathway. Tetraploid wheat has three copies: on chromosome 3A, on 3B and on 5B. Here, we have developed single, double and triple TILLING mutants and a CRISPR mutant, to determine the effect of genes on synapsis and CO formation in the tetraploid wheat cultivar 'Kronos'. We show that disruption of two gene copies in double mutants, results in a 76-78% reduction in COs when compared to wild-type plants. Moreover, when all three copies are disrupted in triple mutants, COs are reduced by over 95%, suggesting that the copy may also affect class II COs. If this is the case, the class I and class II CO pathways may be interlinked in wheat. When duplicated and diverged from chromosome 3B on wheat polyploidization, the new 5B copy, , could have acquired an additional function to stabilize both CO pathways. In tetraploid plants deficient in all three copies, synapsis is delayed and does not complete, consistent with our previous studies in hexaploid wheat, when a similar delay in synapsis was observed in a 59.3 Mb deletion mutant, , encompassing the gene on chromosome 5B. These findings confirm the requirement of for efficient synapsis, and suggest that genes have a stronger effect on synapsis than previously described in Arabidopsis and rice. Thus, in wheat accounts for the two major phenotypes reported for , promotion of homologous synapsis and suppression of homeologous COs.