A separation-of-function ZIP4 wheat mutant allows crossover between related chromosomes and is meiotically stable

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Many species, including most flowering plants, are polyploid, possessing multiple genomes. Duringpolyploidisation, fertility is preserved via the evolution of mechanisms to control the behaviourof these multiple genomes during meiosis. On the polyploidisation of wheat, the major meioticgene ZIP4 duplicated and diverged, with the resulting new gene TaZIP4-B2 being inserted intochromosome 5B. Previous studies showed that this TaZIP4-B2 promotes pairing and synapsis betweenwheat homologous chromosomes, whilst suppressing crossover between related (homoeologous)chromosomes. Moreover, in wheat, the presence of TaZIP4-B2 preserves up to 50% of grain number.The present study exploits a separation-of-function wheat Tazip4-B2 mutant named zip4-ph1d, inwhich the Tazip4-B2 copy still promotes correct pairing and synapsis between homologues (resultingin the same pollen profile and fertility normally found in wild type wheat), but which also allowscrossover between the related chromosomes in wheat haploids of this mutant. This suggests animproved utility for the new zip4-ph1d mutant line during wheat breeding, compared to the previouslydescribed CRISPR Tazip4-B2 and ph1 mutant lines. The results also reveal that loss of suppression ofhomoeologous crossover between wheat chromosomes does not in itself reduce wheat fertility whenpromotion of homologous pairing and synapsis by TaZIP4-B2 is preserved.