Ph1 gene in wheat ensures:
Pairing homoeologous1, Ph1 locus on long arm of chromosome 5B in hexaploid wheat Triticum aestivum encodes ZIP4-B2 protein that suppresses homoeologous chromosome pairing between related A, B, and D subgenomes, enforcing strictly homologous bivalent pairing during meiosis. Without Ph1, homoeologs from different subgenomes would synapse forming multivalents and translocations, causing sterility and aneuploidy, as seen in ph1 mutants where extensive homoeologous pairing occurs and multivalents observed cytologically. Ph1 promotes homolog recognition via telomere bouquet organization and restricts synaptonemal complex formation to true homologs, essentially diploidizing meiosis in polyploid context and ensuring disomic inheritance. This regulatory gene enabled successful evolution of stable fertile hexaploid wheat by preventing meiotic chaos expected from three related genomes coexisting. Breeders temporarily suppress Ph1 using ph1b mutant to induce recombination between wheat and alien chromosomes for introgression of wild relative traits for disease resistance. Ph1 mechanism involves suppression of homoeologous pairing through regulation of cyclin dependent kinase activity and histone modification, delaying telomere bouquet formation; recent CRISPR knockout of TaZIP4-B2 mimics ph1b mutation, enabling homoeologous recombination for alien introgression while maintaining high fertility after transient suppression, revolutionizing wheat alien gene transfer breeding for wild introgressions.
Ref: Riley R & Chapman V 1958 Nature – Ph1 homologous pairing discovery; Griffiths et al. 2006 Nature cloning of Ph1