Structure and Function of Ph1 (Pairing homoeologous 1) Gene of Wheat

Almost all eukaryotes are believed to be polyploids. Maintaining diploid like pairing in the presence of related (homoeologous) chromosomes is essential for sexual propagation of a species. Genes regulating chromosome pairing are not only required in polyploids, but are important in diploids as well. The Ph1 gene regulating chromosome pairing in wheat by differentiating homologs from homoeologs/orthologs was discovered in 1958 but was so far not been cloned. But recently, we reported the identification of a novel gene (C-Ph1 gene) present in the Ph1 locus, transient as well as stable silencing of which resulted in a phenotype similar to that of Ph1 gene mutations including homoeologous chromosome pairing, multivalent formation, and disrupted chromosome alignment on the metaphase I plate. Although with highly conserved DNA sequence, the homoeologs showed dramatically different structure and expression pattern suggesting neofunctionalization of Ph1 gene copies that explains the multiple mechanisms of chromosome pairing control associated with the gene. Furthermore, the unique function of the 5B copy plausibly attained after the event of polyploidization, is due to the 60 bp insertion that is absent in the copies of the wheat diploid progenitors. The gene also has orthologues in other plant species including maize, rice, barley, and Brachypodium that resemble more with the 5D copy suggesting it to be the conserved and ancestral version of the gene. Stable RNAi silencing of the wheat gene in Arabidopsis also showed multivalents and centromere clustering during meiosis I demonstrating conservation of the gene function across plant species.