Structure and Function of Ph1 Genes in Wheat and Other Cereals

More than 50% of all higher plants, including some of the most important crop plants, are polyploids or ancient polyploids.  Recovery of a sexually reproducing, meiotically and hence, reproductively stable polyploid, requires a precise diploidization mechanism in such a way that only bivalents involving homologs are formed during meiosis. The chromosome pairing control genes regulating diploidization have been reported in sexually propagated polyploid species such as wheat, oats, cotton, tobacco, Lolium, and tall fescue. The Ph1 gene in wheat regulating chromosome pairing by differentiating homologous from homoeologous/orthologous pairing was discovered in 1958 but was so far not been cloned. But in the recent PNAS article, 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. The identified gene has three homoeologous copies with different structure and expression patterns suggesting neofunctionalization of Ph1 gene copies. Genes with a function similar to that of chromosome pairing regulators of polyploid species are present in diploid species as well. Stable RNAi silencing of the gene in Arabidopsis also showed the similar phenotype to that of the wheat RNAi silenced plants strengthening the above-mentioned fact. This gene also has orthologs in other diploids and ancient polyploids such as rice, maize, barley, and Brachypodium.