EVOLUTION OF Ph1 GENE AND ITS USE IN WHEAT IMPROVEMENT

Kanwardeep Singh

Wheat (Triticum aestivum) is an allohexaploid (2n=6x=42) that originated after two independent events of polyploidization with three homoeologous sub-genomes that can pair with one another. To maintain the integrity of the nucleus and reduce abnormal meiotic behavior, a homoeologous chromosome pairing control (HECP) mechanism is required for regulation of chromosome pairing between homoeologous chromosomes during meiosis. Identified in 1958, the principle regulator of homoeologous chromosome pairing control (Ph1) in polyploid wheat was recently identified, cloned and characterized using reverse genetic tools. Further, structural differentiation of C-Ph1-5B copy via 29bp deletion or 60bp insertion from its two homoeologs suggested neofunctionalization of the gene after polyploidization. The goal of this study was to understand evolution of the C-Ph1 gene structure and expression, responsible for the evolution of the novel function of differentiating homologs from homoeologs and non-homologs; and to use the gene for developing a novel method of alien introgression. Copy specific expression analysis of C-Ph1 homoeologs in hexaploid wheat and accessions of wild emmer wheat (Triticum turgidum ssp. dicoccoides) segregating for HECP control revealed meiosis specific upregulation of C-Ph1 gene. Further, significantly lower transcript accumulation for C-Ph1-5B copy during meiosis especially prophase-I in accessions without HECP compared to accessions with HECP control suggested a major role of gene expression in evolution of C-Ph1 and HECP control compared to gene structure, that didn’t show any structural variation in accessions segregating for HECP. The second part of the study focused on using the C-Ph1 for alien introgression. The currently available methods are time-consuming, cumbersome and result in the transfer of complete chromosome/arm creating undesirable linkage drag. We are reporting that virus induced transient silencing of the C-Ph1 gene resulted in 26% recombinant gametes between 1BS and 1RS with 1 to 7 alien segments compared to essentially no recombination in controls. Size of the alien transfers ranged from 40kb to 214Mb. A 100kb long recombination hotspot explained 16% of the homoeologous recombination was identified. We were able to induce recombination in a region where usually no recombination was previously observed. Multiple introgression segments present on an arm were segregated out for targeted gene transfers via recombination in the following generation involving interspersing wheat segments.

EVOLUTION OF Ph1 GENE AND ITS USE IN WHEAT IMPROVEMENT

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