Mapping Areas of Low Recombination and Polymorphism; Radiation Hybrid Mapping of Wheat Loci C and Pis1

Precise mapping of wheat genes is often complicated by lack of marker polymorphism or lack of recombination. Radiation hybrid (RH) mapping can address both problems as it relies on radiation-induced breaks rather than recombination to order markers while markers are scored for presence/absence, eliminating the need for polymorphism. We present a method to physically map genes based on deletions generated following γ irradiation of pollen. To demonstrate the potential of this technique, we mapped two genes located on wheat chromosome 2D. Compactum (C) is responsible for club head phenotype in wheat and maps to the low-recombination, pericentromeric region of 2D. It is unknown which side of the centromere C occupies. Three Pistils (Pis1) is a wheat floral mutant producing three pistils per floret instead of the usual single pistil. Pis1 is located on the distal portion of chromosome 2DL. It has proven difficult to map in an F₂ population due to lack of polymorphism. We used γ-irradiated pollen of C and Pis1 hexaploid (AABBDD) wheat lines to pollinate tetraploid (AABB) wheat lines Langdon and Altar, respectively. Quasi-pentaploid (AABBD) F₁ plants carrying radiation-induced fragments of D-genome chromosomes were grown to flowering, phenotyped and genotyped. Absence of dominant C or Pis1 phenotypes in the F₁s indicated radiation-induced deletion of the gene. Independent mapping panels were assembled for both genes. Measures of marker co-retention were used to estimate linkage and build physical maps. Herein we outline the approach and present physical maps of both genes.