Fine Mapping a Major QTL for Pre-Harvest Sprouting (PHS) Resistance by Comparative Analysis in White Wheat.

Pre-harvest sprouting (PHS) is a major constraint to white wheat production. Previously, we mapped a major quantitative trait loci (QTL) for pre-harvest sprouting resistance in a white wheat Rio Blanco using a recombinant inbred line (RIL) population derived from the cross Rio Blanco/ NW97S186 which explained 40% of the phenotypic variation. To dissect and fine map the QTL, a new segregation population consisting of 1874 F₂ lines was developed by selfing the progenies of a RIL (RIL25) that was heterozygous for the three SSR markers in the QTL region. The segregation ratio of PHS resistance in the population fits monogenic inheritance. A comparative genomics approach was used to fine map the QTL for map-based cloning of the QTL. After 114 markers developed based on the physically mapped wheat ESTs on the deletion bin of 3AS0.45-1.00 of Chinese Spring were screened between the two parents and two NIL, and only one was polymorphic. This marker was successfully used to identify the corresponding chromosome regions in rice and Brachypodium. A total of 286 primer pairs were designed based on sequences from the syntenic genome regions in rice and Branchypodium and further used to screen the parents. Thirteen wheat STS markers derived from corresponding wheat ESTs were mapped to the QTL region. The microcolinearity was established between the QTL in wheat and the corresponding region in rice chromosome 1 and Brachypodium chromosome 2 according to the EST sequences. Fine mapping narrowed down the QTL to an interval of 0.4cM. A physical map of the QTL region was constructed by screening a Chinese Spring chromosome 3AS arm-specific BAC library with markers flanking the QTL. Two contigs were identified to span the QTL region and sequence analysis of these contigs is underway.