High-density mapping and comparative analysis of agronomically important traits on wheat chromosome 3A.

Bread wheat chromosome 3A has been shown to contain genes/QTLs controlling grain yield and other agronomic traits. The objectives of this study were to generate high-density physical and genetic-linkage maps of wheat homoeologous group 3 chromosomes and reveal the physical locations of genes/QTLs controlling yield and its component traits, as well as agronomic traits, to obtain a precise estimate of recombination for the corresponding regions and to enrich the QTL-containing regions with markers. Physical mapping was accomplished by 179 DNA markers mostly representing expressed genes using 41 single-break deletion lines. Polymorphism survey of cultivars Cheyenne (CNN) and Wichita (WI), and a substitution line of CNN carrying chromosome 3A from WI [CNN(WI3A)], with 142 RFLP probes and 55 SSR markers revealed that the extent of polymorphism is different among various group 3 chromosomal regions as well as among the homoeologs. A genetic-linkage map for chromosome 3A was developed by mapping 17 QTLs for seven agronomic traits relative to 26 RFLP and 15 SSR chromosome 3A-specific markers on 95 single-chromosome recombinant inbred lines. Comparison of the physical maps with the 3A genetic-linkage map localized the QTLs to gene-containing regions and accounted for only about 36% of the chromosome. Two chromosomal regions containing 9 of the 17 QTLs encompassed less than 10% of chromosome 3A but accounted for almost all of the arm recombination. To identify rice chromosomal regions corresponding to the particular QTL-containing wheat regions, 650 physically mapped wheat group 3 sequences were compared with rice genomic sequences. At an E value of E < or = 10(-5), 82% of the wheat group 3 sequences identified rice homologs, of which 54% were on rice chromosome 1. The rice chromosome 1 region collinear with the two wheat regions that contained 9 QTLs was about 6.5 Mb.