Genetic analysis of a region associated with heat and drought tolerance on chromosome 3B of hexaploid wheat (Triticum aestivum).

Abstract

Drought and heat can occur during the growth cycle of crops and severely reduce yield. A QTL associated with yield and yield-related component was found in four wheat populations (Triticum aestivum L.) on the long arm of chromosome 3B “qYDH.3BL”. The four populations were grown under various climatic conditions including drought, heat and combinations of both in a number of different areas (Australia and Mexico). Linear mixed models that partition and account for genetic and non-genetic or extraneous variation were used to detect loci in single-environment and/or multi-environment QTL analysis using ASReml-R. The alleles carried by RAC875, Excalibur or Drysdale improved grain yield by between 5% and 12.5%. Two doubled haploid populations (RAC875/Kukri and Excalibur/Kukri) and two recombinant inbred line populations (RAC875/Kukri and Gladius/Drysdale) were used to fine map qYDH.3BL and identify candidate gene(s). A total of thirty-seven molecular markers were mapped on one or both genetic maps of chromosome 3B enabling development of a consensus genetic map of the qYDH.3BL region. The markers were selected based on comparisons with a published “neighbour map” of chromosome 3B or designed using either BAC-end, contig or gene sequences from the chromosome 3B sequencing project; 3BSEQ (http://urgi.versailles.inra.fr/) (cv. Chinese Spring). A positional cloning approach was used to identify candidate genes for qYDH.3BL. Molecular markers from the targeted region were assigned to physical contigs by screening the chromosome 3B BAC library experimentally using PCR or in silico by sequence comparison. A total of eight physical contigs containing 85 genes, were anchored to the qYDH.3BL region. Public and in-house resources of wheat transcript sequences were used to restrict the gene list to 65 expressed genes. Based on comparison of the 65 gene sequences to gene probes in a drought transcriptomic database, three genes were found to be differentially expressed between RAC875 and Kukri under drought conditions. Short genomic sequence reads (10× coverage) from each of the five parental lines (RAC875, Kukri, Excalibur, Gladius and Drysdale) were mapped against the 65 genes for polymorphism discovery. One gene exhibited sequence polymorphism between the drought tolerant parents (RAC875, Excalibur and Drysdale) and the drought-sensitive parents (Gladius and Kukri). In addition, presence/absence polymorphisms were consistently detected throughout a region containing 12 genes, indicating that the drought tolerant parents may have a deletion (or alien introgression) in this region. Thus, in this work, we confirmed the genetic effect of qYDH.3BL in multiple environments and multiple populations, saturated the target region with new molecular markers and defined a preliminary list of genes located in the qYDH.3BL region and selected candidate genes for further investigations.