New molecular tools to improve the efficiency of breeding for increased drought resistance

The advent of molecular markers (particularly RFLP- and PCR-derived) for use as probes for genomic DNA has revolutionized the genetic analysis of crop plants and provided not only geneticists, but also physiologists, agronomists and breeders with valuable new tools to identify traits of importance in improving resistance to abiotic stresses. For the breeder, a genetic map saturated with molecular markers allows selection for certain characters to be carried out much more efficiently and effectively than was possible previously. Two areas of molecular marker technology that are proving particularly useful in identifying traits of value for stress resistance and introducing them into improved varieties are in situ hybridization with fluorescent-labelled molecular probes and quantitative trait locus (QTL) analysis with either radioactively- or cold-labelled probes. Fluorescence in situ hybridization (FISH) takes out much of the cytological tedium previously associated with monitoring the introgression of chromosomes and DNA fragments from one species to another. Labelled DNA can be prepared that is specific to a particular species and used to visualize in chromosome preparations the presence of chromosomes or chromosomal fragments from that species amongst the recipient's chromosomes. This is being used to help transfer genes for drought resistance and salt tolerance from alien species into Graminaceous crops. DNA probes showing polymorphism between the donor and recipient species can also be used to monitor the incorporation of alien genes from chromosome addition lines into the recipient species. High density molecular maps allow the location of all major genes regulating the expression of a particular trait to be determined. Statistical methods have been developed to allow QTL for the trait to be identified. Not only does this allow the complexity of genetic control of any trait to be determined, but by comparing the extent to which confidence intervals of QTL for different traits overlap it is possible to examine the likelihood that traits are pleiotropically linked. Thus, the traits most likely to be important in determining yield under droughted conditions can be identified. Examples are given of traits that could be incorporated into breeding programmes to improve drought resistance using techniques of marker-assisted selection.