Utilisation of new or novel traits directly from unadapted sources can impact negatively on breeding programmes due to the inevitable inclusion of inferior traits often present in these accessions. In this pre-breeding programme resistance to biotic stressors in donor accessions is identified and transfered to wheat lines that are well adapted to South African conditions. Currently research is aimed at genetic resistance to the cereal rusts, Fusarium head blight and Russian wheat aphid. Furthermore different production regions have different problems and prevalent stressors. Breeders often need to combine different resistances with each other while keeping adaptation and end-use quality in mind. Specialist pathologists, entomologists and physiologists together with molecular biologists use both marker assisted selection (MAS) and phenotypic screening to pre-breed resistant germplasm.
Wheat genetics is one of the most complicated fields of study in agricultural research. This makes combining specific genetic traits in a single wheat line a complex task. Molecular markers are used to help identify specific genes and the presence of these markers can be used to select wheat lines that contain the trait of interest. A molecular markers is a piece of DNA which forms part of the wheat chromosome and is linked closely to the gene of interest. Scientists can thus extract DNA from the wheat plants, test to see if the marker is present and select only those plants that have the marker. This is a much quicker, and often cheaper way of determining whether a plant is resistant to a particular insect pest or disease than physically testing the plant itself.
To be sure that the genetic marker being used is reliable, the differences observed at a DNA level need to be associated with a phenotypic observation or another measurable trait. It must be possible to mark the observed difference and make the correct selections from the genetic data generated. Molecular biologist therefore work closely with pre-breeders, pathologists and entomologists to validate that the linkage between specific DNA markers and the trait of interest is strong and that the markers can be used for diagnostic purposes.
These identified and validated markers are then used to screen hundreds of young seedlings and those which are rejected can be discarded immediately. This results in many savings with the additional benefit of focused progress in the different pre-breeding programmes. MAS is currently successfully being applied for stem rust, leaf rust, stripe rust and Fusarium head blight resistance. Efforts are being made to implement MAS for other traits such as plant height, aluminium tolerance, Russian wheat aphid resistance and vernalisation requirement.
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