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A Map to Success December, 2000 |
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Differences that you can see or cannot see between wheat plants are encoded by genetic material called DNA. The DNA is arranged as double strands of nucleic acids, called chromosomes. The specific DNA segments in each chromosome are called genes, which determine traits such as height and disease resistance. The objective of plant breeding is to combine as many desirable genes as possible into one plant, or new variety. With as many as 80,000 to 100,000 genes functioning in a wheat plant, that objective is not as easy as it may seem. Times have changed for the better for wheat breeders, thanks to new technology. Traditionally, breeders select plants based on visible or measurable characteristics called phenotypes. Since certain traits can only be measured under certain environments (e.g., rust resistance must be ascertained in the presence of the disease), the selection process can be difficult and slow, and it is often inaccurate due to environmental variation. A relatively new technology called “marker-assisted selection” (MAS) can streamline the breeding process, or make it more accurate, by allowing breeders to select plants based on their DNA constitution, not their appearance in the field. Scientists use molecular markers to identify specific genes. These are pieces of DNA segments located close to the desired gene, which serve as landmarks on the chromosome. A marker and gene closely linked on the same chromosome will tend to stay together as each generation of plants is produced. If a marker shows up in progeny, the progeny most likely contains the desired gene. This relationship helps scientists to predict whether a plant will have a particular gene. As scientists learn where markers occur on chromosomes, and how close they are to specific genes, they can create a “map” of markers and genes on specific chromosomes. These maps show distances from other known genes (see figure below). Using very detailed genetic maps, researchers can analyze a tiny bit of tissue from seedlings to test for several characteristics of interest...in some cases, perhaps grain yield. Breeders are using this technology to select desired plants based on lab analysis of the DNA markers to supplement, or sometimes take the place of, field testing. Eventually, as maps become more developed or complex, this technology can be used to assist selection for several genes simultaneously. In future editions of this column, we will show how these gene maps are being developed by OSU’s Wheat Improvement Team to identify, locate, and select genes of special importance to Oklahoma’s wheat producers. Examples are genes which control tolerance to low pH soils common to central Oklahoma, or genes for longer coleoptile length. We remain dedicated to developing varieties that are more acid-soil tolerant and possess greater capability for emergence in hot soils. Using these special genetic road maps, we should get to where we are going with much less driving time. Submitted by Guihua Bai and Brett Carver, on behalf of OSU’s Wheat Improvement Team.
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