An efficient means to map genes of corn would facilitate understanding the genetic makeup of corn, our major cereal crop. The genome size and complexity of corn is similar to that of humans with a DNA content of about 2.5 billion base pairs. The placement of human genes in their respective chromosomes is greatly facilitated by the use of special Chinese hamster cell lines containing portions of the human genome. An analogous system is proposed for corn gene mapping with the production of oat lines containing portions of the corn genome. Crosses of oats with corn pollen followed by the rescue of early developing embryos on a nutrient medium leads to the recovery of plants with only one corn chromosome together with all the oat chromosomes. Such plants are termed oat-maize chromosome addition lines. The complexity of the corn genome is reduced by 10-fold in these plants. Irradiation of these plants possessing a single corn chromosome can produce an array of progenies with less than the whole corn chromosome present. Such progeny are termed radiation hybrids. A series of radiation hybrids allows the placement of corn genes to short but highly defined regions of the genome. The goal of this project is to complete the series of the 10 different oat-com chromosome addition lines (7 are currently available) and, for each chromosome, produce a series of approximately 100 radiation hybrid lines. Proof of concept has been completed with a radiation hybrid series for chromosome 9. A series of only 12 radiation hybrids allowed chromosome 9 to be delineated into 22 distinct subregions. The average size of these regions, given certain assumptions, is estimated to be 8.7 Mb. About 100 evenly distributed genetic markers and 100 radiation hybrids would be necessary to define corn chromosome 9 regions at the 2 Mb level; 600 markers would be needed to adequately utilize a 400 kb level of resolution. Advantages of this approach - in addition to high resolution mapping -is that any gene can be mapped without the availability of polymorphic forms, the gene's physical chromosome location becomes known, and the system is especially suitable for high-throughput formats. Arrays of DNA samples ultimately will be available to researchers for mapping any gene or DNA fragment to a small region of a given chromosome. Other uses of the radiation hybrids include the cloning of small segments of corn chromosomes separated from the remaining portions of the genome, the isolation of additional markers in a small genomic region perhaps as tags for important traits, and the identification of oat lines with useful traits such as disease resistance or heat stress tolerance incorporated from corn into oat.
