Submitted to: Molecular and General Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/1996
Publication Date: N/A
Interpretive Summary: The identification and isolation of genes for disease resistance is a crucial component of the research efforts to reduce crop losses. The isolation of the Mla6 (powdery mildew resistance) gene in barley is the primary objective of our research project. To isolate a gene, it's location on the plant's genetic map must be determined. A genetic map consists of a number of genes and genetic markers, i.e., small sequences of DNA, whose location relative to each other are used to determine the genetic distance between genes. Unfortunately, the relationship between genetic distance and physical distance, i.e., the actual number of bases separating two genes, varies greatly depending on the location of the gene on the genetic map. Prior to isolating a gene, it is critical to know the physical distance between the gene and the nearest genetic markers. We determined the physical distance between three genetic markers that are very close to the Mla6 gene. In doing so, we have confirmed that the ratio between genetic to physical distance varies greatly in this section of the barley genetic map. These results are the first to confirm the variable nature of the ratio between genetic and physical distance in a region this close to the Mla6 gene. We are currently focusing our research efforts to identifying genetic markers closer to the Mla6 gene. This information will be of most interest to those working to improve disease resistance in barley and in other crops.
Technical Abstract: Variants of pulsed-field gel electrophoresis were used in conjunction with 2-dimensional DNA gel electrophoresis (2-DDGE) to analyze both parental and F4 homozygous lines containing recombination breakpoints in two genetically defined intervals on barley chromosome 1H. 2-DDGE analysis on the parental lines demonstrated that two loci that define one of those intervals reside on a single 450-kb MluI fragment. This result indicates a maximum ratio of physical to genetic distance in this interval of 1500 kb/cM as compared to the overall estimate of 3.7-4.2 mb/cM. HMW DNA restricted with NotI and probed sequentially with MWG068 and BCD249 detected a single band at approximately 2.8 mb in the C.I. 16151 parental line. These results indicate the maximum ratio of physical to genetic distance in the Xmwg068 and Xbcd249.2 interval is 7.8 mb/cM. High molecular weight (HMW) DNA restriction fragment length polymorphisms (RFLPs), as compared to parental lines, were observed in two recombinant lines in the Xbcd249.2-Xmwg068 interval, and in three recombinant lines in the Xbcd249.1-Xbcd249.2 interval. A model is presented proposing these HMW DNA RFLPs represent restriction fragments containing recombination breakpoints. Data from the recombination breakpoint analysis are used to estimate a ratio of genetic to physical distance in this region of the chromosome. The estimates range from 3.16 mb/cM in the Xbcd249.2-Xmwg068 interval to 0.5 mb/cM in the Xbcd249.1-Xbcd249.2 interval. Both physical linkage and recombination breakpoint analysis indicate the Xbcd249.1-Xbcd249.2 interval is physically smaller than the Xbcd249.2-Xmwgo68 interval.