On the structure of a gene for disease resistance in maize.

Disease resistance is an extremely valuable control measure of rusts in cereal crops and is based, almost entirely, on simple procedures of identification and incorporation of major genes for resistance into economic varieties. Usually, several loci that carry these genes are known in each plant species. Frequently, several alleles at a locus can be distinguished on the basis of their reaction to various races of the pathogen. In spite of the vast array of resistance genes now known, their use has been sharply limited by the restricted number of loci in each species. The result has been a never-ending cycle of (1) releasing a variety resistant to prevalent races, (2) finding it susceptible to new races of the pathogen appearing a few years later, (3) breeding resistance to the new races, and (4) releasing the new variety only to find it susceptible, a few years later, to yet new races of the pathogen. Thus, breeding for rust resistance in cereals has been so repetitious that the value and use in disease control of genes that condition resistance to only a portion of the possible rust races is now widely questioned.' Although major genes for rust resistance have been used extensively, little is known about their fine structure. This is probably due to the fact that in crops, where resistance genes have been of maximum importance, genetic analyses requiring large test-cross progenies are not feasible. Economic and biological considerations, however, provide compelling reasons why studies on the structure of genes for rust resistance should be made. The system of genetic resistance to rust, Puccinia sorghi Schw., in maize, Zea mays L., is apparently parallel to that found in other cereal crops. In maize, there are at least five loci (Rpl, Rp3, Rp4, Rp5, and Rp6) at which dominant genes for rust resistance occur.2-6 Rp, is located in the short arm of chromosome 10 and was considered to occupy the terminal position on the genetic map.3'7 Recently, Rpi has been shown to be located between Rp5 and Rpe which are 1.1 and 2.1 map units, respectively, from it.' Orientation of these genes with respect to the centromere is not known. Rp3 and Rp4 are independent of Rpi. On the basis of genetic studies and disease reaction, 14 alleles, Rpia, Rp b. .. to Rpi', all dominant to recessive allele rp1 for susceptibility, have been distinguished at the Rp, locus.5' 6, 8 Allele Rpid is unique in conditioning resistance to all of 59 P. sorghi cultures collected from various regions of the United States and Mexico.9 Other dominant alleles have progressively narrower spectra of resistance, but do not fall into a linear array with respect to dominance relationship. In heterozygotes between dominant alleles, resistance is equal to the sum of the resistance of the two separate alleles. In some instances the resistance is equal to a naturally occurring third allele, which leads to suspicion that Rp, is a complex locus or region.