REACTION OF WHEAT TO COMMON ROOT ROT: LINKAGE OF A MAJOR GENE, Crr, WITH THE CENTROMERE OF CHROMOSOME 5B

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Abstract
Cadet (C), a cultivar of Triticum aestivum L., carries the major recessive gene, Crr, for moderate resistance to common root incited by Cochliobolus sativus (Ito and Kurib.) Drechs. ex Dastur, on the long arm of chromosome 5B. The highly susceptible cultivar, Rescue (R), has the dominant, epistatic allele, crr. The crossover distance from the centromere to this gene was estimated in the genetic background of both ''Cadet'' and ''Rescue''. The ditelosomic for the long arm of each of the lines Ct"5BL (Crr) and Rt"5BL (crr) was crossed by the corresponding reciprocal chromosome 5B substitutions, c-R5B (crr) and R-C5B (Crr). The F1, heterozygous for both the telocentric and the alleles, were then backcrossed by the appropriate recessive lines, ''Cadet'' and R-C5B. Each backcross plant was tested for its reaction to root rot and examined cytologically for the presence of a telocentric chromosome. The recombination frequency of the centromere, marked by the presence or absence of the telocentric, with alleles at the Crr locus was 42.9 .+-. 3.4% in the ''Cadet'' background. In the ''Rescue'' background, the recombination frequency was 36.1 .+-. 3.3%. The difference is attributed to a generally lower chiasma frequency in ''Rescue'' than in ''Cadet''.