A Genetic Study of Leaf Rust Resistance in Wheat

Introduction In the hexaploid wheat, T~iticrmz aestivum (272 = 6x = 42), the tolerance of chromosome deficiency provides a technique by which the resolving power of genetical analysis can be increased enormously. In the variety Chinese Spring, the development of the scries of 21 monotelocentrics by Sears (1954) enables single cl~romosome substitution lines to be produced. These consist of a single homologous pair of cl~romosoines from a donor variety and 20 pairs of chronlosomes from Chinese Spring. This procedure can be carried out for each of the cl~ron~osomes of the donor variety in turn, so that 21 substitution lines are possible between the recipient variety Chinese Spring and the donor. In any form of genetic analysis, the ability to recognise and locate a single gene controlling any aspect of the phenotype depends upon the relative magnitudes of the variation contributcd by this gene and the residual variation brought about by other genes and the environment affecting the samc character. Where a character ij controlled by a number of genes this residual variation is lilcely to be large so that unless one of the genes has a large effect on the phcnotype, methods based on the study of F, and bacltcross generations have little chance of recognising individual components of the system. However, using single chronlosome substitutions only a small component of the 11el.itable variation existing between two varieties is examined at any one time, so that the magnitude of the residi~al genetic variation is reduced considerably. Furthermore, each of the 2 1 substitution lines is true breeding so that adequate replication can be achieved. Inter-varietal chromosome substitutions therefore provide, by comparison with more conventional methods of analysis, a more efficienf way of studying and locating the genes responsible for the differences between varieties of wheat. Single cl~romosome substitution lines, howcver, can only reflect the sum total of the effccts produccd bv the genes carried by a single chromosome. The limit to the resolving power o'f the analysis must therefore be the chromosome. T o overcome this limitation, single chromosonle substitution lines can be crossed to the recipient varietv. In this nay , a hvbrid will be produced in which only the substituted ch;omosome will'be ll~terozygous whereas all the other 20 cl~romoson~es sl~ould bc l~omozygous. Generations following this hybridisation will therefore segregate for -anv allelic diffcrences which occur between the substituted chronlosonle and its l ~ o n ~ o l o ~ u e in the recipient. The present account is a continuation of a study, using a single chrornosome substitution line, in which the genetic differences distinguishing chromosome 7B of the variety Hope from that of Chinese Spring were analysed (Law, 1966a, 1967; Law and Wolfc, 1966). In this paper the character studied is the response to inoculation with Pucci17ia ~eco7zdita (leaf rust). This response is generally considered to bc controlled by genes which determine qualitative