Glossary Plant Breeding

between an avirulent and virulent strains is found to be 3:1. If the host variety is
PPRR (that is, it carries two dominant genes), the ratio of avirulent to virulent will be
15:1, and so on. This suggests that complementary genic systems in the host and
parasite control reaction to the flax rust. Resistance occurs when complementary
genes in both host and parasite are dominant; if either or both the pairs of
complementary genes are recessive, susceptibility results. Thus a variety that carries
no dominant genes for resistance is susceptible to all races of the parasite and a
variety carrying one dominant gene for resistance is resistant to all races carrying the
dominant complementary gene for pathogenicity. The gene-for-gene relationship
probably also operates for host-specific insects but not for insects that have many
hosts. However, the gene-for-gene hypothesis has been criticized on several grounds;
some are: (1) it does not recognize intermediate reaction, and (2) it does not convey
any idea about the strength of the resistant gene. According to this hypothesis, for
each gene conditioning resistance in the host, there is a corresponding gene governing
virulence in the pathogen. Therefore, a minimum of two resistance genes in the host
and two complementary virulence genes in the pathogen are necessary to demonstrate
a gene for gene gene hypothesis. In addition, the disease reaction as demonstrated by
Flor is the most common pattern observed with biotrophic pathogens. With
pathogens, that produce host-specific toxins (such as victorin by Drechslera
victoriae), an opposite pattern is found.
TABLE - 1. Designation of complementary genes for rust resistance in the
host and for virulence and avirulence in the pathogen.
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Genotype of the host Genotype of the pathogen Host reaction
nnpp A N A N A P A P susceptible
NNpp a N a N A P A P susceptible
NNpp A N A N A P A P resistant
nnPP A N A N a P a P susceptible
nnPP A N A N A P A P resistant
NNPP a N a N a P a P susceptible
NNPP A N A N A P A P resistant
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