SPECIFICITY IN PLANT DISEASE 31 



THE GENE -FOR- GENE CONCEPT 



In the gene-for-gene concept we deal with the relationship between 

 host and pathogen. The relationship, the host, and the pathogen may be 

 independently affected by environment. The result of this complex is 

 infection type. If we retain the current definition of disease, then the 

 infection type cannot be considered the disease, since disease is defined 

 in terms of host symptoms, whereas the infection type also includes signs 

 of the pathogen and additional manifestations which are characteristic of 

 the relationship itself. The manifestation of the complex of interactions 

 of a specific host rpathogen : environment system I have called the aegri corpus 

 (Loegering, 1966) to distinguish it from the many views of "disease". 

 The infection type is the character of the aegricorpus. This approach to 

 the gene-for-gene concept, if accepted, strips our minds of the emotional 

 involvement resulting from the expression "a diseased plant". 



CATEGORY III INTERACTION 



Loegering and Powers (1962) published a diagramatic model of a simple 

 gene-for-gene relationship. Later Loegering (1966) modified this model 

 to bring in the word aegricorpus. Figure 1 is a further modification 

 of this model. The model introduces the terminology which I use in dis- 

 cussing the gene-for-gene concept. The concepts which these terms 

 represent have been published (Loegering, 1966; Loegering and Powers, 

 1962). Infection type is the character of the aegricorpus, whereas 

 reaction and pathogenicity, respectively, designate the characters of 

 host and pathogen. The phenotypic expression of infection type, reaction, 

 and pathogenicity is low or high. This pair of terms is comparative. 

 Thus infection type 1 is low compared to infection type 2 but high com- 

 pared to infection type 0. The use of low and high for reaction and 

 pathogenicity replaces the terms resistance and susceptibility, and 

 avirulence and virulence used in earlier published diagrams. This avoids 

 the variable concepts which these latter terms convey to different people 

 and also aids in the development of a new system of genetic symbolization 

 as will be discussed later. The older terms should be reserved for 

 describing varieties and cultures. 



Seldom is it possible to determine the inheritance of reaction or 

 pathogenicity without observing infection type. Infection type is the 

 character of the aegricorpus, not of host or pathogen, but its expression 

 results from interaction of the genes of the two organisms which make up 

 the aegricorpus. Because of this interrelationship it is possible to use 

 infection type to determine host or pathogen genotype by holding one 

 constant . 



The genotypes shown in Figure 1 for host and pathogen are combined 

 in all combinations and are used to show the potential genotypes of the 

 aegricorpus. The illustration uses only one gene pair for reaction in 

 the host and one gene pair for pathogenicity in the pathogen. These pairs 

 are called "corresponding gene pairs" and neither interacts with any other 

 gene pair for pathogenicity or reaction. In examining the genotypes of 

 the aegricorpus we see that low infection type results only when host and 

 pathogen genotypes of corresponding gene pairs are for low reaction and 

 low pathogenicity. When the genotype is for low reaction and high 

 pathogenicity or high reaction and low pathogenicity the infection type 

 is high. This seems incredible to many biologists but it has been demon- 

 strated to be true over and over, and no exceptions have been found to 

 date in studies involving obligate pathogens. 



