THE THEORETICAL BASIS OF RUST RESISTANCE TESTING- - 

 CONCEPT OF GENETIC CAIN IN BREEDING RESISTANT TREES 



K. Stern 



Lehrstuhl fur Forstgenetik vend Forstpflanzenzuchtung, 



Hann. MVcnden, Germany 



.ABSTRACT 



Co-evolution of host and parasite under exclusively natural 

 selection leads to an equilibrium with specific genetic fea- 

 tures on both sides. A genetic equilibrium based on gene- 

 for-gene relationships is not likely to be expected if the 

 host is a long-lived tree species and the parasite a fungus 

 fatal to infected trees. Host-parasite systems of this type 

 might be equilibrated predominantly by means of polygenic 

 systems on both sides. Prediction of genetic gain of selection 

 for higher resistance in such situations involves the risk of 

 biases resulting from counterselection in the parasite popu- 

 lation. Chances for predicting genetic gain of selection in 

 the host are better if the parasite has only recently been 

 introduced and the genetic properties of both populations tend 

 to either the establishment of a genetic equilibrium or the 

 extinction of the parasite by complete resistance of some host 

 genotypes. 



It is most likely that newly-"recruited" genotypes of the host 

 with high or complete resistance are bearers of specific gene 

 combinations, and that genetic variance of resistance in such 

 cases is largely non-additive (epistatic) . Mass selection or 

 related breeding procedures are of limited value here. 



Correlation of resistance in different stages of development 

 or in different ages is of vital importance for the reliability 

 of estimates of genetic gain which are mostly based on results 

 from experiments with younger plants. 



Some discussion is given on how to describe the problem in a 

 general way and how both theoretical and experimental results 

 from biometrical genetics can help to meet the main difficul- 

 ties. The overall conclusion is that more research is required 

 before a satisfactory and sufficiently general model can be 

 presented. 



INTRODUCTION 



Both animal and plant breeders and particularly breeders of long- 

 lived species have to bring their breeding programs close to an economic 



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