MULTIPLE TRAIT SELECTION IN WHITE PINE 595 



a selection index and the need for making appropriate estimates of its 

 parameters. Selection in tandem or by culling levels may be practiced 

 much more simply, but if important genetic correlations or economic 

 values are consequently overlooked, a breeder may risk being deluded 

 about his actual genetic progress. 



Proceeding on the premise that multiple trait selection may be 

 theoretically superior under some conditions, let us examine next whether 

 it may be put into practice without interference between the methods of 

 estimating the genetic parameters. The fact that two pathogens would be 

 interacting with their common host is cause for concern that expressions 

 of resistance to blister rust may be altered by concurrent weevil attack, 

 or vice versa. It would be important to determine for individuals the 

 magnitude and duration of such an effect, if it exists. Its effective 

 magnitude on a population would be smaller in Scheme 1, because the 

 probability of simultaneous infection would be lower unless it were 

 purposely prevented in Scheme 2. The greater control over artificial 

 exposure in Scheme 2 may provide a means of avoiding an interaction of 

 pathogens entirely, if the duration of the effect were brief enough. 

 Either blister rust or weevil attack certainly would reduce height growth, 

 and furthermore it is easy to imagine that any of these relationships 

 could interact with variable environments. It is therefore very likely 

 that interactions would be a source of bias, and it must be decided 

 whether to measure them or to avoid them. 



Other problems arise from the fact that, before growth is measured, 

 most original members of populations will be lost, either gradually in 

 Scheme 1 or quite suddenly in Scheme 2. Blister rust would be the cause 

 of 96% mortality in Scheme 1, compared to 82% in Scheme 2 plus 15% 

 mortality due to weevils. Although it is desirable to have fairly high 

 levels of infection in order to secure high selection intensities, the 

 measurements of growth could be seriously biased if Rg£ or R^- were 

 large. A second consequence is that a large amount of tandem selection 

 already would have been imposed by the pathogens, thus greatly restricting 

 subsequent opportunities for index selection. 



A white pine breeder setting out to improve both rust and weevil 

 resistance is faced with more technical questions than he can resolve 

 all at once. We have explored the impact of some of the problems, but 

 have not even considered such important matters as mating designs, methods 

 of mass-producing improved varieties, and possible changes in pathogeni- 

 city. Therefore, it appears that some means of simplifying this complex 

 situation must be found. 



STRATEGY FOR SEEKING SOLUTIONS 



How can the breeder best take advantage of opportunities for maxi- 

 mizing genetic progress without raising risks of serious mistakes to 

 unacceptable levels? With so many uncertainties facing him, should he 

 postpone all improvement efforts until research has provided complete 

 information upon which to base his decisions? Or should he plunge ahead 

 to select and propagate the phenotypes that please him, ignoring the 

 hazards that he should confront and assuming with blind confidence that 

 his personal judgement can be substituted for facts? 



