458 



BOHUN 



KINLOCH, JR. 



LOW INFECTION 



55 



iTTtw^.. 



fc. 



MODERATE INFECTION 



mShr a 



rrn ) i 



HIGH INFECTION 



7 25 



i -nTM rTl~h 



12 3 2 4 6 8 10 M 6 8 10 12 14 « « 20 



Figure 2 . --Frequency distribution of individual trees in 

 four full-sib families of loblolly pine into categories 

 according to the number of separate infections (galls) 

 sustained by each. The same four families (of a total of 

 55) were planted on each of three different sites that had 

 relatively low, moderate, and high levels of rust infection, 

 Numbers in bold face are the relative ranking in resistance 

 of the same family on each site. 



From this kind of relationship it is tempting to speculate that the 

 number of loci controlling resistance and susceptibility could be roughly 

 inferred from the number of discrete categories detectable in the series. 

 The assumptions required, however, are numerous, and any conclusions of 

 this kind drawn from such crude data would be premature and extremely 

 teneous. The data do provide, on the other hand, a clue to the kinds of 

 genetic interactions that may be occurring in the population. The 

 distribution patterns of individual trees in the population as a whole, 

 as well as individual families within, are suggestive of a heterozygous 

 parent population segregating at several major polymorphic loci condition- 

 ing resistance and susceptibility, similar to theoretical models of host- 

 parasite realtionships in wild populations proposed by Mode (1958) and 

 Person (1959, 1966, 1967). I submit this as a working hypothesis that 

 is testable by the approaches discussed. 



LITERATURE CITED 



Anderson, N. A., and R. L. Anderson. 1965. The susceptibility of jack 



pine and lodgepole pine and their hybrids to sweetfern and eastern 



gall rust. U.S. Dep. Agr. , Forest Serv., Lake States Forest Exp. 



Sta. Res. Note LS-56. 4 p. 

 Bakshi, B. K. 1963. Dangerous forest diseases in India, p. 55-61. In 



Internationally dangerous forest tree diseases. U.S. Dep. Agr. Misc. 



Publ. 939. 

 Barber, J. C. 1964. Inherent variation among slash pine progenies at 



the Ida Cason Calloway Foundation. U.S. Dep. Agr., Forest Serv. 



Southeast. Forest Exp. Sta. Res. Paper SE-10. 90 p. 



