RESISTANCE TO RUSTS IN HARD PINES 453 



By analogy with other rusts, variation in pathogenicity is to be 

 expected, though much more work is needed to determine its extent in 

 present populations of pine rusts and the inherent capacity for change in 

 these populations. The amount of variation in pathogenicity bears 

 critically on the degree of stability of resistance that can be expected 

 in pine populations following selection. 



The potential for change to races of wider virulence depends in large 

 part on the types of sexual behavior and other mechanisms of gene exchange 

 operating in rust populations. If virulence is controlled by several 

 different major genes as, for instance, in gene-for-gene systems, a homo- 

 thallic sexual mechanism presumably would impose some restriction on the 

 range of variability possible and the capacity of the population to 

 synthesize new gene combinations for virulence. Although critical 

 evidence is lacking, present indications are on the side of limited gene 

 exchange in the pine rusts as compared, for example, with the obligately 

 outcrossing wheat stem rust. Hirt (1964) found that cross-fertilization 

 by pycnial exchange was not prerequisite to aeciospore formation in C. 

 ribioola, suggesting that homothallism is at least possible, if not 

 predominant, in this fungus. 



In the autoecious species (P. harknessii, races of P. pini , and 

 probably P. filamentosum) , a sexual stage is either lacking altogether 

 or is highly irregular. Hiratsuka, Morf, and Powell (1966) proposed an 

 endo-type life cycle for P. harknessii. Their conclusions were based 

 primarily on the number of nuclei observed at various stages of aeciospore 

 maturation and germination: immature aeciospores had 2 nuclei, mature 

 aeciospores 1, and germ tubes 1 in each of 2 to 4 septate cells formed. 

 Aeciospores of P. s talacti forme , by contrast (and typical of host- 

 alternating species) remained dicaryotic throughout and lacked septate 

 germ tubes. They interpreted the sequence observed in P. harknessii as 

 indicating nuclear fusion in mature aeciospores followed by meiosis and 

 monocaryotization of germ tube cells. A similar sequence was found in 

 pine-to-pine races of P. vini and interpreted in the same way (Hiratsuka, 

 1968) . 



Evidence of other workers, however, differs in considerable detail, 

 and leaves this conclusion far from definite. True (1938) and 

 Christenson (1968) found that most aeciospores of P. harknessii were 

 dicaryotic, and saw no evidence of nuclear fusion. In a later paper, 

 Christenson (1969) observed that dicaryotic aeciospores of an albino race 

 became uninucleate during maturation, but that subsequent division of 

 nuclei was more characteristic of mitosis than of meiosis. In autoecious 

 races of P. filamentosum, nuclear number is extremely variable, ranging 

 from 1 to 6 in aeciospores and up to 16 in their germ tubes (Christenson, 

 1968). Only 15% of germ tubes became monocaryotic (Krebill, R. G., 

 manuscript in preparation). From these inconsistencies, the question of 

 a sexual stage in autoecious peridermia remains unresolved. The only 

 conclusion warranted presently is that nuclear number and behavior in 

 these species are highly unstable. In any case, mechanisms of gene 

 exchange within the populations appear to be limited. 



