IMPROVED PLANTING STOCK THROUGH SYNTHETIC VARIETIES 573 



the most striking abnormalities were cytoplasmic male-sterile. From 

 cytological study of the abnormalities, they concluded that the mutants 

 had the same or a similar mechanism causing male sterility as that being 

 used in the production of hybrid sorghum. 



BREEDING FOR RUST RESISTANCE IN PINUS 



The objective of pest-resistance improvement should be to select or 

 breed trees that have sufficient resistance to provide a profitable 

 harvest. Since resistance to pests is seldom absolute, it is necessary 

 to set up minimum standards of field resistance acceptable in practice 

 for each host-pest relationship. For biologic and economic reasons, 

 these standards must be flexible in time and place (Schreiner, 1960). 



Field resistance differs from immunity or hypersensitivity in that 

 it is under the control of many gene loci. Polygenic control is believed 

 to be responsible for the greater stability of field resistance because 

 several mutations in the parasite may be necessary to overcome this type 

 of resistance (Williams, 1964, p. 422). 



A few brief remarks on breeding methods are needed as a preface to 

 mass production of rust-resistant pines. Maximum improvement to combine 

 rust resistance with other commercially essential traits will require: 

 (1) Establishment of broad-base gene pools; (2) extensive phenotypic 

 selection in natural populations (and eventually in gene pools) ; (3) 

 large-scale progeny testing; (4) recurrent selection; (5) intraspecific 

 breeding, including selfing and backcrossing; and (6) species hybridiza- 

 tion. 



My suggestions for the establishment of broad-base gene pools have 

 been published (Schreiner, 1968) , and there is now general acceptance of 

 the need for phenotypic (plus tree) selection. 



RECURRENT SELECTION 



Recurrent selection has been defined as "A breeding system involving 

 repeated cycles of selection and recombination with the objective of 

 increasing the frequency of favorable genes for yield or other charac- 

 teristics." Recurrent reciprocal selection is "A recurrent selection 

 breeding system in which two or more geneticlaly divergent groups are 

 maintained and selections from each group are tested for combining ability 

 with the other group or groups in each cycle" (Leonard, Love, and Heath, 

 1968). 



There has been considerable variation in the operational procedure 

 for recurrent selection. In corn breeding, plants selected from a 

 heterozygous source are self-pollinated and are evaluated for some 

 desirable trait or traits. Progenies of the superior plants are propa- 

 gated from the selfed seed and all possible intercrosses among these 

 superior progenies are made by hand or by top-cross or poly-cross methods. 

 The resulting population serves as source material for additional cycles 

 of selection and intercrossing (Allard, 1960). 



Penny, Scott, and Guthrie (1967) reported that, in their recurrent 

 selection for leaf-feeding resistance to the European corn borer in five 

 synthetic varieties, two cycles of selection were sufficient to shift the 



