IMPROVED PLANTING STOCK THROUGH SYNTHETIC VARIETIES 579 



mechanically to form the variety for release to seed growers. Borlaug 

 (1966) reported that multilineal varieties of wheat had been developed by 

 the Mexican and Colombian wheat programs. He also described the possi- 

 bilities for incorporating more lasting rust resistance into wheat through 

 the development of multilineal hybrid varieties by the use of recently 

 found cytoplasmic sterility systems and fertility restorers. 



Sprague (196~) agreed that a mechanical mixture of morphologically 

 similar substrains, each possessing resistance to one or more of the 

 prevalent races of rust, would insure a less rapid buildup of new rust 

 biotypes. He states that, "although this procedure has not been fully 

 evaluated, preliminary' evidence indicates some measure of effectiveness." 



The use of seed blends, analogous to multilineal varieties of rust- 

 resistant cereals, must await the development of practical methods for 

 the production of rust-resistant lines of forest trees. Furthermore, 

 the development of lines resistant to particular races of the rust will 

 not be possible until races of tree rusts have been identified. New 

 races of rust would not be expected to develop until there are extensive 

 plantations of a rust-resistant variety. 



SYNTHETIC VARIETIES 



The product of forest tree seed orchards will be synthetic varieties 

 basically analogous to those developed for outbreeding agricultural crops, 



A synthetic variety can be defined as the "Advanced generation 

 progenies of a number of clones or lines (or of hybrids among them) 

 obtained by open pollination" (Leonard, Love, and Heath, 1968). Allard 

 (1960) has defined the general concept of a synthetic variety in more 

 detail : 



"A synthetic variety is one that has been synthesized from all 

 possible intercrosses among a number of selected genotypes; 

 thereby a population is obtained that is propagated subsequently 

 from open-pcllinated seed. The essential difference between a 

 variety developed by mass selection and a synthetic variety 

 hinges on the way in which the genotypes to be compounded into 

 the new variety are selected. In mass selection, the next genera- 

 tion is propagated from a composite of the seed from phenotyp- 

 ically desirable plants selected from the source population. A 

 synthetic variety is made up of genotypes which have previously 

 been tested for their ability to produce superior progeny when 

 crossed in all combinations. Also, in mass selection, the male 

 gametes represent a, more or less, random sample from the entire 

 previous generation, whereas, in a synthetic variety, pollination 

 is controlled so that the gene frequencies of the selected mate- 

 rials is maintained in the male as well as in the female lineage." 



Sprague and Jenkins (1943) defined a multiple cross as the first 

 generation of a cross containing more than four inbred lines and a 

 synthetic variety as the advanced generations of such a combination 

 maintained by mass selection. 



There is considerable diversity of evidence (and opinion) on the 

 breeding procedure and performance of agricultural synthetics, particu- 

 larly of maize and forage crops. 



