IMPROVED PLANTING STOCK THROUGH SYNTHETIC VARIETIES 575 



INTRASPECIFIC BREEDING 



Selfing 



Selfing will be difficult in some, but not all, Pinus species. On 

 the basis of close observation of 45 Pinus montioola Dougl . trees for 

 periods up to 6 years, Bingham and Squillace (1957) concluded that no 

 phenological barriers to either selfing or crossing existed in the trees 

 under observation. Barnes (1964) reported growth depression from selfing 

 in Pinus montioola, but also highly self-compatible individuals. 



Snyder (1968) is of the opinion that inbreeding will not be practical 

 in Pinus elliottii Engelm. ; among 35 trees, 80 percent produced less than 

 one seedling per self-pollinated flower. He concluded that "...until 

 quicker methods for obtaining homozygous diploids are developed, results 

 from such a system can probably be surpassed by crossing." His opinion 

 was based on the need for two or three generations of selfing followed by 

 the combination of the final lines into Fj's by another generation of 

 breeding. 



One generation of selfing followed by a progeny test is usually 

 sufficient for recurrent selection and for the selection of parents for 

 the establishment of a basic (Syn-0 generation) breeding population for 

 production of a synthetic variety. It is true that homozygosity through 

 selfing or inbreeding is never accomplished in a single step by breeding. 

 In fact it is probable that the complete homozygote can only be obtained 

 by diploidization of a haploid. This would be possible in a short one- 

 step or, at most, two-step process. 



Niizeki and Oono (1968) have produced haploid rice plants from 

 immature pollen grains by culturing anthers on an agar medium about 5 days 

 before anthesis. Hundreds of haploid plants of various species of 

 iHootiana have been produced from cultures of pollen grains by Nitsch 

 and Nitsch (1969). Their method is based on the stimulation of cell divi- 

 sion in immature pollen grains to produce plants from the male prothallus. 



There is increasing interest and research on the production of 

 haploids and their diploidization in forest trees. The role of haploids 

 in tree improvement and forest genetics has been reviewed by Kopecky (1960) 

 Nei (1963), and Stettler (1966). Stettler (personal communication) has 

 reported "successful experimental induction of haploid parthenogenesis..." 

 in Populus trichoaarpa Hook. Winton and Einspahr (1968) have reported on 

 the possible production of aspen haploids using pollen weakened by heat. 

 They note that the recovery of spindly diploid aberrants, as well as 

 glaucous diploids from Populus tremuloides Michx. x Populus alba L. 

 indicates that spontaneous diploidization occurred in maternal haploids. 

 F. A. Valentine (personal communication) has obtained spontaneous 

 diploidization of a monoploid Populus tremuloides. 



Backcross Breeding 



Backcrossing is highly effective for transferring one or two simply 

 inherited characters to an otherwise desirable type. According to Briggs 

 and Allard (1953) : 



