Crossing Success as Measured by Production of 

 Mature Female Cones and Filled Seeds on Srs 



On Si's, as compared with outcrosses, about 20 percent fewer of the second- 

 generation inbred crosses attempted resulted in production of mature cones, 30 percent 

 fewer resulted in production of filled seeds, and less than one-fourth as many filled 

 seeds per cone were produced (table 3). Losses of female strobili on Si trees, between 

 the unopened bud stage and mature cone stage, are twice as great (78 vs. 39 percent) as 

 on the related, outcrossed trees (table 3) . Almost two- thirds of this attrition in 

 female strobili (58 of 78 percent) occurs the first year--either. before or shortly after 

 the scales open and the strobili are pollinated (31 percent), or before "setting" of the 

 strobilus at the time of scale swelling and strobilus closure (27 percent) . Apparently, 

 these losses are not due to pollination failures, but reflect the reduced ability of the 

 Si mother trees to support and mature normal strobili. Again, the effect seems to be 

 least pronounced in the Si's from comparatively self-fertile mother trees (25 and 58, 

 vs. 19). 



Once mature cones are borne, filled seed yield may be nil (6 crosses of Si's of 

 mother tree 19), low but consistent (41 crosses of Si's of tree 58), or even consistently 

 good (18 crosses of Si's of tree 25). The average yield from mature cones produced in 

 79 crosses made on Si trees of 15 different lines was 10.0 filled seed per cone. 



In table 3, the crosses of Si's parents are considered only as a group--they are 

 not classified as S2's, Si X full-sib, etc. In table 4, however, crossing success, 

 attrition of female strobili, and mature cone and seed yields are considered according 

 to type of cross. Besides data for the second-generation inbred crosses, table 4 also 

 provides data for a base outcross population as well as for various types of first- 

 generation inbred crosses (half-sib, full-sib, backcross, and Si matings) . Types of 

 matings are arranged in order of increasing inbreeding coefficient. 



All types of first- and second-generation inbred matings produced some mature 

 cones and filled seeds (table 4). In general, where there is a base of more than a 

 few attempted crosses, at higher levels of inbreeding fewer of the crosses produced 

 mature cones, and filled seed yields were lower. The majority of cones from second- 

 generation inbred matings that reach maturity contain filled seeds, however. There is 

 little failure late in the reproductive process. 



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