Inbreeding in Pinus Radiata – V. The Effects of Inbreeding on Fecundity

A successful inbreeding and hybrid breeding strategy in tree improvement requires that 1) inbreeding (selfing) can produce superior inbred lines (effective purging of deleterious alleles), 2) there is heterosis among crosses of inbred lines, 3) early selection between lines is effective, and 4) inbreeding will not substantially reduce reproductive ability. We have previously reported that inbreeding depression on growth was lower in radiata pine relative to other conifers and segregation in the first two-generations of selfs generated superior inbred trees. In addition, we have observed that early selection among inbred trees (lines) was more effective than in out-crossed populations and there was an apparent heterosis in radiata pine. In this study, the effect of inbreeding on the reproductive ability in young and adult trees of radiata pine has been quantified from five populations of varied inbreeding levels (F =0, 0.125, 0.25, 0.5, and 0.75). It was observed that the effects of inbreeding depression on fecundity was higher at a young age than at older age and inbreeding depression at a young age is due to two factors: 1) a delay of reproductive age (about 8.3, and 8.5% of trees delayed for F =0.5 and F =0.75 populations, respectively) and 2) a true reduction of flowering trees (6.7 and 13.1% more trees having no flowers for F =0.5 and F =0.75 populations than F =0 population, respectively). Despite significant inbreeding depression on the percentage of female reproductive trees and the number of cones on adult trees, overall inbreeding depression on fecundity was low in radiata pine. One founder clone contributed most of the significant inbreeding depression observed for the population of eight founder clones. It was observed that fecundity varied more widely among the eight clones than among the inbreeding level (self and outcross).