Sustainability robustness and efficiency of a multi-generation breeding strategy based on within-family clonal selection

Sustainability and robustness of genetic gain and diversity were analysed f or breeding and production populations managed according to the Swedish bre eding programme for Norway spruce. This strategy is based on double-pair ma ting and balanced, within-family selection with clonal testing within a num ber of reproductively isolated breeding populations. The importance of diff erent characteristics was quantified by computer simulation, using a stocha stic quantitative :infinitesimal genetic model. After ten generations, the linear increase in the additive genetic effect f or the baseline scenario reached 11.0 sigma(A) and status effective number was reduced from 48 to 8.1 in a breeding population with 48 members. Six tr ees with status number 3.9 could be selected as seed orchard parents with a predicted inbreeding in the seed crop of less than 5% resulting in a total gain including inbreeding depression of 11.7 sigma(A). Meanwhile, availabl e gain from a similar inbred clone mix of 6 genotypes was 13.1 sigma(A), ma inly as a result of one more cycle of testing. Provided that populations ha d no fewer than 24 members, increases over ten generations in group coances try, inbreeding and inbreeding depression were not substantial, and the dro p in additive gain per generation was negligible. :Reduction of additive va riance and increased inbreeding had no apparent effect on accumulation of g ain per generation, even after ten breeding cycles. Thus, it is concluded t hat the current programme is sustainable. Clonal testing continued to be efficient throughout ten generations at popu lation sizes down to 24, allowing a substructure of two sublines within a p opulation of 48 members. Non-additive variance reduced the rate of increase of the additive mean. A continuous accumulation of additive effects was mo re important than exploitation of non-additive variation, even when cloned planting stock is deployed. Clonal testing was highly effective and robust, even at low numbers of ramets and weak heritability. Low stochastic variat ion among replicate runs indicated high precision, thus predictions of the breeding programme outcome are reliable within the limits of the model.