General and specific combining ability from partial diallels of radiata pine: implications for utility of SCA in breeding and deployment populations.

Variances for general combining ability (GCA) and specific combining ability (SCA) and the relationship between mid-parental GCA and SCA effects were estimated for tree diameter (DBH) from a series of 20 sets of 6x6 half-diallel mating experiments in radiata pine, planted at ten sites across Australia. Significant SCA variance for DBH was almost equal to GCA variance for the combined analysis of all ten sites. The importance of SCA variance varied among sites, from non-significant to SCA variance accounting for all genetic variation among full-sib families. Significant SCA x site interaction was detected among the ten sites. A significant and positive correlation between mid-parental breeding values and best linear unbiased predictions of the SCA effects was observed. About a quarter of extra genetic gain is achievable through use of SCA variance if selection is based on the best breeding values. To fully exploit genetic gain from SCA variance in a deployment population, positive assortative matings are required for the best parents. It is estimated that the additional deployment gain of 46.0% for ten sites combined, or 52.9% for four sites combined that had significant GCA as well as SCA effects, were achievable relative to gain from GCA only, if all SCA variance within this breeding population was exploited. For a breeding population, selection for breeding values may be sufficient due to positive correlations between breeding values and SCA values. For a deployment population to capture more SCA genetic gain, it is preferable to make more pair-wise mating for parents with higher breeding values.