Selection Response in Subdivided Target Regions

In a small target region, it may be possible to exploit local adaptation to increase gains from selection. However, in a large region more extensive testing is usually possible, resulting in more precise estimation of genotype means. A correlated response model was adapted to determine if division of a large target region is likely to increase gains. Genotypic value in a large region and constituent subregions are considered correlated traits. Correlated response in a subregion to indirect selection across the undivided region, relative to direct response to selection within the subregion, is expressed as a function of heritability in the undivided region (H) and in the subregion (H(i)), and of the genotypic correlation between region and subregion means (r(G'). r(G') depends on the magnitude of the genotype x subregion interaction (sigma2GS)) relative to the genotypic variance (sigma2(G)). sigma2(GS) is the portion of the genotype x location interaction (sigma2(GL)) caused by local adaptation, rather than by random site-to-site variability in genotype means. Subdivision can increase heritability through the addition of sigma2(GS) to the numerator of H(i), but this may be offset by reduced replication across locations within the subregion. Modeling using variance estimates from several cereal programs indicated that, unless sigma2(GL) is large relative to sigma2(G) and at least 30% of sigma2(GL) is due to sigma2(GS), subdivision is unlikely to increase response. These results help explain the success of breeding programs that test broadly.