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 t esting is usually possible, resulting in more precise estimation of genotyp e means. A correlated response model was adapted to determine if division o f a large target region is likely to increase gains. Genotypic value in a l arge region and constituent subregions are considered correlated traits. Co rrelated 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 su bregion means (r(G)) r(G) depends on the magnitude of the genotype x subreg ion interaction tots) relative to the genotypic variance (sigma(G)(2)). sig ma(GS)(2) is the portion of the genotype X location interaction (sigma(GL)( 2)) caused by local adaptation, rather than by random site-to-site variabil ity in genotype means. Subdivision can increase heritability through the ad dition of sigma(GS)(2) to the numerator of H-i, but this may be offset by r educed replication across locations within the subregion, Modeling using va riance estimates from several cereal programs indicated that, unless (TEL i s large relative to sigma(G)(2) and at least 30% of sigma(GL)(2) is due to sigma(GS)(2), subdivision is unlikely to increase response. These results h elp explain the success of breeding programs that test broadly.