Genomewide Selection for Rapid Introgression of Exotic Germplasm in Maize

The length of time needed for prebreeding in adapted x exotic maize (Zea mays L.) crosses has deterred breeders from exploiting exotic germplasm. My objective in this study was to determine, by simulation, the usefulness of genomewide selection for the rapid improvement of an adapted x exotic cross. I simulated F(2), BC(1), and BC(2) populations from an adapted x exotic maize cross. The adapted inbred had the favorable allele at L(Adapted) = 50 quantitative trait loci (QTL), whereas the exotic inbred had the favorable allele at L(Exotic) = 50, 25, 10, or 5 QTL. The joint effects of 512 markers were fitted by best linear unbiased prediction. For L(Exotic) <= 25, the maximum responses (in units of the testcross genetic standard deviation) to multiple cycles of genomewide selection ranged from 0.38 to 3.81. Responses increased as heritability increased and as the number of testcrosses that were phenotyped in Cycle 0 increased from 144 to 288. Overall, the results indicated that a useful strategy for the rapid improvement of an adapted x exotic cross involves 7 to 8 cycles of genomewide selection starting in the F(2) rather than in a backcross population, even when the number of favorable alleles is substantially larger in the adapted parent than in the exotic parent. Assuming three generations can be grown per year in a greenhouse or year-round nursery, this procedure would require only 3 yr beyond the time required to develop and phenotype the Cycle 0 testcrosses.