Should maize doubled haploids be induced among F(1) or F (2) plants?

Maize (Zea mays L.) doubled haploid lines are typically produced from F(1) plants. Studies have suggested that the low frequency of recombinants in doubled haploids may reduce the response to selection. My objective was to determine if, for sustaining long-term response, doubled haploids should be induced in F(1) or F(2) plants during maize inbred development. In simulation experiments, I examined the response to multiple cycles of testcross selection among doubled haploid lines derived from F(1) plants (denoted by DH), doubled haploid lines derived from F(2) plants (DH(F2)), and recombinant inbred (RI) lines derived by single-seed descent. For a trait controlled by 100 or more quantitative trait loci (QTL), the cumulative responses to selection were up to 4-6% larger among DH(F2) lines than among DH lines. The cumulative responses were up to 5-8% larger among RI lines than among DH lines. The QTL become unlinked as the number of QTL in a finite genome decreases, and the responses among RI, DH, and DH(F2) lines were equal or nearly equal when only 20 QTL controlled the trait. Metabolic-flux epistasis reduced the differences in the response among RI, DH, and DH(F2) lines. Overall, the results indicated that doubled haploids should be induced from F(2) plants rather than from F(1) plants. If year-round nurseries are used and new F(1) crosses for inbred development are initially created on a speculative basis, the development of doubled haploids from F(2) rather than F(1) plants should not cause a delay in inbred development.