Forming Double Cross Hybrids Using Two Line Synthetics.

In maize breeding programs in the developing world, double cross hybrids remain of interest due to lower seed costs. However, their production is cumbersome for small seed companies, due to the need to maintain four parental inbreds and to manage the F₁ parent crossing blocks as well as the double-cross seed production field. One way to simplify double cross production is to form hybrids from F₂ bulks or two-line synthetics derived from each single cross parent. In such a scheme, single cross parents would be advanced to F₂  and maintained as two-line synthetics, then crossed to form the certified seed. Allelic frequencies in gametes produced by an F₂ or a two-line synthetic  should be similar to those produced by an equivalent F₁, and therefore the array of genotypes formed from an F₂ x F₂ or two line synthetic cross should be equivalent to that produced by a conventional F₁ x F₁ cross.  Both methods should therefore generate hybrids with similar performance.  We tested this hypothesis in the CIMMYT tropical lowland maize breeding program in 2009. Five conventional double cross hybrids (F1 x F1) and corresponding F2 x F2 crosses were compared in trials at five locations in Southern Mexico. There was no significant difference in yield between F1 x F1 and F2 x F2 crosses. Further trials are being conducted to confirm these findings in 2010 in eastern and southern Africa. Although seed yields on F₂ or two-line synthetic parents will be slightly less than on F₁ seed parents , the method should be of interest to small seed companies in Africa who often have difficulty managing inbred lines, and could help small, local seed companies currently producing only open-pollinated varieties to embark on hybrid seed production.  This, in turn, will increase the availability of low-cost hybrid maize seed to African smallholders.