Optimization of the Marker‐Based Procedures for Pyramiding Genes from Multiple Donor Lines: I. Schedule of Crossing between the Donor Lines

Recent exploitation of DNA markers of desirable trait genes facilitates construction of high‐degree, gene‐pyramided lines via assembling markers from multiple donor lines. In such a program, a plant that has all the target markers in a heterozygous state must be produced first. Efficient procedures for that are discussed. When pyramiding the genes onto the genetic background of a particular recipient line, the backcross should be performed separately for each donor before the crossing between the donors. The plants produced through the backcross should be crossed in a schedule with structure and disposition of the plants as symmetric as possible. When four such plants (A, B, C, and D) are produced, for instance, they should be crossed in a schedule like (A × B) × (C × D) in which the number of target markers of A plus B should be as similar as possible to that of C plus D. Ideal‐type schedules in the presence of four to eight donors are presented. A contrastingly different guideline applies when the donors themselves are crossed without the backcross; they should be crossed in a schedule with completely tandem structure in which donors with fewer target markers enter the schedule in earlier stages. The disposition of donors in the schedule should be modified in the presence of linked or redundant markers. Donors should be disposed in a pattern to minimize the occurrence of repulsion linkages. Formulae for the modification under a high redundancy are presented.