Submitted to: International Crop Science Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: September 15, 2004
Publication Date: October 15, 2004
Citation: Holland, J.B. 2004. Implementation of molecular markers for quantitative traits in a breeding program - challenges and opportunities. In: Fischer, T., et al., editors. New directions for a diverse planet: Proceedings for the 4th International Crop Science Congress. September 26-October 1, 2004. Brisbane, Australia. Interpretive Summary: This paper reviews the use of DNA marker technology in applied plant breeding programs. The most common and successful application of DNA markers to plant breeding has been to rapidly transfer single genes between varieties. Marker-assisted selection has been less successful for traits controlled by many genes because accurate identification of the genes remains difficult. At best DNA markers can be useful aids to conventional selection programs, but they cannot replace traditional breeding programs.
Technical Abstract: Molecular marker assisted selection (MAS) has been touted as a means to improve the efficiency and speed of plant selection programs. The most widespread use of MAS to date is to assist backcrossing of major genes into already proven, elite cultivars. Markers can aid selection for target alleles that are not easily assayed in individual plants, minimize linkage drag around the target gene, and reduce the number of generations required to recover a very high percentage of the recurrent parent genetic background. The use of MAS in forward-selection programs is less common and is mainly restricted to marker loci linked to major-effect genes with consistent linkage phase across numerous populations. MAS has not been widely useful for the improvement of polygenic traits because quantitative trait loci (QTL) mapping techniques remain insufficiently precise and because QTL information cannot be easily extrapolated from mapping populations to other breeding populations. The accuracy of QTL mapping can be improved by increasing population sizes and the number of testing environments, but these same techniques also improve conventional phenotypic selection. Therefore, MAS for polygenic traits is mainly restricted situations where phenotypic selection cannot be easily implemented (such as for traits that are difficult to phenotype on individual plants or in offseason nurseries). MAS will remain a specialized breeding tool until QTL mapping can be extended to estimation of breeding values across many diverse breeding crosses and subpopulations such as those that compose typical plant breeding programs.