Summary and future outlook
MAS has already proven valuable for backcrossing of major genes into elite parents, using both foreground and background selection. This use is expected to increase as new genes and associated markers for economically important traits are identified. As an example of current opportunities for MAS in wheat, protocols for over 20 trait-associated markers are posted on the web site MAS Wheat: Bringing Genomics to the Wheat Fields (http://maswheat.ucdavis.edu/). Knowledge of actual gene sequences and design of PCR primers for specific alleles (e.g., Ellis et al., 2002), will make MAS more powerful and informative across a range of genetic backgrounds.
The use of MAS for multiple QTLs for complex traits is not as clear-cut (Hospital, 2003). This is due to imprecise estimates of QTL location and effect size, as well as interactions of QTLs with environment and genetic backgrounds. Ongoing improvements in statistical methodologies and experimental design (e.g., larger population sizes grown in more environments) will help remedy this situation. In many cases, integration of MAS for QTLs with phenotypic selection would seem a reasonable approach, but the optimum strategies for accomplishing this are not well developed. Efficient incorporation of MAS into an applied breeding program will likely require a rethinking of the structure and implementation of the entire program (Ribaut and Hoisington, 1998).
Finally, economics will be a major driver of the application of MAS (Dreher et al., 2003; Morris et al., 2003). For certain traits that are expensive or logistically difficult to evaluate, MAS is an attractive alternative. Time savings obtained through MAS may be as important as cost savings where there are competitive markets for improved cultivars. Any cost change in DNA extraction or genotyping methods, or on the other hand, in phenotypic evaluation methods, will affect the relative economic benefits of MAS.
In summary, MAS is a methodology that has already proved its value in some plant breeding situations. It is likely to become more valuable as a larger number of genes are identified and their functions and interactions elucidated. Reduced costs and optimized strategies for integrating MAS with phenotypic selection are needed before the technology can reach its full potential.