Tomato

Marker-Assisted Selection

Most economically important traits of crop plants follow a continuous distribution caused by the action and interaction of many genes and various environmental factors. Phenotypic selection (PS) for such "quantitative traits" is the more effective the higher their heritability (h2). For complex characters, such as grain yield, h2 can be improved considerably by increasing the number of test environments and replicates, when evaluating the respective progenies. Molecular marker analysis allows the identification of genome segments, so-called quantitative trait loci (QTL), contributing to the genetic variance of a trait and thus to select superior genotypes at these loci without uncertainties due to genotype by environment interaction and experimental error. Selecting for favorable QTL effects based on marker data (marker-assisted selection, MAS) therefore has great potential for improving quantitative traits. In evaluating the possible impact of MAS it is important to know that in general a quantitative trait is controlled by quite a large number of genes. Evidence for this is provided by long-term selection experiments.

Integration of MAS into Breeding Programs

Presently, MAS can significantly accelerate the improvement of quantitative traits in back cross (BC) programs. It may also be useful for selecting among progenies in advanced generations of a mapping population. In both cases, only verified, important QTL effects should be included in the selection index. Overestimation of environment-specific QTL may reduce the efficacy of the selection index or may even be counter-productive. Including QTL with minor effects adds little to the gain from selection but increases the overall linkage drag (fixation of non-target genes) and thus reduces the potential progress achievable by progeny selection (PS). However, application of MAS to recurrent population improvement or to selection among experimental hybrids or synthetics would require universally (i.e. for a whole population) valid QTL/marker associations. Future progress in functional genomics is expected to provide new approaches.

Marker-assisted selection offers great potential for an accelerated improvement of quantitative traits in crop plants. Theoretical studies showed that (i) combined MAS (based on marker and phenotypic data) is more effective than pure MAS, (ii) the relative efficiency of MAS compared to PS increases as i an p increase and h2 decreases, and (iii) uncertainties of QTL estimation as well as repulsion phase linkage between QTL seriously reduce the superiority of MAS. Experimental studies revealed that (i) MAS is well suited for introgressing exotic germplasm, (ii) MAS is effective in improving materials derived from mapping populations, (iii) pure MAS is equivalent to PS in most practical situations, and (iv) QTL estimates need to be verified before applying them to MAS. Further research is needed (i) to obtain universally valid QTL estimates for elite breeding populations or whole gene pools, (ii) to develop QTL-allele-specific, direct markers based on DNA sequences of the respective genes, and (iii) to optimize the integration of MAS into breeding plans.

Prepared by M. Abhary (mohammedjabri@hotmail.com)

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