Markerassisted Selection

Genetic recombination is a result of crossing-over between homologous chromosomes, and from the independent assortment of maternal and paternal chromosomes during meiosis. One cannot make predictions a priori where crossing-over will occur and what the genotype of the resulting progeny will be. Only by making crosses and developing segregating populations can linkage maps be constructed, and predictions made about the transmission and inheritance of genes. Marker-assisted selection (MAS) is a method in which DNA markers are associated with a specific trait or QTL. Markers are generated by crossing genetically and phenotypi-cally dissimilar parents, evaluating the progeny of a segregating population from that mating, and associating a polymorphic marker with the trait. To be useful, the marker must be in close physical proximity to the gene of interest and, ideally, the marker will bracket the gene. Selection efficiency is increased because the DNA from each individual of the population can be assayed for the marker. Based on the presence or absence of the marker, the individual is either included or excluded from the gene pool. MAS allows culling of genetic material at a much earlier stage and linkage drag is minimized because a clearer relationship between genotype and phenotype is established, thereby increasing efficiency by lowering the number of plants required in each filial population. MAS is especially useful in backcross breeding in which single genes or QTLs are introgressed into elite germplasm that lack the trait of interest (Bernacchi et al., 1998; Tanksley and Nelson, 1996).

A distinct advantage of both conventional plant breeding and marker-assisted selection is that metabolic pathways, enzyme structures, or catalytic mechanisms do not need to be well defined (Schmidt-Dannert et al., 2000). It has been a long-held presumption, with no supporting evidence, that increasing the nutrient content might lower crop yield (Ruel and Bouis, 1998). It should be feasible to alter phytochemi-cal or nutrient content without altering yield, because phytochemicals make up a very small percentage of a plant's dry weight (<0.1%) (DellaPenna, 1999). Most species show two- to three-fold differences of phytochemicals and antioxidants (Ruel and Bouis, 1998; Prior et al., 1998; USDA source of phytochemicals, 1998), yet no significant yield differences have been reported. Further, vitamin and micronutrient concentration can be increased severalfold without detriment to human health, with the notable exception of the toxic heavy metals Zn, Cu, and Ni (DellaPenna, 1999). Because of the expense involved with breeding for phytochem-

icals, more scientifically based data are needed which clearly define the relationship between phytochemicals and human health.

0 0

Post a comment