Home | Business News | Browse by Publication | C | Crop Science

An assessment of phenotype selection for linolenic acid using genetic markers.

Article Excerpt
A DIFFICULT PROBLEM for the plant breeder has been to discern to what extent a trait is heritable and the result of gene action, and to what extent the trait is the result of an environmental influence. The distinction between what is a genetic vs. an environmental effect for quantitative traits is generally difficult to discern. The ideal situation for selection of a quantitative trait is that the trait has high heritability and the phenotype can be observed in all individuals before reproductive age (Dekkers and Hospital, 2002). Unfortunately, the phenotype for most quantitative traits is an imperfect predictor of the breeding value of an individual. Until recently, breeders' selections on economically important quantitative traits have been based on phenotype, without information on the genetic architecture corresponding to the traits. Knowing the number, positions, and effects of all the genes involved could make trait improvement efficient.

Linkages established between nonfunctional or anonymous markers with quantitative trait loci (QTL) are fairly common, but they are subject to losing their association with the desired phenotype through recombination. Molecular markers for causal mutations for quantitative traits are harder to find than simply inherited traits because of the extensive analysis required for positional cloning or the availability of suitable candidate genes. However, these perfect molecular markers lack the ambiguity associated with QTL-type markers, and thus eliminate the problem of disassociation of the trait from a linked anonymous locus. Marker assisted selection allows selection at the genotype level, or specifically the desired alleles, one time, without the need to track and stabilize phenotypes across several generations. Plants with the desired genotype can be identified before pollination with MAS, allowing breeders to more efficiently cross or backcross. Mutation-specific molecular markers can also be used to validate the genetic constituency of phenotype-based selections made on the assumption of a corresponding desirable genotype.

Omega-3 fatty-acid desaturase genes (FAD3) control seed linolenic acid levels in soybean. Previously in our lab we used database homology searches and gene cloning to identify and characterize three soybean microsomal omega-3 fatty-acid desaturase genes that contribute to seed linolenic acid levels (Bilyeu et al., 2003). The complete coding sequences for three soybean microsomal FAD3 genes have been assembled and are available in GenBank (Benson et al., 2002). We screened the soybean FAD3 homologs for mutations, and developed molecular markers for the low linolenic acid soybean line CX1512-44 that contains defects in the GmFAD3A and GmFAD3C genes (Bilyeu et al., 2003, 2005).

Having a set of molecular markers for the GmFAD3A and GmFAD3C genes afforded us a unique opportunity to conduct a retrospective assessment of the effectiveness of breeding selections for low linolenic acid based on the phenotype of this quantitative trait. Our objective was to assess the accuracy of our phenotypic selections in soybean for linolenic acid using molecular markers specific for mutations in two fatty acid desaturase genes, GmFAD3A and GmFAD3C.

MATERIALS AND METHODS

Soybean Lines

The low linolenic acid soybean line CX1512-44 was previously developed from a mutagenesis breeding program as a selection from the...