No abstract

Nutrient solution systems have been considered an alternative method to field evaluations for studies of iron deficiency chlorosis (IDC) and for breeding soybeans with improved iron efficiency. To map genes controlling IDC in nutrient solution, 120 F 24 lines in a Pride B216 × A15 population, and 92 F 24 lines in an Anoka × A7 population were grown in nutrient solution in greenhouse plantings and evaluated for IDC by visual scores and determinations of chlorophyll concentrations. Eighty-nine restriction fragment length polymorphism (RFLP) and 10 simple 1916 LINETAL.sequence repeat (SSR) markers in the Pride B216 x A15 population, and 82 RFLP, 14 SSR and one morphological (hilum color) markers in the Anoka x A7 population were used to construct linkage maps and to locate quantitative trait loci (QTL) controlling IDC. In the Anoka × A7 population, one major gene on linkage group N, and modifying QTL on linkage groups A1 and I previously mapped during field tests also were identified in the nutrient solution test. Different genetic linkage groups in soybean have been identified by letters, and as such will be used throughout the paper. One newly identified QTL was mapped on linkage group B2. In the Pride B216 × A15 population, one QTL previously mapped on linkage group I during field tests was not identified in the nutrient solution test, and two newly identified QTL were mapped on linkage groups A2 and B1. QTL on linkage groups B2, G, H, L, and N were identified in both field and nutrient solution tests. Due to significant interaction between genotype and environment in both field and nutrient solution tests, QTL identifications from multiple environments were used to compare the similarity between field and nutrient solution tests. We concluded that similar QTL are identified in nutrient solution and field tests and therefore, both systems identify similar genetic mechanisms of iron uptake and/or utilization.

An analysis of the genome structure of soybean cultivars was conducted to determine if cultivars are composed of large regions of chromosomes inherited intact from one parent (indicative of minimal recombination) or if the chromosomes are a mixture of one parent's DNA interspersed with the DNA from the other parent (indicative of maximal recombination). Twenty-one single-cross-derived and 5 single-backcross-derived soybean cultivars and their immediate parents (47 genotypes) were analyzed at 89 RFLP loci to determine the minimal number and distribution of recombination events detected. Cultivars derived from single-cross and single-backcross breeding programs showed an average of 5.2 and 8.0 recombination events per cultivar, respectively. A homogeneity Chi-square test based upon a Poisson distribution of recombination events across 13 linkage groups indicated that the number of recombinations observed among linkage groups was random for the single-cross cultivars, but not for the single-backcross-derived cultivars. A twotailed t-test demonstrated that for some linkage groups, the number of recombinations per map unit exceeded the confidence interval developed from a t-distribution of recombinations standardized for map unit distance. Paired t-tests of the number of recombinations observed between linkage-group ends and the mid-portion of the linkage groups indicated that during the development of the cultivars analyzed in this study more recombinations were associated with the ends of linkage groups than with the middle region. Detailed analysis of each linkage group revealed that large portions of linkage groups D, F, and G were inherited intact from one parent in several cultivars. A portion of linkage group G, in contrast, showed more recombination events than expected, based on genetic distance. These analyses suggest that breeders may have selected against recombination events where agronomically favorable combinations of alleles are present in one parent, and for recombination in areas where agronomically favorable combinations of alleles are not present in either parent.