are homozygous and their genotypes can be reproduced by different research groups for repeated experiments Molecular markers provide a rapid approach to breeding for dein a variety of environments (Mather and Jinks, 1977). sired agronomic traits. To use them, it is necessary to determine the linkage between quantitative trait loci (QTLs) and such markers. The

Many agronomic traits of interest to plant breeders are quantitative. Recombinant inbred (RI) lines are particularly useful in genetic mapping studies of quantitative traits. A recombinant inbred population was derived from the Glycine max (L.) Merr. parents ‘Minsoy’ and ‘Noir 1’. This soybean population was used to investigate the genetic basis of several agronomic traits: days to flower (Rl), days to maturity (R8), reproductive period (RS‐R1), plant height, lodging score, height divided by lodging (the ability of tall plants to stand upright), seed protein content, seed oil content, seed size, yield, seed number, yield divided by height (the yield from short plants), leaf width, leaf length, and leaf area. In this RI population, transgressive segregation was observed for all of these traits. As expected, height and lodging were correlated, as were height and maturity; height and maturity with yield; and leaf length and width with leaf area. Height divided by lodging and yield divided by height showed little correlation with other traits, indicating that these traits measured new plant phenotypes. A genetic map was constructed for this population, with restriction fragment length polymorphism markers, simple sequence repeat markers and classical markers. Approximately 2000 cM of linkage was defined. The data were used to identify quantitative trait loci (QTLs) by linking quantitative phenotypes to qualitative genetic markers. for many traits, a few QTLs accounted for a large proportion of the variation observed. QTLs for most of the traits were associated with three linkage groups, often with the same genetic locus within the linkage group. At the level of resolution of the genetic map for this population, it was not possible to determine whether these QTLs have pleiotrophic effects or are clusters of separate, tightly linked genes. The data suggest that separation of QTLs for different traits (such as maturity or lodging and yield) may be difficult, but that this RI population will be useful in resolving questions concerning marker assisted selection of quantitative traits.

Quantitative trait loci (QTL) were mapped in segregating progeny from a cross between two soybean (Glycine max (L.) Merr.) cultivars: 'Minsoy' (PI 27.890) and 'Noir 1' (PI 290.136). The 15 traits analyzed included reproductive, morphological, and seed traits, seed yield and carbon isotope discrimination ratios ((13)C/(12)C). Genetic variation was detected for all of the traits, and transgressive segregation was a common phenomenon. One hundred and thirty-two linked genetic markers and 24 additional unlinked markers were used to locate QTL by interval mapping and one-way analysis of variance, respectively. Quantitative trait loci controlling 11 of the 15 traits studied were localized to intervals in 6 linkage groups. Quantitative trait loci for developmental and morphological traits (R1, R5, R8, plant height, canopy height, leaf area, etc.) tended to be clustered in three intervals, two of which were also associated with seed yield. Quantitative trait loci for seed oil were separated from all the other QTL. Major QTL for maturity and plant height were linked to RFLP markers R79 (31% variation) and G173 (53% variation). Quantitative trait loci associated with unlinked markers included possible loci for seed protein and weight. Linkage between QTL is discussed in relation to the heritability and genetic correlation of the traits.

A large recombinant inbred population of soybean has been characterized for 220 restriction fragmentlength polymorphism (RFLP) unlinked locus (which, by itself, is not associated with trait variation). This type of interaction is shown schematically in Fig. 1. In this example, a difference in height produced by different alleles at locus 1 is conditional upon the presence of a Noir 1 allele at locus 2. Only the RI lines with the Noir 1 allele at locus 2 will show differences in height produced by the first locus. To identify such pairs of loci, we chose as the first locus QTL that had been found to explain significant variation in a quantitative trait. We then scanned through unlinked second loci, dividing the population of RI lines into four genotypic classes by pairing each of the alleles at the first locus with each of the alleles at the second locus. These pairwise comparisons have identified second loci with substantial epistatic effects on plant height and on other traits such as seed yield. MATERIALS AND METHODSThe soybean RI population used here and the measurement of the traits have been described in detail (7). Briefly