A number of molecular genetic maps of the soybean [Glycine max (L.) Merr.] have been developed over the past 10 yr. These maps are primarily based on restriction fragment length polymorphism (RFLP) markers. Parental surveys have shown that most RFLP loci have only two known alleles. However, because the soybean is an ancient polyploid, RFLP probes typically hybridize and map to more than one position in the genome. Thus, the polymorphic potential of an RFLP probe is primarily a function of the frequency of the two alleles at each locus the probe detects. In contrast, simple sequence repeat (SSR) markers are single locus markers with multiple alleles. The polymorphic potential of an SSR marker is dependent on the number of alleles and their frequencies. Single locus markers provide an unambiguous means of defining linkage group homology across mapping populations. The objective of the work reported here was to develop and map a large set of SSR markers. A total of 606 SSR loci were mapped in one or more of three populations: the USDA/Iowa State G. max × G. soja F2 population, the Univ. of Utah Minsoy × Noir 1 recombinant inbred population, and the Univ. of Nebraska Clark × Harosoy F2 population. Each SSR mapped to a single locus in the genome, with a map order that was essentially identical in all three populations. Many SSR loci were segregating in two or all three populations. Thus, it was relatively simple to align the 20+ linkage groups derived from each of the three populations into a consensus set of 20 homologous linkage groups presumed to correspond to the 20 pairs of soybean chromosomes. On the basis of in situ segregation or linkage reports in the literature all but one of the classical linkage groups can now be assigned to a corresponding molecular linkage group.

Previous studies indicate that methylated cyclitols are potentially important osmolytes in plants. In a search for genetic diversity for pinitol (D‐3‐O‐methyl‐chiro‐inositol) accumulation in soybean (Glycine max (L.) Merr.), two‐ to three‐fold differences in pinitol accumulation in leaf blades were found among Chinese plant introductions. Furthermore, it was found that genotypes that accumulated high concentrations of pinitol, when grown under well‐watered conditions, had been selected for performance in regions of China having low rainfall. Among the carbohydrates analysed, pinitol accumulation was uniquely associated with adaptation to dry areas of China. A detailed study of pinitol accumulation in the soybean plant showed two‐ to three‐fold gradients in pinitol concentration from the bottom to the top of the plant. The gradient shifted during plant development, with consistently higher concentrations of pinitol in the uppermost leaves. Pinitol accumulation was not correlated with activity of the key biosynthetic enzyme, inositol methyl transferase. This result and other lines of evidence indicated that shifting patterns of pinitol accumulation were due to translocation of the cyclitol from lower to upper nodes. Pinitol, proline, and sugars accumulated in leaf blades on soybean plants subjected to drought, but the molar concentration of pinitol in stressed plants was greater than the concentrations of proline or sugars. Although the mechanism by which pinitol participates in drought tolerance is not fully known, our results provide additional correlative evidence linking pinitol and drought tolerance in soybean.

Simple sequence repeat (SSR) markers with linkages to the Rps1, Rps2, Rps3, Rps4, Rps5, and Rps6 loci that govern soybean [Glycine max (L.) Merr.] resistance to Phytophthora root rot (caused by Phytophthora megasperma Drechs. f. sp. glycinea Kuan and Ervin) are desired. Near‐isogenic lines (NILs) of Clark or Williams, homozygous resistant (RpsRps) at just one of those Rps loci, were mated to a NIL of Harosoy homozygous susceptible (rpsrps) at all six loci. From the 100 to 120 F2:3 progenies per mating, 20 F3 seedlings were evaluated for resistance (R) or susceptibility (S) following inoculation with the race of P. megasperma affected by the segregating Rps allele. About 15 RpsRps and 15 rpsrps F2 individuals were used to construct contrasting DNA bulks. Presumptive linkage (i.e., SSR marker polymorphism between two bulks) was confirmed or refuted by SSR assay of 15 to 40 F2 individuals within each homozygous class. Recombination values were maximum likelihood estimates from the SSR allelic segregation data of both classes, although the rpsrps class was less prone to phenotypic classification error. SSRs on linkage groups (LGs) N, J, F, and G were identified with linkages to Rps1, Rps2, Rps3, and Rps4, respectively. A skewed R:S segregation in the Rps5 population precluded detection of linked SSRs. The Rps6 locus, whose map position was heretofore unknown, was linked with three SSRs in a region of LG‐G that contains Rps4 and Rps5 SSR–Rps linkages of P < 0.05 could only be identified for the Rps1 alleles because of a paucity of SSR markers and/or parental monomorphism in the genomic regions surrounding other Rps loci.

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