No abstract

Seed weight (SW) is a component of soybean, Glycine max (L.) Merr., seed yield, as well as an important trait for food-type soybeans. Two soybean populations, 120 F4-derived lines of 'Young'xPI416937 (Pop1) and 111 F2-derived lines of PI97100x'Coker 237' (Pop2), were mapped with RFLP makers to identify quantitative trait loci (QTLs) conditioning SW across environments and populations. The genetic map of Pop1 consisted of 155 loci covering 973 cM, whereas Pop2 involved 153 loci and covered 1600 cM of map distance. For Pop1, the phenotypic data were collected from Plains, GA., Windblow, N.C., and Plymouth, N.C., in 1994. For Pop2, data were collected from Athens, GA., in 1994 and 1995, and Blackville, S.C., in 1995. Based on single-factor analysis of variance (ANOVA), seven and nine independent loci were associated with SW in Pop1 and Pop2, respectively. Together the loci explained 73% of the variability in SW in Pop1 and 74% in Pop2. Transgressive segregation occurred among the progeny in both populations. The marker loci associated with SW were highly consistent across environments and years. Two QTLs on linkage group (LG) F and K were located at similar genomic regions in both populations. The high consistency of QTLs across environments indicates that effective marker-assisted selection is feasible for soybean SW.

Brown stem rot (BSR) of soybean [Glycine max (L.) Merr.] is caused by the fungal pathogen Phialophora gregata (Allington & D.W. Chamberlain) W. Gams and occurs in soybean production areas around the world. Brown stem rot resistance genes Rbs1, Rbs2, and Rbs3 have been identified in soybean germplasm and plant introductions through traditional genetic analyses. Resistance to BSR has been shown to reduce yield losses in soybean, but selection for this trait is laborious and confounded by environmental variation. The objectives of this study were to identify molecular markers linked to BSR resistance genes Rbs1 and Rbs2, and map these genes in the soybean genome. Genetic families of populations segregating for Rbs1 and Rbs2 were evaluated in the greenhouse for BSR phenotypic reaction and identified as resistant, segregating, or susceptible. Leaf tissue collected from members of F2:3 families was bulked and DNA simple sequence repeat (SSR) marker analysis was used to identify markers that cosegregated with BSR reaction phenotypes. Five pairs of Rbs2 near‐isogenic lines were subjected to a similar analysis to verify results obtained from marker analysis conducted on the population segregating for Rbs2 Results of marker analyses indicated that SSR markers Satt215 and Satt431 were linked to Rbs1 and that Satt244 and Satt431 were linked to Rbs2 Marker‐assisted selection in the Rbs1 (using Satt431) and Rbs2 (using Satt244) populations would have correctly predicted 88 and 82%, respectively, of the BSR reaction phenotypes. The Rbs1 and Rbs2 loci map to Molecular Linkage Group J and lie in a region known to contain Rbs3 This region also contains loci conditioning resistance to taxonomically diverse fungal pathogens and a locus affecting nodulation in response to a bacterial symbiont.

In the USA, the southern root-knot nematode [Meloidogyne incognita (Kofoid and White) Chitwood] (Mi) is a serious pathogen soybean [Glycine max (L.) Merrill]. The objectives of this study were the following: (i) to use restriction fragment length polymorphlsm (RFLP) to identify markers associated with quantitative trait loci (QTL) conferring Mi resistance, (ii) to estimate the relative contribution to resistance of each QTL, and (iii) to locate each QTL on the molecular map of the soybean genome. P|96354, a plant introduction with a high level of resistance to Mi, was crossed with 'Bossier', a highly susceptible cultivar, and !10 F2 plants were produced and mapped with 121 RFLP markers. A greenhouse screening procedure was used to determine the level of galling for the F2:~ lines. Data were analyzed using single-factor and two-factor analysis of variance and interval mapping. Two QTL were identified which mapped to LG-O and LG-G of the USDA/ARS-|owa State Univ. (USDA/ARS-ISU) soybean RFLP Map. Results showed a major resistance QTL (R 2 = 31%) linked to marker G248A-1 on LG-O. The second QTL (R 2 = 14%) was Iocalted on LG-G in the interval from K493H-I to Cs008D-I and was dominant with respect to resistance. The two QTL explained 39% of the variation in Mi galling in a multiple QTL model. Marker G248A-I may be closely linked to the Rmil gene. When galling data was classified into three distinct classes, it mapped distally to G248A-1 on LG-O. Mi resistance also was linked to G248A-1 through simulated marker analysis. The root-knot nematode resistance QTL identified were found in duplicated DNA segments.

Javanese root-knot nematode [Meioidogyne javanica (Treub) Chitwood] (Mj) is a pathogen of soybean [Glycine max (L). Merr.] in the southern USA. Although breeding for root-knot nematode resistance is an important objective in many plant breeding programs, progress in selection for nematode resistant lines is hampered by laborious screening procedures. The inheritance of resistance to Mj is quantitative and has a moderate to high heritability (0.48-0.76). The objectives of this study were to use restriction fragment length polymorphism (RFLP) markers to identify quantitative trait loci (QTL) conditioning resistance to Mj and to determine the genomic location and the relative contribution to resistance of each QTL. Eighty-four F2 progen~ from a cross between "CNS" and a root-knot nematode resistant soybean plant introduction, PI230977, were used to map 86 RFLP markers and three morphological traits. The 89 markers converged on 18 linkage groups spanning a total of 1053 centimorgan (cM). Sixty-eight of the 84 F2:3 families were assayed for Mj galling in the greenhouse. Analysis of variance and interval mapping were used to identify QTL associated with galling. Two QTL with alleles derived from PI230977 conditioning resistance to Mj were identified on two Linkage Groups (LG). Marker B212-1 on LG-F accounted for 46% of the variation in gall number, whereas A725-2 on LG-D1 accounted for 13%. The additive model best fit the data, and together the two markers accounted for 54% of the variation in gall number. Marker B212-1 is within a cluster of seven other disease resistance loci that span a 5-to 10-cM region on LG-F. T HE ROOT-KNOT NEMATODES, Meloidogyne spp., are distributed worldwide and have a broad host range that includes most crop plants (Sasser, 1977). Root-knot nematodes (RKN) cause serious damage through tissue