North American Soybean Cultivars Research Articles

Introgression of a Danbaekkong high-protein allele across different genetic backgrounds in soybean.

Soybean meal is a major component of livestock feed due to its high content and quality of protein. Understanding the genetic control of protein is essential to develop new cultivars with improved meal protein. Previously, a genomic region on chromosome 20 significantly associated with elevated protein content was identified in the cultivar Danbaekkong. The present research aimed to introgress the Danbaekkong high-protein allele into elite lines with different genetic backgrounds by developing and deploying robust DNA markers. A multiparent population consisting of 10 F5-derived populations with a total of 1,115 recombinant inbred lines (RILs) was developed using "Benning HP" as the donor parent of the Danbaekkong high-protein allele. A new functional marker targeting the 321-bp insertion in the gene Glyma.20g085100 was developed and used to track the Danbaekkong high-protein allele across the different populations and enable assessment of its effect and stability. Across all populations, the high-protein allele consistently increased the content, with an increase of 3.3% in seed protein. A total of 103 RILs were selected from the multiparent population for yield testing in five environments to assess the impact of the high-protein allele on yield and to enable the selection of new breeding lines with high protein and high yield. The results indicated that the high-protein allele impacts yield negatively in general; however, it is possible to select high-yielding lines with high protein content. An analysis of inheritance of the Chr 20 high-protein allele in Danbaekkong indicated that it originated from a Glycine soja line (PI 163453) and is the same as other G. soja lines studied. A survey of the distribution of the allele across 79 G. soja accessions and 35 Glycine max ancestors of North American soybean cultivars showed that the high-protein allele is present in all G. soja lines evaluated but not in any of the 35 North American soybean ancestors. These results demonstrate that G. soja accessions are a valuable source of favorable alleles for improvement of protein composition.

Aggressiveness of isolates of five Pythium species on seeds and seedlings of six North American soybean cultivars

Several species of Pythium cause seed rot and damping-off of soybean in the United States and Canada. The aggressiveness of 14 isolates representing five Pythium species was evaluated on six North American soybean (Glycine max) cultivars based on seed disease in a Petri plate assay and on seedling emergence, plant and root weights, and root rot severity in temperature-controlled greenhouse experiments. In the in vitro assay, the P. aphanidermatum, P. ultimum var. ultimum and P. spinosum isolates caused higher disease severity on germinating seeds than the P. sylvaticum and P. irregulare isolates. In the greenhouse inoculum layer assays, all isolates reduced emergence and tissue weights of at least some cultivars, with significant isolate × cultivar interactions. The aggressiveness of isolates within each species varied significantly based on all or most of the disease parameters measured. At least one isolate from each species reduced emergence by at least 50% on some cultivars, but Pythium ultimum var. ultimum isolate PU 350 caused the most damping-off, stunting and root rot overall. There was a significant negative correlation between root rot severity or seedling emergence (r = −0.93, P < 0.001). ‘Archer’ and ‘Maple Glen’ were more resistant overall than ‘Conrad’, ‘Maple Isle’, ‘Sloan’, ‘Williams’, but some isolates were not very aggressive on any of the cultivars. This study revealed significant variation in the relative ability of the 14 isolates to cause disease and illustrated the importance of identifying aggressive isolates of Pythium species for applications like resistance screening or genetic mapping studies.

Selection of GmSWEET39 for oil and protein improvement in soybean.

Soybean [Glycine max (L.) Merr.] was domesticated from wild soybean (G. soja Sieb. and Zucc.) and has been further improved as a dual-use seed crop to provide highly valuable oil and protein for food, feed, and industrial applications. However, the underlying genetic and molecular basis remains less understood. Having combined high-confidence bi-parental linkage mapping with high-resolution association analysis based on 631 whole sequenced genomes, we mapped major soybean protein and oil QTLs on chromosome15 to a sugar transporter gene (GmSWEET39). A two-nucleotide CC deletion truncating C-terminus of GmSWEET39 was strongly associated with high seed oil and low seed protein, suggesting its pleiotropic effect on protein and oil content. GmSWEET39 was predominantly expressed in parenchyma and integument of the seed coat, and likely regulates oil and protein accumulation by affecting sugar delivery from maternal seed coat to the filial embryo. We demonstrated that GmSWEET39 has a dual function for both oil and protein improvement and undergoes two different paths of artificial selection. A CC deletion (CC-) haplotype H1 has been intensively selected during domestication and extensively used in soybean improvement worldwide. H1 is fixed in North American soybean cultivars. The protein-favored (CC+) haplotype H3 still undergoes ongoing selection, reflecting its sustainable role for soybean protein improvement. The comprehensive knowledge on the molecular basis underlying the major QTL and GmSWEET39 haplotypes associated with soybean improvement would be valuable to design new strategies for soybean seed quality improvement using molecular breeding and biotechnological approaches.

Characterizing the impact of an exotic soybean line on elite cultivar development.

The genetic diversity of North American soybean cultivars has been largely influenced by a small number of ancestors. High yielding breeding lines that possess exotic pedigrees have been developed, but identifying beneficial exotic alleles has been difficult as a result of complex interactions of yield alleles with genetic backgrounds and environments as well as the highly quantitative nature of yield. PI 416937 has been utilized in the development of many high yielding lines that have been entered into the USDA Southern States Uniform Tests over the past ~20 years. The primary goal of this research was to identify genomic regions under breeding selection from PI 416937 and introduce a methodology for identifying and potentially utilizing beneficial diversity from lines prevalent in the ancestry of elite cultivars. Utilizing SoySNP50K Infinium BeadChips, 52 high yielding PI 416937-derived lines as well as their parents were genotyped to identify PI 416937 alleles under breeding selection. Nine genomic regions across three chromosomes and 17 genomic regions across seven chromosomes were identified where PI 416937 alleles were under positive or negative selection. Minimal significant associations between PI 416937 alleles and yield were observed in replicated yield trials of five RIL populations, highlighting the difficulty of consistently detecting yield associations.

Evaluation of Major Ancestors of North American Soybean Cultivars for Resistance to Three Pythium Species that Cause Seedling Blight.

Pythium seedling blight, which can be caused by a number of Pythium spp., is a disease that affects soybean (Glycine max) in the United States and Canada. Pythium ultimum var. ultimum, one of the most common pathogenic species, is favored by cool, wet conditions in early spring and causes seed decay, root rot, and seedling damping-off. In all, 102 major ancestors of modern North American cultivars and "first progeny" cultivars developed directly from ancestral lines were evaluated for resistance to P. ultimum var. ultimum and two other species of Pythium in greenhouse assays. Several ancestors and first progeny cultivars, as well as the resistant check Archer, had varying levels of partial resistance to an Illinois isolate of P. ultimum var. ultimum. In a subsequent experiment, four of the most resistant lines (PI 84637, Maple Isle, Fiskeby III, and Fiskeby 840-7-3) and the susceptible cultivar Kanro were screened for resistance against isolates of P. irregulare and P. sylvaticum, and resistance to P. ultimum var. ultimum was confirmed. The lines that were partially resistant to P. ultimum var. ultimum in the first experiment were also partially resistant to P. irregulare and P. sylvaticum. The P. ultimum var. ultimum isolate was the most aggressive of the three isolates, followed by the P. irregulare and P. sylvaticum isolates. Modern cultivars descended from the soybean lines with partial resistance to these pathogens could be useful sources of resistance to Pythium seedling blight if they are found to have similar levels of resistance.

Identification of Quantitative Trait Loci Conditioning Partial Resistance to Phytophthora sojae in Soybean PI 407861A

ABSTRACTImproving resistance for Phytophthora root and stem rot is an important goal in soybean [Glycine max (L.) Merr.] breeding. The objective of this study was to identify quantitative trait loci (QTLs) conferring partial resistance to Phytophthora sojae in an OX20‐8 × PI 407861A cross, in which PI 407861A was used as the source of partial resistance. One hundred fifty‐seven F7–derived recombinant inbred lines (RILs) were evaluated for partial resistance to P. sojae isolate OH25 using a tray test. Composite interval mapping identified three QTLs on chromosomes 8, 13, and 15 by a genomewide logarithm of odds (LOD) threshold and six QTLs on chromosomes 3, 4, 10, and 18 by chromosomewide LOD threshold. The phenotypic variance explained by each QTL ranged from 2.4 to 8.6%, with a total of 41.6%. Quantitative trait loci on chromosomes 3 and 8 are novel QTLs first reported for partial resistance to P. sojae in this study. All nine QTLs were localized near known resistance gene rich regions or those previously reported for resistance to other soil‐borne pathogens. Genomewide distribution of the QTLs in this population was distinct from the QTLs identified from previous studies with ‘Conrad’ but many were in common with those identified with another South Korean soybean accession. This indicates that the underlying genes for partial resistance in these exotic accessions may be distinct from genes controlling variation for partial resistance observed in Conrad. Plant Introduction 407861A is a promising genetic source of alleles that can be used to increase the genetic diversity and enhance levels of partial resistance to P. sojae in North American soybean cultivars.

Accumulation of Leginsulin, a Hormone‐Like Bioactive Peptide, is Drastically Higher in Asian than in North American Soybean Accessions

ABSTRACTLeginsulin, a peptide made up of 37 amino acids, is homologous to pea (Pisum sativum L.) albumin (PA1b) and belongs to the cysteine‐knot family. Even though the physiological function and three‐dimensional structure of leginsulin have been explored, little is known about its expression, accumulation, and distribution among soybean [Glycine max (L.) Merr.] accessions. An antibody generated against leginsulin was used to screen a diverse array of soybean accessions from the USDA Soybean Germplasm Collection to identify soybean accessions that are enriched in leginsulin. Analysis of 50% isopropanol‐soluble proteins from 485 soybean lines by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS‐PAGE) and western blot analysis enabled the identification of 75 accessions that significantly accumulated leginsulin. Remarkably, all these accessions with the exception of two had their origin from Japan, Korea, or China. Leginsulin was barely detected in most of the commercial North American soybean cultivars used in this study. Western blot analysis revealed low accumulation of leginsulin in the embryonic axis but not in the cotyledons of North American soybean cultivar Williams 82 while in the Chinese PI 458249 leginsulin was abundantly present in both types of tissue. Examination of the soybean cultivar Williams 82 genome sequence revealed the presence of two homologous leginsulin genes (Gm13 g26330 [leginsulin 1] and Gm13 g26340 [leginsulin 2]) on chromosome 13. We have cloned the two leginsulin genes from PI 458249 and found them to be highly similar to that of Williams 82. Northern blot analysis indicated that leginsulin messenger RNA (mRNA) was abundant in Williams 82 embryonic axis but not in the cotyledon. In contrast, leginsulin mRNA was abundantly present in PI 458249, both in the embryonic axis and the cotyledons.

Genetic Diversity of North American Soybean (Glycine max L.) Cultivars as Revealed by RAPD Markers

Genetic Diversity of North American Soybean (Glycine max L.) Cultivars as Revealed by RAPD Markers

Genetic Diversity and Agronomic Improvement of North American Soybean Germplasm

From 1970 to 2008 there were 2242 soybean [Glycine max (L.) Merr.] cultivars registered in North America through U.S. Plant Variety Protection (PVP), U.S. utility patent, and journal registration. Of these, 80% were developed through proprietary and 20% through public programs. Our objective was to characterize the development and genetic diversity of North American soybean cultivars. The most frequently used germplasm for cultivar development were the cultivars Williams (parent used in last cross before inbreeding in 70 cultivars), A3127 (63), Essex (45), Amsoy (38), Corsoy (33), Wayne (30), Forrest (27), Hutcheson (25), MO13404 (23), and Bedford (23). Genetic diversity (1 − coefficient of parentage), estimated from pedigree lineage, was 0.89 overall. Genetic diversity was the same within public (0.89) and proprietary (0.89) cultivars. The cultivar A3127 is a major progenitor of recently developed proprietary cultivars registered from 1999 to 2008. Of these 494 cultivars, 23% have a genetic contribution of at least 25% from A3127. New cultivars were predominantly developed from the following crosses: two‐parent (70% of cultivars developed), complex (12%), three‐parent (5%), one backcross (5%), multiple (two, three, or four) backcrosses (3%), and five or greater backcrosses (2%). In comparisons where both parent and progeny were evaluated together, seed yield increased 3.2% per breeding cycle. In these comparisons, seed yield had a correlation of 0.29 with parental diversity.

Disruption of the Glycine Cleavage System Enables Sinorhizobium fredii USDA257 To Form Nitrogen-Fixing Nodules on Agronomically Improved North American Soybean Cultivars

The symbiosis between Sinorhizobium fredii USDA257 and soybean [Glycine max (L.) Merr.] exhibits a high degree of cultivar specificity. USDA257 nodulates primitive soybean cultivars but fails to nodulate agronomically improved cultivars such as McCall. In this study we provide evidence for the involvement of a new genetic locus that controls soybean cultivar specificity. This locus was identified in USDA257 by Tn5 transposon mutagenesis followed by nodulation screening on McCall soybean. We have cloned the region corresponding to the site of Tn5 insertion and found that it lies within a 1.5-kb EcoRI fragment. DNA sequence analysis of this fragment and an adjacent 4.4-kb region identified an operon made up of three open reading frames encoding proteins of deduced molecular masses of 41, 13, and 104 kDa, respectively. These proteins revealed significant amino acid homology to glycine cleavage (gcv) system T, H, and P proteins of Escherichia coli and other organisms. Southern blot analysis revealed the presence of similar sequences in diverse rhizobia. Measurement of beta-galactosidase activity of a USDA257 strain containing a transcriptional fusion of gcvT promoter sequences to the lacZ gene revealed that the USDA257 gcvTHP operon was inducible by glycine. Inactivation of either gcvT or gcvP of USDA257 enabled the mutant to nodulate several agronomically improved North American soybean cultivars. These nodules revealed anatomical features typical of determinate nodules, with numerous bacteroids within the infected cells. Unlike for the previously characterized soybean cultivar specificity locus nolBTUVW, inactivation of the gcv locus had no discernible effect on the secretion of nodulation outer proteins of USDA257.

IRH1–the first aromatic hybrid rice in Iran

Increasing rice production in Iran, a country with limited land and water resources, is a challenging task, especially with consumer demand largely favoring varieties with superior grain quality. In 2000, Iran imported 1.2 million t of rice. But, in recent years, with increased adoption of improved varieties and with favorable climatic conditions, importation has been reduced to 0.98 million t (FAO 2006). However, with recent fluctuations in the availability of surplus rice from rice-exporting countries and the increasing rice demand in Asia alone, Iran has to initiate a well-drawn plan to attain self-sufficiency and sustain its rice production in the long term. Among the available approaches, the most viable is to pursue hybrid rice technology for Iran, on the basis of success achieved in India, Vietnam, and the Philippines. Hybrid rice technology could easily deliver hybrids with medium quality in the short term, with a 1.5−2.0 t ha–1 advantage over improved check varieties (such as Khazar), while superior quality with high yield can be combined in the hybrids, provided a systematic approach is followed. A comparative yield trial in 2002 involving four promising hybrids and their parents and inbred check variety Khazar revealed exceptional performance of the hybrid combination IR58025A/IR42686R. This hybrid gave the highest standard heterosis (57.9%), heterobeltiosis (53.3%), and commercial heterosis (45.2%) over improved check variety Khazar (Table 1). Furthermore, the number of days to 50% flowering of the parents differed by only 3, with the R line flowering earlier and having a relatively better quality than Khazar (data not shown). A pooled analysis of variance for the adaptive yield trial conducted over two different sites for 2003-04 showed significant yield differences among genotypes and in site-by-genotype-by-year interaction. IR58025A/IR42686R gave an average yield of 9.2 t ha–1 with a yield advantage of 28% over Khazar (Table 1). It was named IRH1. On-farm trials conducted in 2005 at four locations gave an impressive yield of 12 t ha–1, which was 85% higher than Khazar’s 6.5 t ha–1 (Table 2). The hybrids planning breeding programs to control genetic erosion and vulnerability, which is a great possibility because of the observed genetic uniformity. Reshuffling the genetic constituents is also necessary to increase yield potential. References Delannax X, Todgers DM, Palmer RG. 1983. Relative genetic contribution among ancestral lines to North American soybean cultivars. Crop Sci. 23:944-949. Dilday RH. 1990. Contribution of ancestral lines in the development of new cultivars of rice. Crop Sci. 30:905-911. Martin JM, Balke TK, Hockett EA. 1991. Diversity among North American spring barley cultivars based on coefficients of parentage. Crop Sci. 31:1131-1137. St. Martin SK. 1982. Effective population size for the soybean improvement program in maturity groups 00 to IV. Crop Sci. 22:151152.

Resistance to the Soybean Aphid in Soybean Germplasm

With an efficient greenhouse screening method, the first resistance to the soybean aphid (Aphis glycines Matsumura) was found in cultivated soybean [Glycine max (L.) Merr.] germplasm. No resistance was found in 1425 current North American soybean cultivars, 106 Maturity Group (MG) 000 through VII Asian cultivars, and in a set of 11 ‘Clark’ isolines possessing different pubescence traits. Dense pubescence did not provide protection against the soybean aphid. Resistance was discovered and established in three ancestors of North American genotypes: ‘Dowling’, ‘Jackson’, and PI 71506. Expression of resistance in those genotypes was characterized in choice and nonchoice tests. In choice tests, significantly fewer aphids occurred on Dowling, Jackson, and PI 71506 plants compared with susceptible cultivars (P = 0.05). Aphid populations did not develop on Dowling and Jackson in nonchoice tests, indicating that there was a negative impact on aphid fecundity on those cultivars. That evidence combined with observations of aphid mortality on those cultivars suggested that antibiosis‐type resistance contributed to the expression of resistance. Possible donors of resistance to Dowling and Jackson were identified. In nonchoice tests, population development on PI 71506 was not significantly different from development on susceptible cultivars, indicating that antixenosis was more important in that genotype. Resistance was expressed in all plant stages. Dowling provided season‐long protection against aphids equal to the use of the systemic insecticide imidacloprid {1‐[(6‐Chloro‐3‐pyridinyl)methyl]‐N‐nitro‐2‐imidazolidinimine} in a field test. Four other germplasm accessions, ‘Sugao Zarai’, ‘Sato’, ‘T260H’, and PI 230977, had levels of resistance not significantly different from Dowling, Jackson, and PI 71506 in a choice test (P = 0.05).

Resistance to the Soybean Aphid in Soybean Germplasm

With an efficient greenhouse screening method, the first resistance to the soybean aphid (Aphis glycines Matsumura) was found in cultivated soybean [Glycine max (L.) Merr.] germplasm. No resistance was found in 1425 current North American soybean cultivars, 106 Maturity Group (MG) 000 through VII Asian cultivars, and in a set of 11 ‘Clark’ isolines possessing different pubescence traits. Dense pubescence did not provide protection against the soybean aphid. Resistance was discovered and established in three ancestors of North American genotypes: ‘Dowling’, ‘Jackson’, and PI 71506. Expression of resistance in those genotypes was characterized in choice and nonchoice tests. In choice tests, significantly fewer aphids occurred on Dowling, Jackson, and PI 71506 plants compared with susceptible cultivars (P = 0.05). Aphid populations did not develop on Dowling and Jackson in nonchoice tests, indicating that there was a negative impact on aphid fecundity on those cultivars. That evidence combined with observations of aphid mortality on those cultivars suggested that antibiosis-type resistance contributed to the expression of resistance. Possible donors of resistance to Dowling and Jackson were identified. In nonchoice tests, population development on PI 71506 was not significantly different from development on susceptible cultivars, indicating that antixenosis was more important in that genotype. Resistance was expressed in all plant stages. Dowling provided season-long protection against aphids equal to the use of the systemic insecticide imidacloprid {1-[(6-Chloro-3-pyridinyl)methyl]-N-nitro-2-imidazolidinimine} in a field test. Four other germplasm accessions, ‘Sugao Zarai’, ‘Sato’, ‘T260H’, and PI 230977, had levels of resistance not significantly different from Dowling, Jackson, and PI 71506 in a choice test (P = 0.05).

Genetic Diversity of Soybean Cultivars from China, Japan, North America, and North American Ancestral Lines Determined by Amplified Fragment Length Polymorphism

Asian soybean [Glycine max (L.) Merr.] improvement programs have been conducted for many years almost completely independent of U.S. breeding programs. Productive, modern Asian cultivars may be a promising source of new yield genes for U.S. breeding programs. However, this hypothesis has not been tested. The objectives of this study were to determine the level of genetic diversity within and between Asian and North American soybean cultivars (NASC) by amplified fragment length polymorphism (AFLP) analysis and to identify Asian cultivars with significant genetic difference from NASC. The genetic diversity and relationships were assessed among 35 North American soybean ancestors (NASA), 66 high yielding NASC, 59 modern Chinese cultivars, and 30 modern Japanese cultivars. Five AFLP primer‐pairs produced 90 polymorphic (27%) and 242 monomorphic AFLP fragments. Polymorphic information content (PIC) scores ranged from zero to 0.50. Only 53 of the 332 AFLP fragments provided PIC scores ≥0.30. Genetic distance (GD) between pairs of genotypes was calculated on the basis of the similarity indices determined by the 332 AFLP fragments. Within each of the cultivar groups, the average GD between pairs of genotypes was 6.3% among the Japanese cultivars, 7.1% among the NASC, 7.3% among the NASA, and 7.5% among the Chinese cultivars. The average GD between the NASC and the Chinese cultivars was 8.5% and between the NASC and the Japanese cultivars was 8.9%. Although these distances were not significantly different, they were greater than the average GD between all pairs of NASC (7.1%). Clustering and principal coordinate analysis using all 332 fragments showed a separation of the cultivars into three major groups according to their geographic origin. North American soybean ancestors overlapped with all three cultivar groups. The Japanese cultivars were more removed from NASA and NASC than the Chinese cultivars and may constitute a genetically distinct source of useful genes for yield improvement of NASC.

Phenotypic Diversity of Modern Chinese and North American Soybean Cultivars

Chinese and North American (NA) soybean breeding programs have a 70‐yr history of genetic progress in relative isolation from each other. Because both programs rest upon a genetic base that is primarily Chinese in origin, the actual genetic distinctness of Chinese and NA breeding is not clear. The objectives of this study were to (i) develop a phenotypic similarity (PS) index for a large group of Chinese and NA cultivars, on the basis of biochemical, morphological, and agronomic traits, (ii) compare Chinese and NA cultivars for PS through cluster analysis, and (iii) use results to develop guidelines for management of the contrasting Chinese and NA breeding programs as reservoirs of diversity. Chinese (47) and NA (25) cultivars were evaluated for 25 traits in growth chambers. Traits pleiotropic to maturity were avoided. Significant (P &lt; 0.05) differences between Chinese and NA cultivars were noted for leaf and seed traits. Multivariate analysis captured 79% of the total genotypic variation among the 72 cultivars and was used to develop PS estimates. Cluster analysis of PS showed a much greater phenotypic diversity among Chinese than among NA cultivars and a striking distinctness between the two groups. The contrasting nature of Chinese and NA cultivars in this study is theorized to reflect that (i) the NA cultivars may trace to a subset of the Chinese cultivar genetic base, and/or (ii) Chinese and NA cultivars may have diverged phenotypically via breeder selection pressure. Cluster results here, based on PS, agreed roughly with previous cluster analyses, which were derived from pedigree analysis. The physical distinctness of NA and Chinese cultivars shows that introgression of Chinese cultivars into NA breeding should broaden NA germplasm's agronomic, morphological, and biochemical diversity. Introgression may be accomplished most effectively by avoiding matings of Chinese and NA cultivars from the same phenotypic cluster.

Respiration as measured by scanning calorimetry reflects the temperature dependence of different soybean cultivars

Soybean [ Glycine max (L.) Merr.] is grown in many countries and climates. Twenty-two North American soybean cultivars from six different maturity groups were grown from seed under the same conditions. Measurement of metabolic heat rate of leaf tissue with a scanning calorimeter revealed that the slope of heat rate versus temperature when plotted on Arrhenius axes showed abrupt changes reflecting changes in metabolism. The chilling response temperature for all cultivars was near 17.5°C. The maximum tolerable temperature for all cultivars was near 43.5°C. Differences in response to temperatures between the extremes relate to maturity group, follow latitudinal trends, and represent adaptation to different climates. Smaller differences were also observed between cultivars in the same maturity group. Selection of cultivars of soybean for best growth in different climates has resulted in relatively rapid adaptation to local temperatures.

Utilization of Diverse Germplasm for Soybean Yield Improvement

Introgression of diverse germplasm into the current soybean [Glycine max (L.) Merr.J genetic base may increase genetic variability and lead to greater gains from selection. The objective of this research was to evaluate the genetic diversity and agronomic performance of experimental lines derived from plant introductions (Pis) maintained in the USDA Soybean Germplasm Collection. These Pis are known to be genetically distinct from the ancestors of the modern North American soybean cultivars. Experimental lines containing 25 to 100% PI germplasm (based on pedigrees), their parents, and recently released public cultivars were evaluated for yield in seven environments in 1994 and 1995. Data from random amplified polymorphic DNA (RAPD) fragments were collected and genetic relationships among all genotypes were estimated using hierarchical and nonhierarchical cluster analyses. Experimental lines were identified that yielded significantly more than their domestic parent. Comparisons of pairwise distances revealed that many of the high‐yielding experimental lines were more diverse than their domestic parents from the nonparental cultivars. The increased genetic diversity and yield provide evidence that exotic germplasm can contribute genes for high yield.

Automated sizing of fluorescent-labeled simple sequence repeat (SSR) markers to assay genetic variation in soybean

Simple Sequence Repeat (SSR) allele sizing provides a useful tool for genotype identification, pedigree analysis, and for estimating genetic distance between organisms. Soybean [Glycine max (L.) Merr.] cultivars are identified for Plant Variety Protection (PVP) purposes by standard pigmentation and morphological traits. However, many commercial soybeans arise from a limited number of elite lines and are often indistinguishable based on these traits. A system based on SSR markers would provide unique DNA profiles of cultivars. Fluorescent labeling of alleles combined with automated sizing with internal size standards in each gel lane was used as an alternative to standard [32P] labeling to assess genetic variability in soybean. Allelic frequencies at 20 SSR loci were determined in 35 soybean genotypes that account for greater than 95% of the alleles in North American soybean cultivars based upon pedigree analysis. An average of 10.1 alleles per locus (range: 5–17), with a mean gene diversity of 0.80 (range: 0.50 to 0.87) were observed at the 20 SSR loci. The 20 loci successfully distinguished modern soybean cultivars that are identical for morphological and pigmentation traits, as well as 7 soybean genotypes reported to be indistinguishable using 17 RFLP probes. Pedigrees of 7 cultivars were studied to estimate stability of SSRs in soybean across generations. Of the 7 pedigrees 6 had one locus in the progeny with an allele(s) that was not present in either parent. These new alleles are most likely the result of mutation. The mutation rate of SSR alleles in soybean was similar to that reported in humans. To avoid difficulty associated with mutation, DNA fingerprint data should be determined from the bulk of 30-50 plants of a cultivar.

Pod Dehiscence of Soybean: Identification of Quantitative Trait Loci

The dehiscence of pods (shattering) prior to harvest is an undesirable trait of soybean, Glycine max (L.) Merr. Pod dehiscence (PD) is relatively uncommon in modern North American soybean cultivars, but is often observed when unimproved germplasm or the wild species, G. soja Siebold & Zucc., are used as parents to introgress useful genes or to develop genetically diverse breeding populations. In light of the potential for efficient selection using DNA markers, the objective of this study was to identify quantitative trait loci (QTL) that condition resistance to PD. A map of 140 linked restriction fragment length polymorphism (RFLP) markers was constructed using 120 F4-derived lines from a soybean population (Young x PI 416937) that segregated for resistance to PD. These lines were scored for PD on a visual scale of 1 to 10 at both Athens, Georgia, and Windblow, North Carolina, in 1994. Heritability of pod dehiscence was 92%. Associations of marker loci with QTL that condition resistance to PD were tested using homozygous RFLP class means in a single-factor ANOVA. A total of five putatively independent RFLP markers were associated with PD at both locations and in a combined analysis over locations. A single RFLP locus on linkage group J of the USDA/Iowa State University map accounted for 44% of the variation in PD score. Epistasis was observed between one pair of significant marker loci. These results establish the genomic location of one major and a few minor QTL, identify an epistatic interaction, and indicate transgressive segregation which is plausibly the result of susceptibility alleles contributed by the resistant parent.

Genetic Base for North American Public Soybean Cultivars Released between 1947 and 1988

A negative consequence of four decades of modern soybean breeding is the evolution of cultivars with complex pedigrees that tend to obscure the genetic base of applied breeding. A result is that the genetic base of North American soybean [Glycine max (L.) Merr.] has never been described fully. We attempt here to define the genetic base as sets of genotypes that contain 99% of the genes found in modern cultivars. For clarity, the base is defined both in terms of the original plant introductions (ancestors) used for hybridization and in terms of the progeny derived from them. In a first analysis of pedigree data, 80 ancestors were identified and their fractional genetic contributions to 258 cultivars were computed using coefficient of parentage (r) estimates. In a second analysis, six breeding lines and 133 cultivars were identified as first progeny of the 80 ancestors, and their contributions as parents to modern cultivars were computed using r. This analysis revealed that 91 first progeny constituted 99% of the genes found in modern cultivars. Lincoln and Harosoy in the North and Lee and its full sib (D49‐2491) in the South contributed nearly 40% of the genes to North American cultivars. Nearly 75% of the genes in modern cultivars trace to 17 first progeny released before 1960, indicating that breeders have remained dependent upon this early genetic core of breeding material. For practical study of the genetic base of North American soybean, we propose using a combination of the results from the two analyses described here. Twenty‐eight ancestors and seven first progeny were identified which contribute 95% of the genes found in modern soybean cultivars. This group of 35 genotypes would be a useful core collection for evaluating the presence, absence, or distribution of a trait in North American soybean cultivars.