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Published in Crop Sci. 43:1858-1867 (2003).
© 2003 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

PLANT GENETIC RESOURCES

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

George N. Udea, William J. Kenworthy*,a, Jose M. Costaa, Perry B. Creganb and Jennie Alvernazc

a Dep. of Natural Resource Sciences and Landscape Architecture, Univ. of Maryland, College Park, MD 20742
b USDA-ARS, Soybean Genomics and Improvement Lab., Beltsville, MD 20705
c Dep. of Crop and Soil Sciences, Univ. of Georgia, Athens, GA 30602

* Corresponding author (wk7{at}umail.umd.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
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.

Abbreviations: AFLP, amplified fragment length polymorphism • GD, genetic distance • NASA, North American soybean ancestors • NASC, North American soybean cultivars • PIC, polymorphic information content • RAPD, random amplified polymorphic DNA • RFLP, restriction fragment length polymorphism


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
SOYBEAN is one of the world's most important oil and protein crops. By selection and hybridization, breeders in the USA have increased soybean yield by at least 20% (Fehr, 1984). More than 300 publicly developed cultivars have been released in North America in the past 50 yr (Thompson and Nelson, 1998b). However, it has been observed that the use of only a few plant introductions and intensive plant breeding have narrowed the genetic diversity among North American elite soybean cultivars (Gizlice et al., 1994; Sneller, 1994).

The genetic similarity among NASC has reached a level that could limit continued breeding success. Introduction of new sources of germplasm into the breeding pool may provide the genetic variability to permit continued progress in developing high yielding cultivars. Though plant introductions (PIs) provide genetic variability, they are less frequently used as sources of new yield genes than current cultivars and elite lines because they often yield less. Populations developed from crossing cultivars with PIs, which have been selected for good phenotypic traits, generally have a lower mean yield and lower frequency of desirable lines than those populations developed from crossing elite parents (Vello et al., 1984; Ininda et al., 1996). Recent studies have used molecular markers to help identify genetically diverse PIs to use in crosses in cultivar improvement programs (Thompson and Nelson, 1998a, b; Thompson et al., 1998; Narvel et al., 2000). These studies have had more success than conventional selection programs in producing productive lines from PI crosses with elite genotypes. Modern Asian cultivars, which share no ancestors with NASC, represent a potential reservoir of new alleles available for improving U.S. soybean yield, and is a different approach than using other germplasm in crosses with NASC.

Acquisition of soybean germplasm from Asia has increased over time, though not all the introduced cultivars or germplasm have been assessed for their usefulness in soybean improvement. There is a need for extensive evaluation of new germplasm from Asia to determine its genetic diversity and to identify Asian lines to serve as sources of unique genes for U.S. soybean yield improvement.

Conventional molecular marker analysis using restriction fragment length polymorphism (RFLP) (Apuya et al., 1988), ribosomal DNA (Doyle and Beachy, 1985), and random amplified polymorphic DNAs (RAPDs) (Williams et al., 1990) have identified only low levels of genetic diversity in cultivated soybean. Microsatellite markers can detect higher levels of genetic diversity among soybean cultivars but this marker system requires the synthesis of primers and construction of genomic libraries (Maughan et al., 1996). AFLP is a PCR-based, molecular technique that detects high numbers of polymorphic bands (Powell et al., 1996). AFLPs are detected frequently in soybean, are inherited in a stable Mendelian fashion, and exhibit high levels of diversity (Maughan et al., 1996). The objectives of this study were to determine the level of genetic diversity within and between Asian and NASC by AFLP analysis and to identify Asian cultivars with significant genetic difference from NASC.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
A sample of 59 Chinese and 30 Japanese cultivars that have no known NASC in their ancestry were compared with 66 high yielding NASC and 35 NASA genotypes to estimate the level of genetic variability between and within the groups. Twenty micrograms each of the 190 soybean DNA samples were extracted according to the procedure of Keim et al. (1988). Some of the soybean genotypes (Table 1) used in this study were also evaluated in field plots as part of a cooperative effort by USDA-ARS, North Carolina State University, Pioneer Hi-Bred International, Asgrow Seed Company, and the Universities of Arkansas, Georgia, Illinois, Maryland, and Minnesota (Manjarrez-Sandoval et al., 1997).


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  		Table 1. Name, code, PI number, maturity group, country of origin, average genetic distance (AGD), classification, and clusters based on the UPGMA clustering of the 190 soybean lines studied.

 
The AFLP procedure was performed according to Lin et al. (1996) with the AFLP primer starter kit and the core reagent kit supplied by Life Technologies Inc., Gaithersburg, MD. Primary template DNA was prepared by completing a restriction enzyme digest followed by an adaptor ligation. Five hundred nanograms of DNA from each of the 190 genotypes was digested with 2 µL of EcoRI/MseI (1.25 units of EcoRI/µL and 1.25 units of MseI/µL) at 37°C for 2 h, and then heated to 70°C for 10 min to inactivate the enzymes. In addition to the DNA and enzymes, the following were added to a 1.5-mL microcentrifuge tube: 5 µL of 5x reaction buffer and AFLP-grade water to a final volume of 25 µL. The DNA fragments were ligated to EcoRI and MseI adapters provided in the kit. The ligation mixture (containing fragments with adapters at both ends) was diluted 10-fold with sterile distilled water and held at -20°C in a freezer until used.

The 10-fold diluted ligation mixture was preamplified by 20 PCR cycles. The PCR reaction was performed in a thermal cycler with the following temperature profile: 94°C for 30 s, 56°C for 60 s, and 72°C for 60 s using EcoRI+A(5'GACTGCGTACCAATTC+A3') and MseI+C(5'GATGAGTCCTGAGTAA+C3') primers (provided in the kit) described by Vos et al. (1995). Five primer combinations, E-ACT/M-CAT, E-ACC/M-CAA, E-AAG/M-CTT, E-ACA/M-CAC, and E-AGC/M-CTC, were chosen from a pool of primer combinations that produced seven or more polymorphic bands among the parents of the cross PI290136 x BARC-2 (Rj4) (Ude et al., 1999) and were used for fingerprinting the 190 genotypes in this study.

One hundred-twenty microliters of distilled water was added to 5 µL of each of the preamplified DNA to make a 1:24 dilution from which 5 µL was used for selective amplification. Selective amplification was conducted in 5 µL-aliquots of the diluted preamplified fragments with 32P-ATP labeled EcoRI+3 primer with an unlabeled MseI+3 primer. Amplification was done by PCR with one temperature cycle at 94°C for 30 s, 65°C for 30 s, and 72°C for 60 s, followed by lowering the annealing temperature each cycle 0.7°C for 12 cycles. At the end of the 12 cycles, the reaction was programmed to amplify for 23 cycles at 94°C for 30 s, 56°C for 30 s, and 72°C for 60 s. The reaction products were loaded on a 5% (w/v) polyacrylamide DNA sequencing gel containing 7.5 M urea. Ten base pair (bp) DNA ladder (Cat. No. 10821-015) purchased from Life Technologies Inc., Gaithersburg, MD, was used as a molecular weight standard in every gel. Autoradiography was performed by exposing Kodak Bio-Max MR-2 film (Eastman Kodak Co., Rochester, NY) to the dried gel at room temperature for 48 h.

The gel autoradiographs were scored visually for polymorphism. A band was considered polymorphic if it was present in at least one soybean genotype and absent in others. A matrix was generated in which each band was scored as "1" if present and as "0" if absent for each genotype. Polymorphic information content (PIC); the fraction of polymorphic loci (ß); the arithmetic mean heterozygosity (Hav); the effective multiplex ratio (ER); the marker index (MI), and the average expected heterozygosity for polymorphic markers [Hav(p)] for each of the primer combinations were estimated according to Powell et al. (1996).

The sum of polymorphic heterozygosity ({sum}Hp) is the sum of the polymorphic information content for all loci for each primer pair ({sum}Hp = {sum}PIC) and the fraction of polymorphic loci (ß) is the number of polymorphic loci (np) divided by the sum of polymorphic (np) and nonpolymorphic loci (nnp) [ß = np/(np + nnp)]. The arithmetic mean heterozygosity (Hav) is the product of the fraction of polymorphic loci (ß) and the polymorphic heterozygosity (Hp) divided by the number of polymorphic loci (np) (Hav = ß {sum}Hp/np).

The effective multiplex ratio (E) is defined as the product of the total number of loci per primer (n) and the fraction of polymorphic loci (ß) (E = nß). The marker index (MI) is the product of the total number of loci per primer pair (n) and the arithmetic mean heterozygosity (Hav) (MI = nHav). The marker index (MI) can also be defined as the product of effective multiplex ratio (E) and the average expected heterozygosity [Hav(p)] for the polymorphic markers [MI = EHav(p) where Hav(p) = MI/(n x ß) and Hav(p) = MI/E].

Genetic similarities between pairs of genotypes were estimated with 332 monomorphic and polymorphic bands by means of simple matching coefficients (Powell et al., 1996) in the NTSYS-pc software package version 2.02f (Rohlf, 1998). Genetic distances were calculated by subtracting the similarity indices from 1 and multiplying the result by 100. Student's t tests (P = 0.05) were used to compare the average genetic distances within and among the groups of soybeans studied. A dendrogram based on the similarity coefficient matrix and unweighted pair group method of the arithmetic average clustering was produced. Principal coordinate analysis was also done to show multiple dimensions of the distribution of the genotypes in a scatter-plot (Keim et al., 1992). Genetic distances calculated with monomorphic and polymorphic markers are about one third that calculated with only polymorphic bands (Becker et al., 1995).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
Primer Utility
The AFLP primer pairs used in this study were selected on the basis of our previous soybean studies (Ude et al., 1999). The five primer pairs revealed a total of 332 different bands that were of sufficient intensity to score (Table 2). The band sizes ranged from 50 to 500 bp but only 90 (27%) were polymorphic. The PIC scores ranged from zero for nonpolymorphic loci to 0.50 (Table 2). Average PIC score for the 332 AFLP bands was 0.10. Fifty-three polymorphic bands showed PIC scores >=0.30 indicating that only 16% of the 332 AFLP bands contributed significantly to the genetic discrimination of the 190 soybean genotypes studied. A PIC score >=0.30 has been used previously by Keim et al. (1992) and Lorenzen et al. (1995) with RFLP probes and by Thompson and Nelson (1998b) with RAPD fragments to determine usefulness in other soybean germplasm diversity studies.


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  		Table 2. Total number of bands, proportion of polymorphic bands, average expected heterozygosity for polymorphic markers, polymorphic information content, the effective multiplex ratio, and the marker index, for each primer pair used in the analysis of the 190 soybean lines.

 
The average expected heterozygosity estimate for polymorphic markers [Hav(p)] for each primer pair ranged from 0.15 to 0.37 with an average of 0.27 per primer (Table 2). The overall average expected heterozygosity estimate [Hav(p)] for the 90 polymorphic AFLP markers was 0.30. The values of the average expected heterozygosity [Hav(p)] for the markers are in agreement with those previously reported in soybean for AFLPs (Powell et al., 1996), RFLPs (Keim et al., 1992), and RAPDs (Thompson and Nelson, 1998a). The primers E-ACC/M-CAA and E-ACT/M-CAT showed the highest average expected heterozygosity and produced the most informative DNA fragments for distinguishing among the genotypes. On the basis of this criterion, the primers E-AGC/M-CTC [Hav(p) = 0.27] and E-AAG/M-CTT [Hav(p) = 0.26] showed less discriminatory power than E-ACC/M-CAA and E-ACT/M-CAT, although they were better than E-ACA/M-CAC [Hav(p) = 0.15]. Multiplex ratio, which is the number of different genetic loci that may be scored in a gel using a primer combination, ranged between 46 and 83. Effective multiplex ratio, which is the number of polymorphic loci per primer combination, ranged from 7 to 34 (Table 2).

Marker index (MI) is the statistic used to calculate the overall utility of a marker system and is the product of expected heterozygosity and multiplex ratio. The primers E-AGC/M-CTC and E-ACA/M-CAC had low marker indices (2.16 and 1.11, respectively), while the other primers [E-AAG/M-CTT (9.14), E-ACC/M-CAA (8.24), and E-ACT/M-CAT (6.12)] showed high MI values. Just as the Hav(p) analysis indicated, these primers were more useful than E-AGC/M-CTC and E-ACA/M-CAC in discriminating between the soybean genotypes in this study. Average MI for the five-primer pairs used in this study was 5.35, and it was similar to the MI reported by Powell et al. (1996) for the AFLP marker system.

Genetic Relationships
Average genetic distance among the 190 soybean genotypes was 8.1%, and the range of genetic distance (GD) was 0.9 to 15.0% (Table 3). There were no significant differences between the genetic distance means of any of the four genotype groupings. The average GD was lowest among Japanese cultivars (6.3%) whereas the Chinese cultivars had the highest average GD estimate (7.5%). The average GD of NASA and NASC were also not significantly different. The average GD for all possible pairings of the 66 NASC with Chinese cultivars and with Japanese cultivars were 8.5 (GD range 3.6 to 13.9) and 8.9% (range 4.8 to 14.5), respectively. The most diverse cross between a NASC and an Asian cultivar would have a genetic distance of 14.5%.


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  		Table 3. Mean, standard deviation, and range (in parenthesis) of the genetic distances (%) between all pairings of the North American soybean ancestors (NASA), North American soybean cultivars (NASC), Chinese cultivars, and Japanese cultivars.

 
Average genetic distance within germplasm groups of 36, 31, 32, and 26% has been estimated for these same NASA, NASC, Chinese, and Japanese cultivars, respectively, on the basis of 121 RFLP probes (Alvernaz et al., 1998). The average GD (with RFLP data) for all possible pairings of the 66 North American with all Asian cultivars from the RFLP study was 35% for Chinese and 37% for Japanese cultivars (Alvernaz et al., 1998). These RFLP results are similar to the AFLP data, which were produced with only five primer combinations. The difference in the magnitude of these two sets of genetic distances exists because of the use of only polymorphic markers in the RFLP analysis, whereas polymorphic and monomorphic markers were used in the AFLP analysis.

The UPGMA-derived dendrogram assigned the 190 genotypes into four major clusters (Fig. 1) designated as ‘a’, ‘b’, ‘c’, and ‘d’. The NASA were primarily in cluster ‘a’, the NASC in cluster ‘b’, the Japanese cultivars in cluster ‘c’, and the Chinese cultivars in cluster ‘d’ (Fig. 1). In general, 85% of the 190 soybean lines clustered between 90 and 95% similarity distance.




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  		Fig. 1. Dendogram of 190 soybean lines produced by UPGMA clustering method based on the genetic similarity matrix derived from 332 AFLP markers. The letter and the numbers after the plus sign (+) in the cultivar names are codes from Table 1. A- = North American soybean ancestor; U- = North American soybean cultivar; Us- = Southern USA cultivar; C- = Chinese cultivar; J- = Japanese cultivar.

 
Although the major clusters were related to the geographic origin of the genotypes, smaller clusters of cultivars and genotypes with known pedigree relationships were evident. Cluster ‘a’ included 29 NASA (83% of all the NASA), 15 NASC, three Chinese cultivars, and two Japanese cultivars. Five subgroups were observed among the genotypes in cluster ‘a’. Ogden, Roanoke, and Jackson, which account for 24% of the genetic base of cultivars developed in the southern USA, were placed in the same subgroup. A similar cluster of Ogden, Roanoke, and Jackson was also identified by Kisha et al. (1998) and Brown-Guedira et al. (2000). The third subgroup showed that A.K. (Harrow) and Illini were very similar and both were closer to S-100 than to any other accessions in the study. Previous researchers (Kisha et al., 1998; Thompson et al., 1998; Brown-Guedira et al., 2000) had placed Lincoln in the A.K. (Harrow) cluster with Illini and S-100, but the present study distinguished it from that group and clustered it with other ancestors, Anderson and Flambeau (Fig. 1). S-100 is thought to be a selection from Illini or a progeny of Illini (Thompson et al., 1998). Gizlice et al. (1994) suggested that A.K. (Harrow) and Illini may be identical. These two genotypes were among the most similar in our analysis (Fig. 1).

The soybean ancestors Arksoy, Ralsoy, Mejiro, and Mukden were grouped and they clustered with seven NASC in subgroup 4 of cluster ‘a’. Arksoy and Ralsoy were always grouped together in studies by Brown-Guedira et al. (2000) and give an additional example of how these groupings agree with previous studies.

Mandarin (Ottawa) was the only NASA in subgroup 5 of cluster ‘a’. Also contained in subgroup 5 were eight Canadian cultivars, three Chinese cultivars, and two Japanese cultivars. The grouping of Mandarin (Ottawa) with other cultivars from North America, China, and Japan suggests that it has a broad genetic base that has some similarity to elite cultivars from both Asian regions. It has been reported that Mandarin (Ottawa), which was originally introduced from China, is a major ancestor of the Canadian and North American cultivars having contributed between 18 and 55% of their genomes (Lohnes and Bernard, 1991; Kisha et al., 1998).

Cluster ‘b’ consisted of four subgroups made of three NASA, 49 North American, and three Chinese cultivars. In this cluster, the NASC Hutcheson (Buss et al., 1988) and Narow (Caviness et al., 1985) grouped very closely. Although they are both in maturity group V and they have several common ancestors in their pedigree, this level of similarity was unexpected. Carter et al. (1993) reported a genetic similarity estimate of 0.529 between Hutcheson and Narow where 1.0 is defined as genetic identity. Since these cultivars also differ for several morphological traits, it seems unlikely that the observed level of similarity is correct and a more likely explanation is that the original DNA samples were mislabeled.

Cluster ‘c’ was composed of eight Chinese and 26 Japanese cultivars. This suggests that the Asian cultivars in cluster ‘c’ were derived from soybean ancestors that are very different from the ancestors of the NASC. A recent report by Cui et al. (2000) identified a very minor NASA (Mamotan) in the pedigrees of three of the Chinese cultivars in subgroup 1 of cluster ‘c’ (Yu dou 11, Zheng 77249, and Zhong dou 19), but essentially this cluster has no NASC or NASA.

The fourth cluster ‘d’ consisted of three NASA (Peking, PI88788, and Korean), two NASC, 45 Chinese, and two Japanese cultivars. The three soybean ancestors Peking, PI88788, (both collected from China), and Korean (collected from North Korea) were very different from the other NASA. Gizlice et al. (1993) also observed Peking to be very different from other NASA based on the 10 metric traits they measured on plants grown in a phytotron. In general, PI88788 and Peking [two sources of soybean cyst nematode (Heterodera glycines Ichinohe) resistance genes], Manokin, Jin dou 14, Kitakomachi, and Yuuhime constituted the most divergent group from all the soybean accessions used in our study.

It is not clear on the basis of their pedigrees why Manokin (Kenworthy et al., 1996) and Harlon would be in cluster ‘d’. Manokin (maturity group IV) was the most genetically different within the NASC, with an average GD of 8.8% from other NASC. A previous pedigree analysis by Sneller (1994) did not identify Manokin as having a unique coefficient of parentage. Manokin was derived from a cross of parents representing northern U.S. by southern U.S. germplasm. Manokin has cyst nematode resistance that was derived from Peking, an ancestor also in this cluster. Pedigree information (Lohnes and Bernard, 1991) on Harlon (maturity group I) indicates that it was selected from the cross of ‘Blackhawk’ x ‘Harosoy 63’ and has four ancestors from China-Mandarin (Ottawa), Mukden, Richland, and A.K. (Harrow) which contributed 37, 25, 25, and 13% of its genome, respectively. However, Harlon's pedigree would be similar to other NASC grown in the northern USA and has no obvious uniqueness.

Principal coordinate analysis (PCO) was used to identify multidimensional relationships that describe portions of the genetic variance in a data set. The first two principal coordinates of the AFLP data explained 15.4% of the total variance (Fig. 2). Principal coordinate analysis separated the germplasm into four broad groups corresponding to the UPGMA clusters (‘a’, ‘b’, ‘c’, and ‘d’) on the basis of the geographical origin of the accessions. The PCO scatter plot, however, showed overlap between accessions from different geographic origins. The NASA lines occupied a central position among North American, Chinese, and Japanese cultivars and overlapped each of them (Fig. 2). With the exception of Kitakomachi and Yuuhime, the rest of the Japanese cultivars were well separated from the North American cultivars and ancestors. The Chinese cultivars were widely scattered in all clusters and they also appeared as a bridge between the North American accessions (cultivar and ancestor) and the Japanese cultivars.



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  		Fig. 2. Principal coordinate graph of the 190 soybean lines composed of the first and second principal coordinates derived from the analysis of 332 AFLP markers. Soybean lines in the scatter are identified by codes from Table 1. A- = NASA; U- = North American soybean cultivar (NASC); Us- = Southern USA cultivar; C- = Chinese cultivar; and J- = Japanese cultivar.

 
Breeding Implications
The AFLP genetic distance clearly formed a distinct grouping of cultivars on the basis of their origin. Even though NASC were derived from Chinese and Japanese introductions, subsequent breeding efforts have resulted in the development of rather distinct gene pools in each country. The Japanese cultivars in this study had the lowest average GD of the three groups of cultivars. They were also the most genetically different from NASC indicating separate ancestors for the elite cultivars in the two regions. Although a few Japanese cultivars, Yuuhime, Kitakomachi, Fukunagaha, and Kitahomare, showed close relationship to some NASA, the remaining 26 Japanese cultivars were very genetically different from both NASC and NASA. This suggests that some Japanese elite cultivars may serve as sources of exotic genes for the genetic improvement of North American soybean cultivars.

Agronomic data and yield for all of the Asian cultivars in this study are available from the Soybean Asian Variety Evaluation Project (Project SAVE) report by Manjarrez-Sandoval et al. (1997). Seven of the Asian cultivars yielded at least 80% of the NASC checks of similar maturity in at least 1 yr of the 2-yr SAVE project. Those cultivars and their yield as a percentage of the NASC checks (2-yr average) are Akishirome (76%), Hyuuga (83%), Misuzu Daizu (83%), Nakasennari (87%), Nasu Shirome (68%), Otsuru (75%), and Tachinagaha (70%) (Manjarrez-Sandoval et al., 1997).

The U.S. soybean breeders have been slow to utilize diverse genetic material in cultivar improvement programs. Gizlice et al. (1993) found that many recent U.S. cultivars were as closely related as half-sibs. Asian breeders have utilized a different strategy in their breeding programs. Cui et al. (2000) reported that Chinese breeders avoid mating related parents, and continue to introduce new germplasm in cultivar development programs. Chinese breeders have successfully introgressed U.S. germplasm into Chinese cultivars, but U.S. germplasm contributes only about 7% of the total genetic base of Chinese cultivars. Similarly, Zhou et al. (2000) reported that U.S. and Chinese cultivars have been utilized by Japanese breeders in their cultivar development programs. Intermating cultivars from these three major gene pools should provide new genetic recombinations to exploit in cultivar development programs. The information presented here should assist breeders in the selection of sources of new genes for the yield improvement of NASC.


   ACKNOWLEDGMENTS
 
R.L. Nelson (USDA-ARS, Univ. of Illinois) and T.E. Carter, Jr. (USDA-ARS, North Carolina State Univ.) obtained the pedigree information and the original seed of the Asian cultivars used in this study. Both served in leadership roles and as principal investigators in conducting the overall research project funded by the United Soybean Board, and their many contributions to the success of this project are gratefully acknowledged.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
This research was supported in part by a grant from the United Soybean Board. The research was part of a dissertation submitted by the senior author in partial fulfillment of the Ph.D. degree.

Received for publication February 5, 2002.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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