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Published online 1 September 2007
Published in Crop Sci 47:1947-1954 (2007)
© 2007 Crop Science Society of America
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PLANT GENETIC RESOURCES

Genetic Diversity of Canadian Soybean Cultivars and Exotic Germplasm Revealed by Simple Sequence Repeat Markers

Yong-Bi Fua,*, Gregory W. Petersona and Malcolm J. Morrisonb

a Plant Gene Resources of Canada, Saskatoon Research Centre, Agriculture and Agri-Food Canada, 107 Science Pl., Saskatoon, SK, Canada S7N 0X2
b Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, K.W. Neatby Building, Ottawa, ON, Canada K1A 0C6

* Corresponding author (fuy{at}agr.gc.ca).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
Genetic diversity assessment of improved crop germplasm can facilitate the expansion of the genetic base in a plant breeding program, but little effort has been made to assess the Canadian soybean [Glycine max (L.) Merr.] gene pool established over the past century. Simple sequence repeat (SSR) markers were applied to assess the genetic diversity of 45 Canadian soybean cultivars released from 1934 to 2001 and 37 exotic germplasm accessions. Thirty-seven SSR primer pairs were applied and 234 polymorphic bands were scored for each accession. The frequencies of the scored bands ranged from 0.01 to 0.90 and averaged 0.17. The proportion of total SSR variation occurring between exotic and Canadian germplasm was 9%; among the Canadian cultivars of three breeding periods 10%; and between the cultivars of maturity groups 0 and 00 4%. More diversity was found for exotic germplasm than the Canadian. More diversity was observed in the cultivars of the recent breeding period than the early. The Canadian cultivars were clustered into seven major groups, partially congruent to the known pedigrees, and they were more related to germplasm from Russia, Sweden, and Ukraine and less to the Asian germplasm. The six genetically most distinct cultivars were PS86 RR, Gaillard, Manitoba Brown, Beechwood, Maple Isle, and 92B91. These findings are useful for the selection of genetically distinct or less related soybean materials to improve the genetic background of the soybean gene pool.

Abbreviations: AD, average dissimilarity • AMOVA, analysis of molecular variance • EST, expressed sequence tag • MG, maturity group • PCR, polymerase chain reaction • PGRC, Plant Gene Resources of Canada • PIC, polymorphic information content • SSR, simple sequence repeat

Genetic Diversity of Canadian Soybean Cultivars and Exotic Germplasm Revealed by Simple Sequence Repeat Markers

Yong-Bi Fua,*, Gregory W. Petersona and Malcolm J. Morrisonb

a Plant Gene Resources of Canada, Saskatoon Research Centre, Agriculture and Agri-Food Canada, 107 Science Pl., Saskatoon, SK, Canada S7N 0X2
b Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, K.W. Neatby Building, Ottawa, ON, Canada K1A 0C6

* Corresponding author (fuy{at}agr.gc.ca).

Genetic diversity assessment of improved crop germplasm can facilitate the expansion of the genetic base in a plant breeding program, but little effort has been made to assess the Canadian soybean [Glycine max (L.) Merr.] gene pool established over the past century. Simple sequence repeat (SSR) markers were applied to assess the genetic diversity of 45 Canadian soybean cultivars released from 1934 to 2001 and 37 exotic germplasm accessions. Thirty-seven SSR primer pairs were applied and 234 polymorphic bands were scored for each accession. The frequencies of the scored bands ranged from 0.01 to 0.90 and averaged 0.17. The proportion of total SSR variation occurring between exotic and Canadian germplasm was 9%; among the Canadian cultivars of three breeding periods 10%; and between the cultivars of maturity groups 0 and 00 4%. More diversity was found for exotic germplasm than the Canadian. More diversity was observed in the cultivars of the recent breeding period than the early. The Canadian cultivars were clustered into seven major groups, partially congruent to the known pedigrees, and they were more related to germplasm from Russia, Sweden, and Ukraine and less to the Asian germplasm. The six genetically most distinct cultivars were PS86 RR, Gaillard, Manitoba Brown, Beechwood, Maple Isle, and 92B91. These findings are useful for the selection of genetically distinct or less related soybean materials to improve the genetic background of the soybean gene pool.

Abbreviations: AD, average dissimilarity • AMOVA, analysis of molecular variance • EST, expressed sequence tag • MG, maturity group • PCR, polymerase chain reaction • PGRC, Plant Gene Resources of Canada • PIC, polymorphic information content • SSR, simple sequence repeat


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
SOYBEAN [Glycine max (L.) Merr.] breeding in Canada began in 1893 with the goal to improve forage yield of early introductions (Zavitz, 1927). Since then, breeding efforts have gone through several major stages from early selection and adaptation, to expansion, to commercialization (Beversdorf et al., 1995). Great improvements have been made in disease control, maturity, seed yield, and quality traits (Voldeng et al., 1997; Morrison et al., 2000). Hundreds of soybean cultivars have been developed and released, many of which have had significant impacts on Canadian agriculture (Beversdorf et al., 1995). There are still concerns about the narrowing of the North American soybean gene pool as a result of the small number of parental lines used since soybean breeding began (Gizlice et al., 1994; Carter et al., 2004; Hyten et al., 2006). Canadian soybean cultivars are unique in their geographic and climatic ranges of adaptation, yet there has been no assessment of the genetic diversity of the germplasm.

Molecular characterization of germplasm developed across the years of plant breeding should be more informative for parental selection to widen the gene pool than traditional pedigree analyses based on many rarely met assumptions (Cox et al., 1985; Gizlice et al., 1994; Cornelious and Sneller, 2002; Carter et al., 2004). DNA microsatellite (or simple sequence repeat [SSR]) markers have proven to be important tools in soybean genetics (Akkaya et al., 1992; Cregan et al., 1999) and have been widely applied in the genetic diversity studies of soybean germplasm (e.g., Brown-Guedira et al., 2000; Narvel et al., 2000; Abe et al., 2003; Wang et al., 2006; Hyten et al., 2006). These diversity studies not only provide useful information for understanding the genetic bases of various soybean gene pools established in different geographic regions, but also facilitate the selection of sources of new genes for yield and quality improvements to soybean.

The objective of this study was to assess the genetic distinctiveness, relationships, and diversity changes of 45 Canadian soybean cultivars released from 1934 to 2001 and 37 exotic germplasm accessions from eight countries using mapped SSR markers.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
Plant Materials
Forty-five Canadian soybean cultivars and 37 exotic accessions (Table 1 ) were selected from the soybean germplasm collection maintained at Plant Gene Resources of Canada (PGRC), Agriculture and Agri-Food Canada, Saskatoon. Currently the PGRC soybean collection consists of 367 active accessions originating from nine countries. There are 256 accessions originating from Canada, 58 from South Korea, 22 from Russia (or former Soviet Union), 20 from China, seven from Hungary, and one accession from each of four countries (Ukraine, Sweden, Netherlands, USA). Cultivar selection was made based on pedigree analyses, agronomic importance, and representation of the soybean breeding efforts over the past century. Several Canadian soybean breeders and researchers were consulted regarding the cultivar selection. To facilitate the diversity analysis, these cultivars representing three maturity groups (MG0, MG00, and MG000) were classified into three groups reflecting three major breeding periods (P1 = early selection and adaptation, before 1970; P2 = expansion, from 1970 to 1989; and P3 = commercialization, from 1989 to present) (Table 1). Exotic accessions were selected through a random sampling stratified with respect to country of origin (excluding Canada) and with the size equal to the natural logarithm frequency of accessions for a country.


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  		Table 1. Soybean germplasm assayed and the average dissimilarity (AD).

 
DNA Extraction
Approximately 10 to 15 seeds of each accession were randomly selected from the PGRC soybean collection and grown in the greenhouse at the Saskatoon Research Centre. Young leaves were collected from 10 2- to 3-wk-old seedlings of each accession, bulked, freeze-dried in a Labconco Freeze Dry System for 5 to 7 d, and stored at –20°C. From each bulked sample, dry leaves were ground to a fine powder in a 2-mL Eppendorf tube with three 3-mm glass beads in a multidirectional shaker. Genomic DNA was extracted using DNeasy Plant Mini Kit (Qiagen Inc., Mississauga, ON) according to the manufacture's directions. Extracted DNA was quantified by fluorimetry using Hoechst 33258 stain (Sigma Chemical Co., St. Louis, MO), followed by the dilution to 10 ng mL–1 for SSR analysis.

SSR Analysis
Based on reported polymorphism and genome coverage (Cregan et al., 1999; Song et al., 2004), 60 genomic and 38 expressed sequence tag (EST)-derived SSR primer pairs were selected and screened on four selected accessions: CN 30317 and CN 30318 from China, CN 30391 from Russia, and CN 31719 from the Netherlands. All polymerase chain reactions (PCRs) were performed in an MJ Research DYAD thermocycler (BioRad, Mississauga, ON) using the PCR conditions described by Cregan et al. (1999) with the exception of the PCR extension temperature and time changed to 72°C for 50 s. The PCR products were separated on a 1.5-mm-thick 6% (w/v) nondenaturing acrylamide/bis-acrylamide (19:1) gel in 1x TBE buffer with 0.5 mg L–1 ethidium bromide for 2 to 2.5 h and recorded on a digital gel documentation system. Based on the screening, 23 genomic and 14 EST-derived SSR primer pairs (Table 2 ) were selected to genotype all 82 accessions.


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  		Table 2. Microsatellite allelic counts and polymorphic information contents (PIC) at individual loci for Canadian soybean cultivars from three breeding periods and exotic germplasm.

 
Data Analysis
To generate a dataset of SSR allele counts for each accession, DNA fragments amplified by SSR primer pairs were identified based on their sizes in base pairs measured with a 10-bp DNA ladder (Invitrogen, Carlsbad, CA) and compared with the fragment sizes reported in the literature (Cregan et al., 1999; Song et al., 2004). Frequencies of the scored alleles were calculated with respect to primer, germplasm type (exotic and cultivars), and breeding period. To assess the informativeness of each marker, the polymorphic information content (PIC) was calculated for each locus, as described in Roussel et al. (2004).

To assess the genetic distinctiveness of the soybean accession, the similarities of each accession with the remaining accessions assayed were calculated using the simple matching coefficient (Sokal and Michener, 1958) as: Sij = (a + d)/(a + b + c + d), where Sij is the SSR similarity between the accession i (i = 1 to n) and the other accession j [j = 1 to (n – 1)], a is the number of alleles (from all SSR loci) shared in both i and j, b is the number of alleles present in i but not shared in j, c is the number of alleles present in j but not shared in i, and d is the number of alleles absent from both i and j. The SSR dissimilarity for each pair of accessions can be defined as 1 – Sij. The average SSR dissimilarity for the accession i can be obtained by averaging all of the n – 1 SSR dissimilarities that the accession was associated with. This average dissimilarity measures the overall genetic difference present between the accession (i) of interest and the remaining accessions assayed. A higher average dissimilarity obtained from unlinked markers means that the accession has a genetic background more distinct from the other accessions (Fu, 2006). This assessment was done using a specific SAS program written in SAS IML (SAS Institute, 2004).

To assess the genetic relationships of the Canadian cultivars, the dissimilarity matrix of pairwise cultivars was calculated using simple matching coefficient and clustered using TREECON software (Van de Peer and De Wachter, 1994) with the algorithm of unweighted pair-group methods using arithmetic averages (UPGMA). The support for clustering was assessed using 1000 bootstrapped replicates. A principal component analysis of 82 accessions was conducted using NTSYS-PC 2.01 (Rohlf, 1997) based on the similarity matrix of 234 SSR bands, and plots of the first three resulting principal components were made to assess the accession associations and to identify genetically distinct accessions.

To compare the SSR variation among soybean accessions of various groups, the numbers of alleles detected at each locus were calculated. To assess the significance of the difference in allelic count between any two groups of unequal numbers of soybean accessions, the same random permutation procedure as described in Fu et al. (2003) was applied. An analysis of molecular variance (AMOVA; Excoffier et al., 1992) that was based on the dissimilarity matrix of pairwise accessions was also performed using Arlequin version 3.1 (Excoffier et al., 2005) to assess the genetic structure of the soybean germplasm. This analysis not only allowed the partition of the total SSR variation into within- and among-group variation components, but also provided a measure of intergroup genetic distance as the proportion of the total SSR variation residing between any two groups (phi statistic; Excoffier et al., 1992; Huff et al., 1998). Three models of structuring were examined (Canadian vs. exotic germplasm; germplasm of three breeding periods; and germplasm of two major maturity groups excluding the cultivar with MG000). Significance of resulting variance components and intergroup genetic distances was tested with 10,100 random permutations.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
SSR Variation
The 37 SSR primer pairs used detected a total of 37 loci on 18 homologous linkage groups (Table 2), presumably representing 18 (out of 20) pairs of soybean chromosomes (Cregan et al., 1999; Song et al., 2004). No SSR markers were assayed for linkage groups G and Y. Fourteen loci detected by EST-derived SSR markers should represent transcribed chromosomal regions, while the other 23 loci detected by genomic SSR markers may largely sample nongenic chromosomal segments. A total of 234 SSR alleles were detected in this study, but they could include some null alleles, because it was difficult to separate nonamplification due to experimental errors from null alleles. The number of detected alleles per primer pair ranged from two for four primer pairs (AW620774, AW781285, AW186493, and BE823543) to 14 for Satt458, with an average of 6.3 alleles per primer pair. Values of each marker PICs ranged from 0.31 for the marker detected by AW781285 to 0.89 for the marker by Satt458 with an average of 0.63. This variation was significantly associated with the number of alleles detected at each locus. The three most informative loci were Satt458 on linkage group D2, Satt005 on linkage group D1b, and Satt417 on linkage group K. Genomic SSR markers appear to be more informative with an average PIC of 0.69 than EST-derived SSR markers with an average PIC of 0.54. Thus, the SSR markers revealed a relatively large amount of variation in the sampled genome.

The observed occurrence frequencies of the 234 alleles ranged from 0.01 to 0.90 with an average of 0.17. There were two alleles with an occurrence frequency of 0.80 or larger in the assayed accessions and 16 alleles of >0.50, while 150 alleles were detected with frequencies of ≤0.17, 86 alleles of ≤0.05, and 32 alleles of ≤0.02. Some of the rare alleles may be useful as diagnostic markers for some of the assayed soybean cultivars.

Genetic Distinctiveness of Soybean Germplasm
The genetic distinctiveness of a soybean cultivar was measured by the average dissimilarity (AD) of the cultivar against the remaining cultivars assayed. The higher the AD was, the greater the distinctiveness of the genetic background. The AD of the cultivars ranged from 0.161 for OAC Frontier to 0.216 for PS 86 RR with a mean of 0.185 (Table 1). Such levels of AD were relatively lower than those reported with genetic similarity of the other North American soybean cultivars (e.g., Narvel et al., 2000). The six most distinct cultivars were PS86 RR, Gaillard, Manitoba Brown, Beechwood, Maple Isle, and 92B91. With respect to exotic germplasm, the AD of a cultivar ranged from 0.205 for OAC Eramosa to 0.250 for Acme with a mean of 0.221 for all the cultivars assayed (Table 1). These AD values were generally larger than those with respect to the other cultivars, reflecting the distinct backgrounds of the exotic germplasm. The eight most distinct cultivars were Acme, Maple Glen, Maple Presto, 9071, KG20, Apache, Maple Donovan, and PS 86 RR. The AD of the exotic accessions ranged from 0.199 for a South Korean accession (CN 35342) to 0.243 for a Chinese accession (CN 43603) with a mean of 0.219 for all exotic accessions (Table 1). Overall, the ranges of within-group AD values were small, indicating the genetic narrowness of the assayed germplasm (Carter et al., 2004; Hyten et al., 2006).

The ADs shown in Table 1 are limited to only those cultivars or accessions assayed. It would be more informative if more soybean cultivars released in Canada were assessed. This method can recognize the distinctiveness, but not necessarily the relatedness, of cultivars (Fu, 2006). For example, two closely related cultivars that were quite distinct from the remaining cultivars could have similar higher levels of AD than the others and both cultivars would have been identified as genetically distinct cultivars. In spite of these limitations, the relative measure of genetic distinctiveness reported here could provide a guide for selecting specific cultivars with distinct genetic backgrounds for soybean breeding program.

Genetic Relationships of Soybean Cultivars
Similarities among the 45 cultivars reflected in the 234 SSR alleles were calculated and grouped into seven major clusters (Fig. 1 ). The largest, cluster I, consisted of 21 cultivars, followed by two relatively large clusters, IV with 7 cultivars and III with 6. Three other clusters, II, VI, and VII, had three cultivars each, while cluster V had two. The cultivars developed from different breeding programs over various breeding periods were well distributed across the clusters. The cultivars representing maturity groups were also widely spread over the clusters, although most of the MG00 cultivars were mainly located in the largest cluster I (not shown). Assessments on the grouping with respect to pedigree revealed congruency of some clusters with known pedigree. For example, five cultivars (Comet, Pagoda, Mandarin, Portage, and Crest) sharing common parentage were grouped in cluster I. Similarly, KG20, McCall, Nordet, and OAC Scorpio, having common parentage McCall, were grouped in cluster I. Altona, with a parentage of Flambeau was clustered with Flambeau, while AC Dundas was clustered with one of its parents, AC Harmony. There were also many inconsistencies between grouping and known pedigree. For example, cultivars 9063 released from CEF and Gaillard developed from Semico were grouped into cluster V (Fig. 1), but they did not share any common parents. The parentage of 9063 was A3127/Maple Isle, while Gaillard's parents were Ozzie/X1000-2-B-9. Cultivar PS42 with a parentage of Calland/Altona/2/840-7-3/3/Premier was clustered together with OAC Eramosa of parentage Baron/OAC Libra. Similarly, Apache with a parentage of PI 232997/2/Altona/Calland was clustered with 9071 of parentage 9061/9181.


Figure 1
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  		Figure 1. Unweighted pair-group methods using arithmetic averages (UPGMA) dendrogram reflecting genetic relationships of 45 Canadian soybean cultivars based on their simple sequence repeat (SSR) dissimilarities. Bootstrap values higher than 50% (out of 1000 replicates) are indicated above the nodes. The label in parenthesis following each cultivar consists of the breeding program and breeding period specified in Table 1. Seven major clusters are also labeled.

 
Inconsistencies between cultivar clustering and known pedigrees should not be surprising. The selected cultivars were expected to have had more diverse genetic backgrounds because they were selected to represent the efforts of various breeding periods, and thus they were less related genetically. Also, these clusters were not well supported by the statistical assessment with bootstrapping. Many clusters were not stable in association with bootstrap values less than 50%. Moreover, the limited coverage of the soybean genome revealed by 37 SSR loci may contribute to such inconsistencies. Applications of more mapped markers across the genome would increase the resolution to the genetic relationships of these cultivars. In spite of these inconsistencies, the estimated relationships should still be useful for soybean breeding, as they are more informative to parental selection (Cornelious and Sneller, 2002) than those obtained from a traditional pedigree analysis (Cox et al., 1985; Gizlice et al., 1994).

The genetic associations of the 45 cultivars revealed by the principal component analysis (Fig. 2 ) were largely consistent with the genetic relationships described above, although these two principal components accounted for only 24.2% of the total SSR variation. These cultivars representing various breeding programs and breeding periods were widely spread over the plot. No marked cultivar associations with respect to maturity group were observed (not shown). The Canadian cultivars were more related to the soybean accessions from Russia, Sweden, and Ukraine and less to the Asian accessions. Such close relationships were expected, as several Swedish and Russian lines originated in northern China were introgressed into the Canadian breeding programs in the mid-1960s (Beversdorf et al., 1995).


Figure 2
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  		Figure 2. Plot of the first two principal component scores based on the Euclidean distances converted from the simple matching coefficient matrix of 234 simple sequence repeat (SSR) bands for 82 soybean accessions. These two components accounted for 13.7 and 10.5% of the total SSR variance, respectively. Individual accessions of seven groups were separately labeled, including one Swedish accession.

 
Genetic Structures of Soybean Germplasm
The proportion of the total SSR variation which resided between Canadian and exotic germplasm was 9.1% (Table 3 ), but more diversity was observed within the exotic germplasm than within the Canadian cultivars, as revealed by the average pairwise difference within each group (Table 4 ). The proportion of the total SSR variation that resided among the Canadian cultivars of three breeding periods was 10.4% (Table 3). It appears that recently released cultivars (i.e., of the third breeding period) had more diversity than those from the two previous breeding periods, as the average pairwise difference was 36.9 for the cultivars of the first breeding period, 40.0 for the second breeding period, and 42.5 for the third breeding period (Table 4). The MG00 cultivars appeared to show 4.3% more SSR variation than the MG0 cultivars (Table 4). The largest genetic distance measured by the proportion of the total SSR variation residing between two groups of soybean germplasm was observed between exotic germplasm and cultivars released in the first breeding period. The largest genetic distance was also observed among the cultivars of the first and third breeding periods (Table 4).


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  		Table 3. Results for analysis of molecular variance (AMOVA) for 82 soybean germplasm accessions with three models of structuring.

 

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  		Table 4. Results for analysis of molecular variance (AMOVA) of 234 microsatellite alleles for 37 exotic accessions and 45 Canadian cultivars of three breeding periods. The average pairwise difference among accessions of a given group is given on the diagonal. The proportion of the total simple sequence repeat (SSR) variation residing between any two groups is given below the diagonal.{dagger}

 
Based on allelic counts, exotic germplasm not only had more alleles (207) than the Canadian cultivars (173), but also possessed more unique alleles (61 vs. 27) (Table 2). These differences were statistically significant, based on the permutation test for unequal sampling sizes of various groups (Table 2). Exotic germplasm was genetically more distinct than the Canadian cultivars as revealed by the differences in AD magnitude (0.219 for exotic and 0.185 for cultivar, Table 1). After accounting for unequal sample size among the breeding periods, there was a significant difference in AD between P1 and P3 at the Satt409 locus (Table 2).

These results indicate exotic germplasm had significantly more SSR variation than the Canadian cultivars assayed. No significant diversity reduction was found between early and recent Canadian cultivars. This finding may also reflect some sampling bias of cultivar selection, as many important public cultivars such as OAC 211 released in 1922 and Harosoy in 1951 were not assayed in this study. The difficulty in obtaining germplasm released in the past 20 yr from private companies may also contribute to such a sampling bias. Moreover, bias might exist in the selection of exotic germplasm, as the current PGRC soybean collection is not well representative of the genetic diversity or the extent of soybean germplasm worldwide. Thus, it is important to assay more soybean germplasm.

Implications for Canadian Soybean Breeding
Our results indicate that Canadian cultivars have maintained a broad degree of genetic diversity. This is revealed by the fact that cultivars released after 1990 had slightly more diversity than those before 1970. Substantial genetic variation still exists within the Canadian soybean gene pool and selection within the gene pool is still feasible. The genetic relationships and distinctiveness obtained for the cultivars in this study should facilitate the selection of less related germplasm for intercrossing. A close relationship of Canadian cultivars to germplasm from northeast Europe appears to support the continuous search from this region for new adapted germplasm for yield improvement. Less genetic diversity of the Canadian soybean germplasm than the selected exotic germplasm underlines the need for a continuous effort to diversify the Canadian soybean gene pool from exotic germplasm to ensure sustainable breeding programs in the future.


   ACKNOWLEDGMENTS
 
The authors would like to thank Ken Richards, Elroy Cober, and Harvey Voldeng for their encouraging discussions on the project including cultivar selection; Dallas Kessler for his assistance in sampling seeds; Brian Couture for seed handling and preparation; and Stephen Molnar, Elroy Cober, and three anonymous reviewers for their helpful comments on the early version of the manuscript.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
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Received for publication January 2, 2007.


   REFERENCES
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 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 




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