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a Horticultural Sciences Dep., Univ. of Florida, Institute of Food and Agricultural Sciences, Gainesville, FL 32611-0690
b Dep. of Crop Sciences, Univ. of Illinois, Urbana, IL 61801
* Corresponding author (ekabelka{at}ifas.ufl.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: ANOVA, analysis of variance • LG, linkage group • NIL, near isogenic line • PCR, polymerase chain reaction • PI, plant introduction • QTL, quantitative trait loci • SCN, soybean cyst nematode • SSR, simple sequence repeat
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Molecular marker studies have shown that the G. max sources of SCN resistance have genes in common (Diers and Arelli, 1999; Concibido et al., 2004). Plant introduction 437654 (Webb et al., 1995), PI 209332, PI 88788, PI 90763, PI 89772, and Peking (Concibido et al., 1996, 1997; Chang et al., 1997; Yue et al., 2001a) all have the major SCN resistance gene, rhg1, on linkage group G (Cregan et al., 1999). This locus controls a large portion of the total variation for resistance and is effective against several HG Types of SCN (Concibido et al., 1996, 1997, 2004). In addition, Peking (Matson and Williams, 1965; Mahalingam and Skorupska, 1995; Chang et al., 1997), PI 209332 (Concibido et al., 1994), and PI 437654 (Webb et al., 1995) have the resistance gene Rhg4 that maps near the I locus (black seed-coat pigmentation) on linkage group A2 (Cregan et al., 1999).
It has been estimated that soybean cultivars selected for resistance to SCN, derived from G. max, yield 5 to 10% less than susceptible cultivars when grown in environments with low SCN pressure (Noel, 1992). This was noted by Chen et al. (1999) who reported that soybean cultivars with SCN resistance from PI 88788 yielded on average 161 kg ha–1 less than susceptible cultivars in noninfested field trials in Minnesota during 1999. The reduced yield associated with SCN resistance in noninfested, or low SCN pressure environments, can be attributed to pleiotropic effects of the SCN resistance gene(s) on yield or linkage and coinheritance of genes effecting yield.
Linkage between SCN resistance and reduced yield was reported by Mudge et al. (1996). In their study, populations segregating for SCN resistance derived from G. max PI 209332 revealed yield reducing quantitative trait loci (QTL) alleles in coupling linkage with the SCN resistance gene rhg1. These yield reducing alleles mapped approximately 10 cM from each other and a difference of 296 kg ha–1 for the QTL distal to rhg1 and 632 kg ha–1 for the QTL proximal to rhg1 was measured when homozygous resistant and susceptible lines were compared. This region was also associated with an increase in height and lodging, later maturity, and a decrease in seed protein and oil content. Kopisch-Obuch et al. (2005) tested for linkage between SCN resistance and reduced yield in near isogenic line (NIL) populations developed from soybean cultivars with resistance derived from G. max PI 88788. Five NIL populations segregated for resistance at rhg1 and two segregated for resistance at cqSCN-003 on LG J. In multiple field studies at locations with low SCN pressure, NILs carrying the SCN resistance allele yielded significantly (P < 0.05) less (118 kg ha–1) than NILs carrying the susceptible alleles in one population segregating for rhg1 and in one population segregating for cqSCN-003 (76 kg ha–1). Molecular marker analysis of the regions flanking the resistance genes suggested the presence of a yield reducing allele distal to rhg1 and possibly another yield reducing allele linked or pleiotropic to cqSCN-003. In several populations, an association between SCN resistance with maturity, height, and lodging was measured, but differences were small in magnitude. Brucker et al. (2005) tested two of the NIL populations that segregated for rhg1 in fields moderately to highly infested with SCN. They found that the resistance allele at rhg1 was associated with significantly greater yield (273 kg ha–1) than the susceptible allele for one population. In both populations, they observed that the rhg1 resistance allele was associated with reduced SCN reproduction compared with the susceptible allele.
Glycine soja, the wild ancestor of soybean, has genetic diversity not present within G. max soybean germplasm (Keim et al., 1989; Maughan et al., 1995). In 2001, Wang et al. (2003) mapped two major loci that conferred resistance to PA3, a HG Type 0 (Niblack et al., 2002) (Race 3) SCN isolate, in a population of F2–derived lines developed from a cross with G. soja PI 468916. One locus mapped in the interval between Satt598 and Satt491 on LG E and the second locus mapped in the interval between Satt288 and Satt472 on LG G. The locus on LG G mapped to a position where no SCN resistance loci has been reported and is over 72 cM from the major SCN resistance gene, rhg1, from G. max [http://soybase.ncgr. org (verified 20 November 2005; Concibido et al., 2004]. However, a SCN resistance locus has been reported from G. max PI 438489B at the same location as the LG E locus from G. soja (Yue et al., 2001b). Kabelka et al. (2005) recently confirmed the G. soja SCN resistance loci in a population of BC4F3–derived lines referred to as LDX01–1. G. soja PI 468916 was used as the donor parent and the experimental line A81–356022 was the recurrent parent during backcrossing. Using a PA3 HG Type 0 SCN isolate, they observed significant associations (P < 0.05) between a greenhouse SCN bioassay of the LDX01–1 population and markers linked to the SCN resistance loci on LGs E and G. Additional molecular markers were added within the regions containing the SCN resistance loci by using amplified fragment length polymorphism (AFLP) markers and bulked segregant analysis (BSA). The positions of the resistance loci also were better defined by developing and testing several backcross populations that segregated for different genetic regions where the resistance loci map.
While studies have confirmed the G. soja PI 468916 SCN resistance loci, the effect of each locus on seed yield has not been investigated. Therefore, the objective of this research was to determine the effect of each G. soja PI 468916 SCN resistance gene on seed yield and other agronomic traits. This association was tested in two populations that each segregated for SCN resistance derived from G. soja PI 468916.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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A second population of 100 F3–derived lines, referred to as LDX01–2, was developed by crossing the maturity group II cultivar Dwight with a BC3F1 plant carrying the G. soja SCN resistance QTL alleles. Dwight possesses the LG G rhg1 resistance allele from G. max PI 88788 (Nickell et al., 1998). The BC3F1 parent was developed by backcrossing the two G. soja SCN resistance alleles into the background of A81–356022 as part of the same backcrossing program that developed LDX01–1. F1 plants produced through crossing the BC3F1 plant with Dwight were tested with markers and an F1 that was heterozygous for all three resistance loci was selected. An F3 population was developed from this selected plant through single-seed descent. F3 plants were sown and individually harvested to develop F3:4 lines. Seed of each line was sown and bulk harvested for use in the 2003 (F3:5) and 2004 (F3:6) field tests.
Field Trials
LDX01–1 was field tested at three locations in 2003, designated Numbers 1 (Crop Science Main Farm, Urbana, IL, planted 13 May), 2 (Ivesdale, IL, planted 17 May), and 3 (East Grein Tract, Urbana, IL, planted 16 May), and at two locations in 2004, designated Numbers 4 (Cruse Farm Tract, Urbana, IL, planted May 6) and 5 (Ivesdale, IL, planted 5 May). LDX01–2 was field tested at two locations in 2003, designated Numbers 1 (Ivesdale, IL, planted 14 May) and 2 (East Grein Tract, Urbana, IL, planted 16 May), and at two locations in 2004, designated Numbers 3 (Cruse Farm Tract, Urbana, IL, planted 6 May) and 4 (Ivesdale, IL, planted 5 May). The two populations were evaluated in separate tests by a randomized complete-block design with two replications at each location. In the LDX01–2 field trial Number 3, only one replication could be evaluated because of heavy rains and flooding. Cultivars Dwight (SCN resistant) and IA2052 (SCN susceptible) and experimental line A81–356022 (SCN susceptible) were included as checks. At all locations, 1.5- x 3.2-m two-row plots were planted at 395 000 seeds ha–1. Conventional tillage practices were followed, fields were maintained weed-free, and recommended fertilization levels were applied at all locations.
Plots were evaluated for days to maturity, plant height, lodging, and seed yield. Days to maturity was recorded as the number of days after planting when approximately 95% of the pods had reached mature pod color (R8; Fehr et al., 1971). Plant height was measured at maturity in centimeters as the average distance from soil surface to the apex of the main stem. Lodging was scored at maturity on a scale of 1 to 5 with 1 designated as plants standing erect and 5 as plants prostrate. Seed yield expressed as kilograms per hectare was adjusted to 130 g kg–1 moisture.
Soil Samples and Soybean Cyst Nematode Egg Densities
Soil samples were taken at the R1 soybean growth stage (Fehr et al., 1971) from 10 randomly chosen plots in each replication per location and population. Each 2.5-cm diameter soil sample core was taken at approximately 80 mm from the plant row at a depth of 0.2 m and stored at 4°C until processed. Cyst extraction and staining to determine SCN egg density in a 100 cm3 soil sample was performed according to Kopisch-Obuch et al. (2005).
Genetic Characterization of LDX01–1 and LDX01–2
Lines in LDX01–1 were tested with the simple sequence repeat (SSR) markers, Satt598 and Satt491 on LG E and Satt288 and Satt472 on LG G. These markers span the genetic regions conferring SCN resistance introgressed into LDX01–1 and LDX01–2 from G. soja PI 468916 (Fig. 1
). Lines within LDX01–1 had been previously genotyped with the above SSR markers (Kabelka et al., 2005). Lines within LDX01–2 were genotyped with SSR markers Satt491, Satt288, Satt472, and Satt309. Satt309 maps approximately 0.4 cM from the rhg1 SCN resistance allele from Dwight (Cregan et al., 1999) (Fig. 1). Chi-square analyses revealed that each SSR marker segregated according to expected ratios within populations LDX01–1 and LDX01–2.
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Statistical Analysis
Agronomic traits were subject to analysis of variance (ANOVA) by the PROC GLM functions of SAS (Statistical Analysis System version 9.1, SAS Institute, Cary, NC). Lines within populations were considered as a fixed effect whereas environments, replications within environments, and environment x line interactions were considered as random effects. Each location–year combination was considered an environment in the analysis. Significance was determined by an F test with the numerator and denominator degrees of freedom approximated as described by Satterthwaite (1946). Least significant differences (LSD) between entry means were determined at a 5% significance level. Estimates of variance components were obtained using the REML method of PROC VARCOMP of SAS. Single-factor ANOVA was used to identify marker association with each agronomic trait using PROC GLM in SAS. Marker genotypes, used as class variables, were considered as fixed effects whereas environments, replications, and lines were considered random effects. Pair-wise comparisons of least square means were used to test the significance (P < 0.05) of each marker class. While all SSR markers were evaluated in this study, only the most significant markers associated with SCN resistance, across environments, were listed (Tables 1 and 2).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Across environments in the LDX01–1 field trials, the average seed yields of A81–356022 (3071 kg ha–1) and lines in LDX01–1 (3097 kg ha–1) were at least 6% less than the average seed yields of Dwight (3454 kg ha–1) or IA2052 (3299 kg ha–1) (Table 4). Within the moderate to high SCN infested environments (Numbers 1, 2, and 3), and across all environments, the highest yielding lines within LDX01–1 did not yield significantly different from either Dwight or IA2052. However, within the low SCN infested environments (Numbers 4 and 5), lines were present that yielded at least 11% higher than the checks. These high yielding lines tended to mature later than the checks, however. On average, Dwight and IA2052 matured 6 to 9 d earlier, were shorter in plant height, and lodged the least compared with either A81–356022 or the mean of lines in LDX01–1 (Table 5).
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Association between Soybean Cyst Nematode Resistance Loci and Agronomic Traits within LDX01–1
Across environments, the introgressed G. soja regions on LG E, marked by Satt491, and LG G, marked by Satt472, were associated with enhanced seed yield ranging from 74 to 94 kg ha–1 (Table 1). Within the low SCN infested environments (Numbers 4 and 5), the two regions either had no significant effect on seed yield or enhanced it up to 155 kg ha–1. Within the moderate to high SCN infested environments, (Numbers 1, 2, and 3), the two regions either had no significant effect on seed yield or enhanced it up to 94 kg ha–1. Across and within certain environments, the genetic regions on LGs E and G were also associated with either increasing days to maturity by up to 1 d, plant height by up to 4.6 cm, or lodging scores by up to 0.4 units.
Lines within LDX01–1 that carried both G. soja SCN resistance loci averaged, across environments, 3163 kg ha–1 in yield, matured at 127 d, were 106 cm tall, and had a lodging score of 2.6. In comparison, lines that carried both the G. max SCN susceptible alleles at these loci averaged, across environments, 2936 kg ha–1, matured at 125 d, were 105 cm tall, and had a lodging score of 2.1. A line recently released from LDX01–1, designated LDX01–1–65 (Diers et al., 2005), averaged, across environments, 3265 kg ha–1 in yield, matured at 128 d, was 107 cm tall, and had a lodging score of 2.9.
Association between Soybean Cyst Nematode Resistance Loci and Agronomic Traits within LDX01–2
Within and across environments, the genetic region introgressed from G. soja on LG E, marked by Satt491, did not have a significant effect on seed yield (Table 2). However, this region, across and within certain environments, was associated with increasing days to maturity by up to 2 d, plant height by up to 3.5 cm and lodging scores by up to 0.2 units. The introgressed G. soja region on LG G, marked by Satt472, within the low SCN infested environment (Number 4), and across all environments, was significantly associated with enhancing seed yield by up to 208 kg ha–1. Within the moderate to high SCN infested environments (Numbers 1, 2, and 3), this region either had no significant effect on seed yield or enhanced it up to 168 kg ha–1. The introgressed G. soja LG G region, across and within certain environments, was also associated with increasing days to maturity by 1 d and lodging scores by 0.1 units. Segregation at the G. max rhg1 SCN locus on LG G marked by Satt309, across environments and within the low (Number 4) and high (Number 5) SCN infested environments, did not have a significant association with seed yield. However within the environments considered to have moderate SCN infestation (Numbers 2 and 3), the resistance allele was associated with a seed yield increase of up to 242 kg ha–1. This resistance allele was also associated with increasing lodging scores by up to 0.4 units within and across all environments.
Lines within LDX01–2 carrying all three SCN resistance loci averaged, across environments, 3051 kg ha–1 in yield, matured at 121 d, were 84 cm tall, and had a lodging score of 1.5. Lines within LDX01–2 that carried the SCN susceptible alleles at these loci averaged, across environments, 2853 kg ha–1, matured at 120 d, were 86 cm tall, and had a lodging score of 1.4.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The positive yield response observed for the G. soja LG E locus in the backcross population is consistent with results reported by Wang et al. (2004) who also used PI 468916 as a donor parent of backcross populations. They tested BC2 populations for yield and other agronomic traits across four environments and found that the G. soja allele at Satt491 was associated with 53 kg ha–1 greater seed yield than the allele from ‘IA2008’, the recurrent parent. Although SCN infestation levels were not quantified for their field environments, high infestation levels were not observed at any location.
The results from our study indicate that the G. soja SCN resistance alleles can be used in soybean improvement without major concerns of linkage drag reducing yield. When genes are introgressed from exotic sources, linked drag often hinders breeding progress (Tanksley and McCouch, 1997). The introgression of the two G. soja resistance genes had a nondetectable to a positive effect on yield, suggesting that negative yield effects were not linked to these genes. We cannot resolve with certainty whether the positive yield effects were the result of SCN resistance, or linked genes that have a positive impact on yield. This is especially difficult to determine because there was no clear pattern between SCN infestation levels and the effect of these regions on yield. To resolve this, the populations would need to be tested in fields that have no SCN infestations; which are extremely rare in Illinois.
A factor that may have contributed to the lack of association between yield and SCN infestation levels is diversity in the pathogenicity of SCN populations in the field locations. The SCN populations in some locations may be poorly controlled by the G. soja resistance loci, which would have resulted in little or no yield benefit from these genes. A line carrying both resistance loci that was released from the backcross LDX01–1 population, designated LDX01–1–65, was evaluated in the greenhouse with three SCN isolates (Diers et al., 2005). The female index of this line was 14 for PA 3, a HG type 0 SCN isolate; 57 for PA2, a HG type 1.2.5.7 isolate; and 3 for PA5, a HG type 2.5.7 isolate. If we had an SCN population similar to the HG type 1.2.5.7 isolate in our field, little yield benefit may have resulted from the resistance gene(s). In retrospect, we should have greenhouse-tested lines from the populations with SCN samples from each field environment to quantify the control we achieved with the resistance.
This study also revealed that the SCN resistance allele rhg1 derived from G. max PI 88788 within LDX01–2 either had no effect on seed yield or enhanced it by up to 7% in environments with moderate SCN infestations. The rhg1 locus was also found, within and across environments, to be associated with increasing lodging score by up to 0.4 units. Why rhg1 was not effective under the high SCN infestation environment in this study is not entirely clear. In fact, rhg1 has been shown to be effective in environments with even greater SCN infestation levels than observed in our Environment 1 (Brucker et al., 2005). The most likely explanation for the lack of association observed in Environment 1 is that, as SCN populations can differ for their virulence genes, it is possible the SCN population and its elevated levels at this site overcame the PI 88788 rhg1 allele that segregated in the LDX01–2 population. However, it is also possible that rhg1 alone could have had a positive effect on yield but because of the two G. soja resistance genes, also segregating in this population, the effect of rhg1 may have been diluted enough so that its influence on yield was not significant.
Comparison of our rhg1 findings with those of others (Mudge et al., 1996; Chen et al., 1999; Kopisch-Obuch et al., 2005) continues to suggest that the association between the G. max SCN resistance locus rhg1 and reduced seed yield in environments without high SCN infestations can be broken. In Kopisch-Obuch et al. (2005), the presence of a yield depression locus at least 3 cM distal to G. max rhg1 on LG G was identified. Their data also revealed possible breakage of the linkage between rhg1 and this yield depression locus in two out of three populations evaluated that segregated at this genetic region. Possibly in our LDX01–2 population, linkage between rhg1 and a yield reduction locus has been broken. Additional genetic analysis within and around rhg1 in LDX01–2 would be necessary to confirm this.
We detected an unfavorable association between all three SCN resistance alleles and greater lodging, which is a concern in cultivar development programs. This association was notable in LDX01–1, where the marker alleles from the resistance sources were associated with 0.2 (LG E) to 0.4 (LG G) greater lodging scores, across and within certain environments, than the susceptible alleles. The increased lodging score association with rhg1 (LG G) in LDX01–2 ranged from 0.1 to 0.4 units within and across all environments. This unfavorable lodging association may need to be resolved before or during cultivar development efforts.
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ACKNOWLEDGMENTS |
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Received for publication June 15, 2005.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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