Genetic mapping revealed two loci for soybean aphid resistance in PI 567301B

The soybean aphid (Aphis glycines Matsumura) is the most damaging insect pest of soybean [Glycine max (L.) Merr.] in North America. New soybean aphid biotypes have been evolving quickly and at least three confirmed biotypes have been reported in USA. These biotypes are capable of defeating most known aphid resistant soybean genes indicating the need for identification of new genes. Plant Introduction (PI) 567301B was earlier identified to have antixenosis resistance against biotype 1 and 2 of the soybean aphid. Two hundred and three F_7:9 recombinant inbred lines (RILs) developed from a cross of soybean aphid susceptible cultivar Wyandot and resistant PI 567301B were used for mapping aphid resistance genes using the quantitative trait loci (QTL) mapping approach. A subset of 94 RILs and 516 polymorphic SNP makers were used to construct a genome-wide molecular linkage map. Two candidate QTL regions for aphid resistance were identified on this linkage map. Fine mapping of the QTL regions was conducted with SSR markers using all 203 RILs. A major gene on chromosome 13 was mapped near the previously identified Rag2 gene. However, an earlier study revealed that the detached leaves of PI 567301B had no resistance against the soybean aphids while the detached leaves of PI 243540 (source of Rag2) maintained aphid resistance. These results and the earlier finding that PI 243540 showed antibiosis resistance and PI 567301B showed antixenosis type resistance, indicating that the aphid resistances in the two PIs are not controlled by the same gene. Thus, we have mapped a new gene near the Rag2 locus for soybean aphid resistance that should be useful in breeding for new aphid-resistant soybean cultivars. Molecular markers closely linked to this gene are available for marker-assisted breeding. Also, the minor locus found on chromosome 8 represents the first reported soybean aphid-resistant locus on this chromosome.     

Mapping soybean aphid resistance genes in PI 567598B

The soybean aphid (Aphis glycines Matsumura) has been a major pest of soybean [Glycine max (L.) Merr.] in North America since it was first reported in 2000. Our previous study revealed that the strong aphid resistance of plant introduction (PI) 567598B was controlled by two recessive genes. The objective of this study was to locate these two genes on the soybean genetic linkage map using molecular markers. A mapping population of 282 F_4:5 lines derived from IA2070 × E06902 was evaluated for aphid resistance in a field trial in 2009 and a greenhouse trial in 2010. Two quantitative trait loci (QTLs) were identified using the composite and multiple interval mapping methods, and were mapped on chromosomes 7 (linkage group M) and 16 (linkage group J), respectively. E06902, a parent derived from PI 567598B, conferred resistance at both loci. In the 2010 greenhouse trial, each of the two QTLs explained over 30 % of the phenotypic variation. Significant epistatic interaction was also found between these two QTLs. However, in the 2009 field trial, only the QTL on chromosome 16 was found and it explained 56.1 % of the phenotypic variation. These two QTLs and their interaction were confirmed with another population consisting of 94 F_2:5 lines in the 2008 and 2009 greenhouse trials. For both trials in the alternative population, these two loci explained about 50 and 80.4 % of the total phenotypic variation, respectively. Our study shows that soybean aphid isolate used in the 2009 field trial defeated the QTL found on chromosome 7. Presence of the QTL on chromosome 16 conferred soybean aphid resistance in all trials. The markers linked to the aphid-resistant QTLs in PI 567598B or its derived lines can be used in marker-assisted breeding for aphid resistance.     

QTL underlying the resistance to soybean aphid (<Emphasis Type="Italic">Aphis glycines</Emphasis> Matsumura) through isoflavone-mediated antibiosis in soybean cultivar ‘Zhongdou 27’

Soybean aphid (Aphis glycines Matsumura) results in severe yield loss of soybean in many soybean-growing countries of the world. A few loci have been previously identified to be associated with the aphid resistance in soybean. However, none of them was via isoflavone-mediated antibiosis process. The aim of the present study was to conduct genetic analysis of aphid resistance and to identify quantitative trait loci (QTL) underlying aphid resistance in a Chinese soybean cultivar with high isoflavone content. One hundred and thirty F_5:6 derived recombinant inbred lines from the ‘Zhongdou 27’ × ‘Jiunong 20’ cross were used. Two QTL were directly associated with resistance to aphid as measured by aphid damage index. qRa_1, close to Satt470 on soybean linkage group (LG) A2 (chromosome 8), was consistently detected for 3- and 4-week ratings and explained a large portion of phenotypic variations ranging from 25 to 35%. qRa_2, close to Satt144 of LG F (chromosome 13), was detected for 3- and 4-week ratings and could explain 7 and 11% of the phenotypic variation, respectively. These two QTL were highly associated with high isoflavone content and both positive alleles were derived from ‘Zhongdou 27’, a cultivar with higher isoflavone content. The results revealed that higher individual or total isoflavones contents in soybean lines could protect soybean against aphid attack. These two QTL detected jointly provide potential for marker-assisted selection to improve the resistance of soybean cultivars to aphid along with the increase of isoflavone content.     

Genetic analysis and mapping of genes for resistance to multiple strains of Soybean mosaic virus in a single resistant soybean accession PI 96983

Soybean mosaic virus (SMV) is one of the most broadly distributed soybean (Glycine max (L.) Merr.) diseases and causes severe yield loss and seed quality deficiency. Multiple studies have proved that a single dominant gene can confer resistance to several SMV strains. Plant introduction (PI) 96983 has been reported to contain SMV resistance genes (e.g., Rsv1 and Rsc14) on chromosome 13. The objective of this study was to delineate the genetics of resistance to SMV in PI 96983 and determine whether one gene can control resistance to more than one Chinese SMV strain. In this study, PI 96983 was identified as resistant and Nannong 1138-2 was identified as susceptible to four SMV strains SC3, SC6, SC7, and SC17. Genetic maps based on 783 F₂ individuals from the cross of PI 96983 × Nannong 1138-2 showed that the gene(s) conferring resistance to SC3, SC6, and SC17 were between SSR markers BARCSOYSSR_13_1114 and BARCSOYSSR_13_1136, whereas SC7 was between markers BARCSOYSSR_13_1140 and BARCSOYSSR_13_1185. The physical map based on 58 recombinant lines confirmed these results. The resistance gene for SC7 was positioned between BARCSOYSSR_13_1140 and BARCSOYSSR_13_1155, while the resistance gene(s) for SC3, SC6, and SC17 were between BARCSOYSSR_13_1128 and BARCSOYSSR_13_1136. We concluded that, there were two dominant resistance genes flanking Rsv1 or one of them at the reported genomic location of Rsv1. One of them (designated as “Rsc-pm”) conditions resistance for SC3, SC6, and SC17 and another (designated as “Rsc-ps”) confers resistance for SC7. The two tightly linked genes identified in this study would be helpful to cloning of resistance genes and breeding of multiple resistances soybean cultivars to SMV through marker-assisted selection (MAS).     

Genetic mapping of a novel gene for soybean aphid resistance in soybean (Glycine max [L.] Merr.) line P203 from China

The soybean aphid (SA: Aphis glycines Matsumura) is a worldwide pest of soybean (Glycine max [L.] Merr.). The objectives of this study were to identify the type of aphid resistance and the resistance phenotype in soybean line ‘P203’, and to map the relative position of the gene involved. Compared with cultivars ‘P746’ and ‘Dongnong 47’, P203 was demonstrated to possess antixenosis resistance. P203 prevented aphids from reproducing in a choice test, but the resistance level decreased significantly in a no-choice test at 11 and 21 days after infestation. Analysis of 273 Dongnong 47/P203 F₂ plants and confirmed using 260 F_2:3 families revealed that a single dominant gene from P203 was positioned between marker loci Sat_377 and Satt409 on chromosome 8. The gene was further mapped to a 1.57 Mb interval flanked by marker loci BARCSOYSSR_08_1451 and BARCSOYSSR_08_1527. We developed five new SSR markers in the target interval and the resistance locus mapped between new markers SSR_08_75 and SSR_08_88 with the genetic distance of 1.1 and 1.0 cM corresponding to a physical distance of 192 kb on the Williams 82 8X draft genome assembly (Glyma1.01). A single serine/threonine protein kinase gene is present in this region, suggesting that the SA resistance mechanism in P203 may be different from those previously reported. Therefore, the resistance gene could very well be novel, and could be valuable in soybean aphid resistance breeding programs.     

Inheritance of soybean aphid resistance in 21 soybean plant introductions

Key Message

The Rag2 region was frequently identified among 21 F ₂ populations evaluated for soybean aphid resistance, and dominant gene action and single-gene resistance were also commonly identified.

Abstract

The soybean aphid [Aphis glycines Matsumura (Hemiptera: Aphididae)] is one of the most important insect pests of soybean [Glycine max (L.) Merr] in the northern USA and southern Canada, and four resistance loci (Rag1–rag4) have been discovered since the pest was identified in the USA in 2000. The objective of this research was to determine whether resistance expression in recently identified soybean aphid-resistant plant introductions (PIs) was associated with the four Rag loci using a collection of 21 F₂ populations. The F₂ populations were phenotyped with soybean aphid biotype 1, which is avirulent on plants having any of the currently identified Rag genes, using choice tests in the greenhouse and were tested with genetic markers linked to the four Rag loci. The phenotyping results indicate that soybean aphid resistance is controlled by a single dominant gene in 14 PIs, by two genes in three PIs, and four PIs had no clear Mendelian inheritance patterns. Genetic markers flanking Rag2 were significantly associated with aphid resistance in 20 PIs, the Rag1 region was significantly identified in five PIs, and the Rag3 region was identified in one PI. These results show that single dominant gene action at the Rag2 region may be a major source for aphid resistance in the USDA soybean germplasm collection.     

A novel locus for soybean aphid resistance

The soybean aphid (Aphis glycines Matsumura) is an important pest on soybean [Glycine max (L.) Merr.] in North America. Aphid resistance has recently been found on plant introduction (PI) 567543C, but little is known about its genetic control. The objectives of this study were to identify the resistance genes in PI 567543C with molecular markers and validate them in a different genetic background. A mapping population of 249 F₄ derived lines from a cross between PI 567543C and a susceptible parent was investigated for aphid resistance in both the greenhouse and the field. The broad sense heritability of aphid resistance in the field trial was over 0.95. The segregation of aphid resistance in this population suggests a major gene controlling the resistance. Bulked segregant analysis with molecular markers revealed a potential genomic region. After saturating this putative region with more markers, a genetic locus was mapped in an interval between Sat_339 and Satt414 on chromosome 16 (linkage group J) using the composite interval mapping method. This locus explained the majority of the phenotypic variation ranging from 84.7% in the field trial to 90.4% in the greenhouse trial. Therefore, the aphid resistance in PI 567543C could be mainly controlled by this gene. This aphid resistance gene was mapped on a different chromosome than the other resistance genes reported previously from other resistant germplasms. This gene appears to be additive based on the aphid resistance of the heterozygous lines at this locus. Thus, a new symbol Rag3 is used to designate this gene. Moreover, Rag3 was confirmed in a validation population. This new aphid-resistance gene could be valuable in breeding aphid resistant cultivars.     

Identification of QTL in soybean underlying resistance to herbivory by Japanese beetles (Popillia japonica, Newman)

Soybean [Glycine max (L.) Merr.] was one of the most important legume crops in the world in 2010. Japanese beetles (JB; Popillia japonica, Newman) in the US were an introduced and potentially damaging insect pest for soybean. JBs are likely to spread across the US if global warming occurs. Resistance to JB in soybean was previously reported only in plant introductions. The aims here were to identify loci underlying resistance to JB herbivory in recombinant inbred lines (RILs) derived from the cross of Essex × Forrest cultivars (EF94) and to correlate those with loci with factors that confer insect resistance in soybean cultivars. The RIL population was used to map 413 markers, 238 satellite markers and 177 other DNA markers. Field data were from two environments over 2 years. Pest severity (PS) measured defoliation on a 0–9 scale. Pest incidence (PI) was the percentage of plants within each RIL with beetles on them. Antibiosis and antixenosis data were from feeding assays with detached leaves in petri plates. Five QTL were detected for the mean PS field trait (16% < R ² < 27%). The loci were within the intervals Satt632–A2D8 on linkage group (LG) A2 (chromosome 8); Satt583–Satt415 on LG B1 (11); Satt009–Satt530 on LG N (3); and close to two markers OB02_140 (LG E; 20 cM from Satt572) and OZ15_150 LG (19 cM from Satt291 C2). Two QTL were detected for the mean PI field trait (16% < R ² < 18%) close to Satt385 on LG A1 and Satt440 on LG I. The no choice feeding studies detected three QTL that were significant; two for antixenosis (22% < R ² < 24%) between Satt632–A2D8 on LG A2 (8) and Sat_039–Satt160 on LG F (13); and a major locus effect (R ² = 54%) for antibiosis on LG D2 (17) between Satt464–Satt488. Therefore, loci underlying resistance to JB herbivory were a mixture of major and minor gene effects. Some loci were within regions underlying resistance to soybean cyst nematode (LGs A2 and I) and root knot nematode (LG F) but not other major loci underlying resistance to nematode or insect pests (LGs G, H and M).     

Novel quantitative trait loci for partial resistance to Phytophthora sojae in soybean PI 398841

Phytophthora root and stem rot caused by Phytophthora sojae Kaufmann and Gerdemann is one of the most severe soybean [Glycine max (L.) Merr] diseases in the USA. Partial resistance is as effective in managing this disease as single-gene (Rps gene)-mediated resistance and is more durable. The objective of this study was to identify quantitative trait loci (QTL) associated with partial resistance to P. sojae in PI 398841, which originated from South Korea. A population of 305 F_7:8 recombinant inbred lines derived from a cross of OX20-8 × PI 398841 was used to evaluate partial resistance against P. sojae isolate C2S1 using a tray test. Composite interval mapping using a genome-wide logarithm of odd (LOD) threshold detected three QTL on chromosomes 1, 13, and 18, which individually explained 4–16 % of the phenotypic variance. Seven additional QTL, accounting for 2–3 % of phenotypic variance each, were identified using chromosome-wide LOD thresholds. Seven of the ten QTL for resistance to P. sojae were contributed by PI 398841. Seven QTL co-localized with known Rps genes and previously reported QTL for soil-borne root pathogens, isoflavone, and seed oil. Three QTL on chromosomes 3, 13, and 18 co-localized with known Rps genes, but PI 398841 did not exhibit an Rps gene-mediated resistance response following inoculation with 48 different isolates of P. sojae. PI 398841 is potentially a source of novel genes for improving soybean cultivars for partial resistance to P. sojae.     

Reduced Fitness of Virulent Aphis glycines (Hemiptera: Aphididae) Biotypes May Influence the Longevity of Resistance Genes in Soybean

Sustainable use of insect resistance in crops require insect resistance management plans that may include a refuge to limit the spread of virulence to this resistance. However, without a loss of fitness associated with virulence, a refuge may not prevent virulence from becoming fixed within a population of parthenogenetically reproducing insects like aphids. Aphid-resistance in soybeans (i.e., Rag genes) prevent outbreaks of soybean aphid (Aphis glycines), yet four biotypes defined by their capacity to survive on aphid-resistant soybeans (e.g., biotype-2 survives on Rag1 soybean) are found in North America. Although fitness costs are reported for biotype-3 on aphid susceptible and Rag1 soybean, it is not clear if virulence to aphid resistance in general is associated with a decrease in fitness on aphid susceptible soybeans. In laboratory assays, we measured fitness costs for biotype 2, 3 and 4 on an aphid-susceptible soybean cultivar. In addition, we also observed negative cross-resistance for biotype-2 on Rag3, and biotype-3 on Rag1 soybean. We utilized a simple deterministic, single-locus, four compartment genetic model to account for the impact of these findings on the frequency of virulence alleles. When a refuge of aphid susceptible was included within this model, fitness costs and negative cross-resistance delayed the increase of virulence alleles when virulence was inherited recessively or additively. If virulence were inherited additively, fitness costs decreased the frequency of virulence. Combined, these results suggest that a refuge may prevent virulent A. glycines biotypes from overcoming Rag genes if this aphid-resistance were used commercially in North America.     

Performance and prospects of Rag genes for management of soybean aphid

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an invasive insect pest of soybean [Glycine max (L.) Merr. (Fabaceae)] in North America, and it has led to extensive insecticide use in northern soybean‐growing regions there. Host plant resistance is one potential alternative strategy for managing soybean aphid. Several Rag genes that show antibiosis and antixenosis to soybean aphid have been recently identified in soybean, and field‐testing and commercial release of resistant soybean lines have followed. In this article, we review results of field tests with soybean lines containing Rag genes in North America, then present results from a coordinated regional test across several field sites in the north‐central USA, and finally discuss prospects for use of Rag genes to manage soybean aphids. Field tests conducted independently at multiple sites showed that soybean aphid populations peaked in late summer on lines with Rag1 or Rag2 and reached economically injurious levels on susceptible lines, whereas lines with a pyramid of Rag1 + Rag2 held soybean aphid populations below economic levels. In the regional test, aphid populations were generally suppressed by lines containing one of the Rag genes. Aphids reached putative economic levels on Rag1 lines for some site years, but yield loss was moderated, indicating that Rag1 may confer tolerance to soybean aphid in addition to antibiosis and antixenosis. Moreover, no yield penalty has been found for lines with Rag1, Rag2, or pyramids. Results suggest that use of aphid‐resistant soybean lines with Rag genes may be viable for managing soybean aphids. However, virulent biotypes of soybean aphid were identified before release of aphid‐resistant soybean, and thus a strategy for optimal deployment of aphid‐resistant soybean is needed to ensure sustainability of this technology.     

Mapping and validation of a gene for soybean aphid resistance in PI 567537

The soybean aphid (Aphis glycines Matsumura) is a major pest on soybean [Glycine max (L.) Merr.] in North America. Aphid resistance has been found on plant introduction (PI) 567537, but its genetic characterization is unknown. The objectives of this study were to identify the resistance genes in PI 567537 using molecular markers and validate them in a different genetic background. A mapping population of 86 F4 lines from a cross between PI 567537 and a susceptible parent E00003 was investigated for aphid resistance in both greenhouse and field trials. A genomic region associated with the aphid resistance in PI 567537 was revealed on chromosome 16 (linkage group J) with molecular markers. This locus was coincidently located in the same region as Rag3 and explained most of the phenotypic variation, ranging from 87.4 % in the greenhouse trial to 78.9 % in the field trial. This resistance gene was further confirmed in an F2 population derived from a cross of PI 567537 × Skylla. The segregation of the F2 population indicated that the aphid resistance in PI 567537 was most likely controlled by a single dominant gene, which was the one we mapped in the F4-derived population. This gene was designated Rag3b since it is located in the same region as Rag3. The mapping of the aphid resistance gene in PI 567537 could be useful in marker-assisted selection when employing PI 567537 as an aphid resistance source.     

Molecular mapping of two genes conferring resistance to Phytophthora sojae in a soybean landrace PI 567139B

Phytophthora root and stem rot (PRR), caused by the soil-borne oomycete pathogen Phytophthora sojae, is one of the most destructive diseases of soybean. PRR can be effectively controlled by race-specific genes conferring resistance to P. sojae (Rps). However, the Rps genes are usually non-durable, as populations of P. sojae are highly diverse and quick to adapt, and can be overcome 8–15 years after deployment. Thus, it is important to identify novel Rps genes for development of resistant soybean cultivars. PI 567139B is a soybean landrace carrying excellent resistance to nearly all predominant P. sojae races in Indiana. A mapping population consisting of 245 F₂ individuals and 403 F_2:3 families was developed from a cross between PI 567139B and the susceptible cultivar ‘Williams’, and used to dissect the resistance carried by PI 567139B. We found that the resistance in PI 567139B was conferred by two independent Rps genes, designated RpsUN1 and RpsUN2. The former was mapped to a 6.5 cM region between SSR markers Satt159 and BARCSOYSSR_03_0250 that spans the Rps1 locus on chromosome 3, while the latter was mapped to a 3.0 cM region between BARCSOYSSR_16_1275 and Sat_144, approximately 3.0–3.4 cM upstream of Rps2 on chromosome 16. According to the ‘Williams 82’ reference genome sequence, both regions are highly enriched with NBS-LRR genes. Marker assisted resistance spectrum analyses of these genes with 16 isolates of P. sojae, in combination with the mapping results, suggested that RpsUN1 was likely to be a novel allele at the Rps1 locus, while RpsUN2 was more likely to be a novel Rps gene.     

Characterization and genetics of multiple soybean aphid biotype resistance in five soybean plant introductions

Key message

Five soybean plant introductions expressed antibiosis resistance to multiple soybean aphid biotypes. Two introductions had resistance genes located in the Rag1, Rag2, and Rag3 regions; one introduction had resistance genes located in the Rag1, Rag2, and rag4 regions; one introduction had resistance genes located in the Rag1 and Rag2 regions; and one introduction had a resistance gene located in the Rag2 region.

Abstract

Soybean aphid (Aphis glycines Matsumura) is the most important soybean [Glycine max (L.) Merr.] insect pest in the USA. The objectives of this study were to characterize the resistance expressed in five plant introductions (PIs) to four soybean aphid biotypes, determine the mode of resistance inheritance, and identify markers associated with genes controlling resistance in these accessions. Five soybean PIs, from an initial set of 3000 PIs, were tested for resistance against soybean aphid biotypes 1, 2, 3, and 4 in choice and no-choice tests. Of these five PIs, PI 587663, PI 587677, and PI 587685 expressed antibiosis against all four biotypes, while PI 587972 and PI 594592 expressed antibiosis against biotypes 1, 2, and 3. F₂ populations derived from PI 587663 and PI 587972 were evaluated for resistance against soybean aphid biotype 1, and populations derived from PIs 587677, 587685, and 594592 were tested against biotype 3. In addition, F_2:3 plants were tested against biotypes 2 and 3. Genomic DNA from F₂ plants was screened with markers linked to Rag1, Rag2, Rag3, and rag4 soybean aphid-resistance genes. Results showed that PI 587663 and PI 594592 each had three genes with variable gene action located in the Rag1, Rag2, and Rag3 regions. PI 587677 had three genes with variable gene action located in the Rag1, Rag2 and rag4 regions. PI 587685 had one dominant gene located in the Rag1 region and an additive gene in the Rag2 region. PI 587972 had one dominant gene located in the Rag2 region controlling antixenosis- or antibiosis-type resistance to soybean aphid biotypes 1, 2, or 3. PIs 587663, 587677, and 587685 also showed antibiosis-type resistance against biotype 4. Information on multi-biotype aphid resistance and resistance gene markers will be useful for improving soybean aphid resistance in commercial soybean cultivars.

     

Soybean aphid intrabiotype variability based on colonization of specific soybean genotypes

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is one of the most destructive insect pests on soybeans in the United States. One method for managing this pest is through host plant resistance. Since its arrival in 2000, 4 aphid biotypes have been identified that are able to overcome soybean aphid resistance (Rag) genes. A soybean aphid isolate collected from Moline, Illinois readily colonized soybean plants with the soybean aphid resistance gene Rag2, unlike biotypes 1 and 2, but similar to soybean aphid biotype 3. Two no‐choice experiments compared the virulence of the Moline isolate with biotype 3. In both experiments, differences in aphid population counts were not significant (P > 0.05) on soybean genotypes LD08–12957a (Rag2) and LD11–5413a (Rag2), but the aphid counts for the Moline isolate were significantly (P < 0.05) lower than the aphid counts for the biotype 3 isolate on the soybean genotypes Dowling (Rag1), LD05–16611 (Rag1), LD11–4576a (Rag1), and PI 567598B (rag1b and rag3). The Moline isolate was a variant of aphid biotype 3, which is the first report showing that soybean aphid isolates classified as the same biotype, based on virulence against specific Rag genes, can differ in aggressiveness or ability to colonize specific host genotypes.     

Two simple sequence repeat markers to select for soybean cyst nematode resistance coditioned by the rhg1 locus

The soybean cyst nematode (SCN) (Heterodera glycines Inchinoe) is the most economically significant soybean pest. The principal strategy to reduce or eliminate damage from this pest is the use of resistant cultivars. Identifying resistant segregants in a breeding program is a difficult and expensive process which is complicated by the oligogenic nature of the resistance and genetic variability in the pathogen. Fortunately, resistance at one SCN-resistance locus, rhg1, is generally accepted as a necessity for the development of resistant genotypes using any source of resistance and when challenged by any SCN race. Thus, the development of SCN resistant cultivars would be expedited if an effective and rapid system were available to identify breeding lines carrying a resistance allele at the rhg1 locus. In this study we report two simple sequence repeat (SSR) or microsatellite loci that cosegregate and map 0.4 cM from rhg1. Allelic variation at the first of these loci, BARC-Satt309, distinguished most, if not all, SCN-susceptible genotypes from those carrying resistance at rhg1 derived from the important SCN-resistance sources ’Peking’, PI 437654, and PI 90763. BARC-Satt309 was also effective in distinguishing SCN resistance sources PI 88788 and PI 209332 from many, but not all, susceptible genotypes. BARC-Satt309 cannot be used in marker-assisted selection in populations developed from typical southern US cultivars crossed with the important resistance sources PI 88788 or PI 209332 because these genotypes all carry the identical allele at the BARC-Satt309 locus. A second SSR locus, BARC-Sat_168, was developed from a bacterial artificial chromosome (BAC) clone that was identified using the primers to BARC-Satt309. BARC-Sat_168 distinguished PI 88788 and PI 209332 from southern US cultivars such as ’Lee’, ’Bragg’ and ’Essex’. Both BARC-Satt309 and BARC-Sat_168 were used to assay lines from SCN-susceptible×SCN-resistant crosses and proved to be highly effective in identifying lines carrying rhg1 resistance from those carrying the allele for SCN susceptibility at the rhg1 locus. Received: 5 November 1998 / Accepted: 3 February 1999     

Pyramiding different aphid-resistance genes in elite soybean germplasm to combat dynamic aphid populations

The soybean aphid (Aphis glycines Matsumura), an invasive species, has posed a significant threat to soybean [Glycine max (L.) Merr.] production in North America since 2001. Use of resistant cultivars is an effective tactic to protect soybean yield. However, the variability and dynamics of aphid populations could limit the effectiveness of host-resistance gene(s). Gene pyramiding is a promising way to sustain host-plant resistance. The objectives of this study were to determine the prevalent aphid biotypes in Michigan and to assess the effectiveness of different combinations of aphid-resistance genes. A total of 11 soybean genotypes with known resistance gene(s) were used as indicator lines. Based on their responses, Biotype 3 was a major component of Michigan aphid populations during 2015–2016. The different performance of Rag-“Jackson” and Rag1-“Dowling” along with the breakdown of resistance in plant introductions (PIs) 567301B and 567324 may be explained by Biotype 3 or an unknown virulent biotype establishing in Michigan. With the assistance of flanking markers, 12 advanced breeding lines carrying different aphid-resistance gene(s) were developed and evaluated for effectiveness in five trials across 2015 to 2017. Lines with rag1c, Rag3d, Rag6, Rag3c?+?Rag6, rag1b?+?rag3, rag1c?+?rag4, rag1c?+?rag3?+?rag4, rag1c?+?Rag2?+?rag3?+?rag4, and rag1b?+?rag1c?+?rag3?+?rag4 demonstrated strong and consistent resistance. Due to the variability of virulent aphid populations, different combinations of Rag genes may perform differently across geographies. However, advanced breeding lines pyramided with three or four Rag genes likely will provide broader and more durable resistance to diverse and dynamic aphid populations.     

Genetic analysis of shoot fresh weight in a cross of wild (<Emphasis Type="Italic">G. soja</Emphasis>) and cultivated (<Emphasis Type="Italic">G. max</Emphasis>) soybean

Shoot fresh weight (SFW) is one of the parameters, used to estimate the total plant biomass yield in soybean. In the present study, a total of 188 F_5:8 recombinant inbred lines (RIL) derived from an interspecific cross of PI 483463 (Glycine soja) and Hutcheson (Glycine max) were investigated for SFW variation in the field for three consecutive years. The parental lines and RILs were phenotyped in the field at the R6 stage by measuring total biomass in kg/plot to identify the QTLs for SFW. Three QTLs qSFW6_1, qSFW15_1, and qSFW19_1 influencing SFW were identified on chromosome 6, 15, and 19, respectively. The QTL qSFW19_1 flanked between the markers BARC-044913-08839 and BARC-029975-06765 was the stable QTL expressed in all the three environments. The phenotypic variation explained by the QTLs across all environments ranged from 6.56 to 21.32 %. The additive effects indicated contribution of alleles from both the parents and additive × environment interaction effects affected the expression of SFW QTL. Screening of the RIL population with additional SSRs from the qSFW19_1 region delimited the QTL between the markers SSR19-1329 and BARC-29975-06765. QTL mapping using bin map detected two QTLs, qSFW19_1A and qSFW19_1B. The QTL qSFW19_1A mapped close to the Dt1 gene locus, which affects stem termination, plant height, and floral initiation in soybean. Potential candidate genes for SFW were pinpointed, and sequence variations within their sequences were detected using high-quality whole-genome resequencing data. The findings in this study could be useful for understanding genetic basis of SFW in soybean.     

Identification and molecular mapping of two soybean aphid resistance genes in soybean PI 587732

Soybean [Glycine max (L.) Merr.] continues to be plagued by the soybean aphid (Aphis glycines Matsumura: SA) in North America. New soybean resistance sources are needed to combat the four identified SA biotypes. The objectives of this study were to determine the inheritance of SA resistance in PI 587732 and to map resistance gene(s). For this study, 323 F₂ and 214 F₃ plants developed from crossing PI 587732 to two susceptible genotypes were challenged with three SA biotypes and evaluated with genetic markers. Choice tests showed that resistance to SA Biotype 1 in the first F₂ population was controlled by a gene in the Rag1 region on chromosome 7, while resistance to SA Biotype 2 in the second population was controlled by a gene in the Rag2 region on chromosome 13. When 134 F₃ plants segregating in both the Rag1 and Rag2 regions were tested with a 1:1 mixture of SA Biotypes 1 and 2, the Rag2 region and an interaction between the Rag1 and Rag2 regions were significantly associated with the resistance. Based on the results of the non-choice tests, the resistance gene in the Rag1 region in PI 587732 may be a different allele or gene from Rag1 from Dowling because the PI 587732 gene showed antibiosis type resistance to SA Biotype 2 while Rag1 from Dowling did not. The two SA resistance loci and genetic marker information from this study will be useful in increasing diversity of SA resistance sources and marker-assisted selection for soybean breeding programs.