A transgressive segregation factor (RKN2) in Gossypium barbadense for nematode resistance clusters with gene rkn1 in G. hirsutum

Host plant resistance is an important strategy for managing root-knot nematode (Meloidogyne incognita) in cotton (Gossypium L.). Here we report evidence for enhanced resistance in interspecific crosses resulting from transgressive segregation of clustered gene loci. Recently, a major gene, rkn1, on chromosome 11 for resistance to M. incognita in cv. Acala NemX was identified using an intraspecific G. hirsutum cross with susceptible cv. Acala SJ-2. Using interspecific crosses of Acala NemX × susceptible G. barbadense cv. Pima S-7, F₁, F₂, F_2:3, backcross, and testcross Acala NemX × F₁ (Pima S-7 × SJ-2), parental entries and populations were inoculated in greenhouse tests with M. incognita. Genetic analyses based on nematode-induced root galling and nematode egg production on roots, and molecular marker analysis of the segregating interspecific populations revealed that gene rkn1 interacted with a gene (designated as RKN2) in susceptible Pima S-7 to produce a highly resistant phenotype. RKN2 did not confer resistance in Pima S-7, but when combined with rkn1 (genotype Aa or aa), high levels of resistance were produced in the F₁ and segregating F₂, F₃, and BC₁F₁ populations. One SSR marker MUCS088 was identified tightly linked to RKN2 within 4.4 cM in a NemX × F₁ (Pima S-7 × SJ-2) testcross population. Using mapped SSR markers and interspecific segregating populations, MUCS088 linked to the transgressive gene from the susceptible parent and was located in the vicinity of rkn1 on chromosome 11. Diverse genome analyses among A and D genome diploid and tetraploid cottons revealed that marker MUCS088 (165 and 167 bp) is derived from G. arboreum, A₂ diploid genome. These results demonstrated that a highly susceptible parent contributed to nematode resistance via transgressive segregation. Derived highly resistant lines can be used as improved resistance sources in cotton breeding, and MUCS088 can be used to monitor RKN2 introgression in diverse populations. The close genomic location of the transgressive resistance determinants provides an important model system for studying transgressive segregation and epistasis in plants.     

Phenotypic Expression of rkn1-Mediated Meloidogyne incognita Resistance in Gossypium hirsutum Populations

The root-knot nematode Meloidogyne incognita is a damaging pest of cotton (Gossypium hirsutum) worldwide. A major gene (rkn1) conferring resistance to M. incognita was previously identified on linkage group A03 in G. hirsutum cv. Acala NemX. To determine the patterns of segregation and phenotypic expression of rkn1, F₁, F₂, F_2:3, BC₁F₁ and F_2:7 recombinant inbred lines (RIL) from intraspecific crosses between Acala NemX and a closely related susceptible cultivar Acala SJ-2 were inoculated in greenhouse tests with M. incognita race 3. The resistance phenotype was determined by the extent of nematode-induced root galling and nematode egg production on roots. Suppression of root galling and egg production was highly correlated among individuals in all tests. Root galling and egg production on heterozygous plants did not differ from the susceptible parent phenotype 125 d or more after inoculation, but were slightly suppressed with shorter screening (60 d), indicating that rkn1 behaved as a recessive gene or an incompletely recessive gene, depending on the screening condition. In the RIL, rkn1 segregated in an expected 1 resistant: 1 susceptible ratio for a major resistance gene. However, within the resistant class, 21 out of 34 RIL were more resistant than the resistant parent Acala NemX, indicating transgressive segregation. These results suggest that rkn1-based resistance in G. hirsutum can be enhanced in progenies of crosses with susceptible genotypes. Allelism tests and molecular genetic analysis are needed to determine the relationship of rkn1 to other M. incognita resistance sources in cotton.     

SSR markers closely associated with genes for resistance to root-knot nematode on chromosomes 11 and 14 of Upland cotton

Molecular markers closely linked to genes that confer a high level of resistance to root-knot nematode (RKN) [Meloidogyne incognita (Kofoid & White) Chitwood] in cotton (Gossypium hirsutum L.) germplasm derived from Auburn 623 RNR would greatly facilitate cotton breeding programs. Our objectives were to identify simple sequence repeat (SSR) markers linked to RKN resistance quantitative trait loci (QTL) and map these markers to specific chromosomes. We developed three recombinant inbred line (RIL) populations by single seed descent from the crosses of RKN-resistant parents M-240 RNR (M240), developed from the Auburn 623 RNR source, moderately resistant Clevewilt 6 (CLW6), one of the parents of Auburn 623 RNR, and susceptible parent Stoneville 213 (ST213). These crosses were CLW6 × ST213, M240 × CLW6, and M240 × ST213. RILs from these populations were grown under greenhouse conditions, inoculated with RKN eggs, scored for root gall index, eggs plant⁻¹, and eggs g⁻¹ root. Plants were also genotyped with SSR markers. Results indicated that a minimum of two major genes were involved in the RKN resistance of M240. One gene was localized to chromosome 11 and linked to the marker CIR 316-201. This CIR 316-201 allele was also present in CLW6 but not in Mexico Wild (MW) (PI593649), both of which are parents of Auburn 623 RNR. A second RKN resistance gene was localized to the short arm of chromosome 14 and was linked to the SSR markers BNL3545-118 and BNL3661-185. These two marker alleles were not present in CLW6 but were present in MW. Our data also suggest that the chromosome 11 resistance QTL primarily affects root galling while the QTL on chromosome 14 mediates reduced RKN egg production. The SSRs identified in this study should be useful to select plants with high levels of RKN resistance in segregating populations derived from Auburn 623 RNR.     

Identification of QTL regions and SSR markers associated with resistance to reniform nematode in <Emphasis Type="Italic">Gossypium barbadense</Emphasis> L. accession GB713

The identification of molecular markers that are closely linked to gene(s) in Gossypium barbadense L. accession GB713 that confer a high level of resistance to reniform nematode (RN), Rotylenchulus reniformis Linford & Oliveira, would be very useful in cotton breeding programs. Our objectives were to determine the inheritance of RN resistance in the accession GB713, to identify SSR markers linked with RN resistance QTLs, and to map these linked markers to specific chromosomes. We grew and scored plants for RN reproduction in the P₁, P₂, F₁, F₂, BC₁P₁, and BC₁P₂ generations from the cross of GB713 × Acala Nem-X. The generation means analysis using the six generations indicated that one or more genes were involved in the RN resistance of GB713. The interspecific F₂ population of 300 plants was genotyped with SSR molecular markers that covered most of the chromosomes of Upland cotton (G. hirsutum L.). Results showed two QTLs on chromosome 21 and one QTL on chromosome 18. One QTL on chromosome 21 was at map position 168.6 (LOD 28.0) flanked by SSR markers, BNL 1551_162 and GH 132_199 at positions 154.2 and 177.3, respectively. A second QTL on chromosome 21 was at map position 182.7 (LOD 24.6) flanked by SSR markers BNL 4011_155 and BNL 3279_106 at positions 180.6 and 184.5, respectively. Our chromosome 21 map had 61 SSR markers covering 219 cM. One QTL with smaller genetic effects was localized to chromosome 18 at map position 39.6 (LOD 4.0) and flanked by SSR markers BNL 1721_178 and BNL 569_131 at positions 27.6 and 42.9, respectively. The two QTLs on chromosome 21 had significant additive and dominance effects, which were about equal for each QTL. The QTL on chromosome 18 showed larger additive than dominance effects. Following the precedent set by the naming of the G. longicalyx Hutchinson & Lee and G. aridum [(Rose & Standley) Skovsted] sources of resistance, we suggest the usage of Ren ^barb1 and Ren ^barb2 to designate these QTLs on chromosome 21 and Ren ^barb3 on chromosome 18.     

A SNP haplotype associated with a gene resistant to Xanthomonas axonopodis pv. malvacearum in upland cotton (Gossypium hirsutum L.)

An F_4:5 population of 285 families with each tracing back to a different F₂ plant, derived from a cotton bacterial blight resistant line ‘DeltaOpal’ and a susceptible line ‘DP388’, was artificially inoculated with bacterial blight race 18 (Xanthomonas axonopodis pv. malvacearum) to assay their resistance or susceptibility to the disease. The segregation in the F_4:5 population indicates that the resistance was conditioned by a single dominant gene designated B _12. Simple sequence repeat (SSR) markers identified as putatively linked to the resistance gene by bulked segregant analysis were confirmed on the entire F_4:5 population. Three SSR markers, CIR246, BNL3545 and BNL3644 on chromosome 14, were found closely linked to B ₁₂ . The association between CIR246 and B ₁₂ was validated among 354 plants of 16 diverse varieties. Based on Monsanto SSR/single nucleotide polymorphism (SNP) consensus map, SNP markers closely linked to CIR246 were used to screen ‘DeltaOpal’ and ‘DP388’ for polymorphism. The polymorphic SNP markers were run on the F_4:5 population and the four SNP markers spanning 3.4 cM were found to flank the resistance gene on chromosome 14. The linkage between B ₁₂ and the 4-SNP marker haplotype was validated using 18 elite cotton lines. This 4-SNP marker haplotype can be used for marker assisted selection for bacterial blight resistance breeding programs or for screening germplasm collections for this locus rapidly.     

Fine mapping QMi-C11 a major QTL controlling root-knot nematodes resistance in Upland cotton

The identification and utilization of a high-level of host plant resistance is the most effective and economical approach to control root-knot nematode (Meloidogyne incognita). In an earlier study, we identified a major quantitative trait locus (QTL) for resistance to root-knot nematode in the M-120 RNR Upland cotton line (Gossypium hirsutum L.) of the Auburn 623 RNR source. The QTL is located in a 12.9-cM interval flanked by the two SSR markers CIR069 and CIR316 on the distal segment of chromosome 11. To construct a fine map around the target region, a bulked segregation analysis was performed using two DNA pools consisting of five individuals, with each being homozygous for the two parental alleles. From a survey of 1,152 AFLP primer combinations, 9 AFLP markers closely linked to the target region were identified. By screening an additional 1,221 F₂ individuals developed from the initial mapping population, the Mi-C11 locus was delimited to a 3.6-cM interval flanked by the SSR marker CIR069 and the AFLP marker E14M27-375. These results further elucidate the genetic fine structure of the Mi-C11 locus and provide the basis for map-based isolation of the nematode resistance gene in M-120 RNR.     

Association of AFLP and SSR markers with agronomic and fibre quality traits in <Emphasis Type="Italic">Gossypium hirsutum</Emphasis> L.

Molecular markers linked to QTL contributing to agronomic and fibre quality traits would be useful for cotton improvement. We have attempted to tag yield and fibre quality traits with AFLP and SSR markers using F₂ and F₃ populations of a cross between two Gossypium hirsutum varieties, PS56-4 and RS2013. Out of 50 AFLP primer combinations and 177 SSR primer pairs tested, 32 AFLP and four SSR primers were chosen for genotyping F₂ individuals. Marker-trait associations were studied for eight agronomic and five fibre quality traits through simple and multiple regression analysis (MRA) using a set of 92 AFLP polymorphic loci and four SSR markers. Simple linear regression analysis (SLRA) identified 23 markers for eight different traits whereas multiple regression analysis identified 30 markers for at least one of the 13 traits. SSR marker BNL 3502 was consistently identified to be associated with fibre strength. While all the markers identified in SLRA were also detected in MRA, as many as 16 of the 30 markers were identified to be associated with respective traits in both F₂ and F₃ generations. The markers explained up to 41 per cent of phenotypic variation for individual traits. A number of markers were found to be associated with multiple traits suggesting clustering of QTLs for fibre quality traits in cotton.     

Identification and molecular mapping of a Fusarium wilt resistant gene in upland cotton

Fusarium wilt (FW) is one of the most economically damaging cotton diseases worldwide, causing yellowing, wilting, defoliation, vascular tissue damage and ultimately death. Identification of molecular markers linked to FW genes is vital to incorporate resistance into elite cotton cultivars. An intraspecific F₂ in Gossypium hirsutum L. was developed by crossing with a highly resistant cultivar Zhongmiansuo 35 (ZMS35) and a susceptible cultivar Junmian 1 to screen simple sequence repeats (SSRs) closely linked to the FW resistance gene. FW was identified in F_2:3 families by evaluating seedling leaf symptoms and vascular tissue damage at plant maturity under natural field infection conditions over 2 years. The results showed that FW resistance segregated in a 3:1 ratio as a simple monogenic trait in F_2:3 families. Molecular mapping identified a FW resistance gene closely linked with the SSR marker JESPR304₋₂₈₀ in chromosome D3(c17). We proposed to name this gene FW ^R . A composite interval mapping method detected four QTLs for FW resistance in Chr.A7(c7), D1(c15), D9(c23) and D3, respectively. Among them, one major QTL (LOD > 20) was tagged near marker JESPR304 within an interval of 0.06–0.2 cM, and explained over 52.5–60.9% of the total phenotypic variance. The data confirmed the existence of a major gene in Chr.D3. This is the first report of molecular mapping of a major gene contributing FW resistance in cotton. The present research therefore provides an opportunity to understand the genetic control of resistance to FW and conduct molecular marker-assisted selection breeding to develop FW resistant cultivars.     

Mapping of SMV resistance gene Rsc-7 by SSR markers in soybean

Soybean mosaic virus (SMV) is one of the most prevalent pathogens that limit soybean production. In this study, segregation ratios of resistant plants to susceptible plants in P₁, P₂, F₁, F₂ populations of Kefeng No. 1 (P₁)×Nannong 1138-2 (P₂) and derived RIL populations, were used to study the inheritance of resistance to the SMV strain SC-7. Populations Kefeng No. 1 and F₁ were found to be completely resistant to this SMV strain while Nannong 1138-2 was susceptible to it. The F₂ and RIL populations segregated to fit a ratio of 3:1 and 1:1for resistant plants to susceptible ones, respectively. These results indicated that a single dominant gene, designated as Rsc-7, controlled resistance to the SMV strain SC-7 in Kefeng No.1. SSR markers were used to analyze the RIL population and MAPMAKER/EXP 3.0b was employed to establish linkage between markers and this resistance gene. Combining the data of SSRs and resistance identification, a soybean genetic map was constructed. This map, covering 2625.9 cM of the genome, converged into 24 linkage groups, consisted of 221 SSR markers and the resistance gene Rsc-7. The Rsc-7 gene was mapped to the molecular linkage group G8-D1b+W. SSR markers Satt266, Satt634, Satt558, Satt157, and Satt698 were found linked to Rsc-7 with distances of 43.7, 18.1, 26.6, 36.4 and 37.9 cM, respectively.     

Identification and genomic location of a reniform nematode (Rotylenchulus reniformis) resistance locus (Renari) introgressed from Gossypium aridum into upland cotton (G. hirsutum)

In this association mapping study, a tri-species hybrid, [Gossypium arboreum × (G. hirsutum × G. aridum)²], was crossed with MD51ne (G. hirsutum) and progeny from the cross were used to identify and map SSR markers associated with reniform nematode (Rotylenchulus reniformis) resistance. Seventy-six progeny (the 50 most resistant and 26 most susceptible) plants were genotyped with 104 markers. Twenty-five markers were associated with a resistance locus that we designated Ren ^ari and two markers, BNL3279_132 and BNL2662_090, mapped within 1 cM of Ren ^ari . Because the SSR fragments associated with resistance were found in G. aridum and the bridging line G 371, G. aridum is the likely source of this resistance. The resistance is simply inherited, possibly controlled by a single dominant gene. The markers identified in this project are a valuable resource to breeders and geneticists in the quest to produce cotton cultivars with a high level of resistance to reniform nematode.     

RFLP genetic linkage maps from four F(2.3) populations and a joinmap of Gossypium hirsutum L

An RFLP genetic linkage joinmap was constructed from four different mapping populations of cotton (Gossypium hirsutum L.). Genetic maps from two of the four populations have been previously reported. The third genetic map was constructed from 199 bulk-sampled plots of an F_2.3 (HQ95–6×’MD51ne’) population. The map comprises 83 loci mapped to 24 linkage groups with an average distance between markers of 10.0 centiMorgan (cM), covering 830.1 cM or approximately 18% of the genome. The fourth genetic map was developed from 155 bulk-sampled plots of an F_2.3 (119– 5 sub-okra×’MD51ne’) population. This map comprises 56 loci mapped to 16 linkage groups with an average distance between markers of 9.3 cM, covering 520.4 cM or approximately 11% of the cotton genome. A core of 104 cDNA probes was shared between populations, yielding 111 RFLP loci. The constructed genetic linkage joinmap from the above four populations comprises 284 loci mapped to 47 linkage groups with the average distance between markers of 5.3 cM, covering 1,502.6 cM or approximately 31% of the total recombinational length of the cotton genome. The linkage groups contained from 2 to 54 loci each and ranged in distance from 1.0 to 142.6 cM. The joinmap provided further knowledge of competitive chromosome arrangement, parental relationships, gene order, and increased the potential to map genes for the improvement of the cotton crop. This is the first genetic linkage joinmap assembled in G. hirsutum with a core of RFLP markers assayed on different genetic backgrounds of cotton populations (Acala, Delta, and Texas plain). Research is ongoing for the identification of quantitative trait loci for agronomic, physiological and fiber quality traits on these maps, and the identification of RFLP loci lineage for G. hirsutum from its diploid progenitors (the A and D genomes). Received: 23 February 2001 / Accepted: 8 June 2001     

Genetic Mapping of the Tph1 Gene Controlling Beta-tocopherol Accumulation in Sunflower Seeds

Tocopherols are a family of fat soluble antioxidants of great value for both nutritional and technological properties of seed oils. The four naturally occurring tocopherols (alpha-, beta-, gamma- and delta-tocopherol) widely differ for their relative in vivo (vitamin E) and in vitro antioxidant properties. Sunflower (Helianthus annuus L.) seeds mainly contain alpha-tocopherol (95% of the total tocopherols), which has a great vitamin E value but a low in vitro activity. Conversely, beta-tocopherol shows more balanced in vitro and in vivo antioxidant properties, which is desired for specific uses of the oil. The sunflower line T589 is characterised by an increased beta-tocopherol content in the seeds ( >30%), which is determined by the single gene Tph1. The objectives of this study were to map the Tph1 gene by molecular markers (SSRs) and to develop a linkage map of the Tph1-encompassing region. High performance liquid chromatography (HPLC) was used to phenotype 103 F₂ and 67 F₃ progeny from the mapping population CAS-12 × T589, which segregates for Tph1. Bulk segregant analysis identified two SSR markers on linkage group (LG) 1 linked to Tph1. A large linkage group was constructed by genotyping additional SSRs and INDEL markers. Tph1 mapped to the upper end of LG 1 and cosegregated with the SSR markers ORS1093, ORS222, and ORS598. The availability of tightly linked PCR-based markers and the location of the Tph1 gene on the sunflower genetic map will be useful for marker-assisted selection in sunflower and provides a basis for the physical mapping and positional cloning of this gene.     

QTL mapping for resistance to root-knot nematodes in the M-120 RNR Upland cotton line (Gossypium hirsutum L.) of the Auburn 623 RNR source

Root-knot nematodes Meloidogyne incognita (Kofoid and White) can cause severe yield loss in cotton (Gossypium hirsutum L.). The objectives of this study were to determine the inheritance and genomic location of genes conferring root-knot nematode resistance in M-120 RNR, a highly resistant G. hirsutum line with the Auburn 623 RNR source of resistance. Utilizing two interspecific F₂ populations developed from the same M-120 RNR by Gossypium barbadense (cv. Pima S-6) cross, genome-wide scanning with RFLP markers revealed a marker on Chromosome 7 and two on Chromosome 11 showing significant association with the resistant phenotype. The association was confirmed using SSR markers with the detection of a minor and a major dominant QTL on Chromosome 7 and 11, respectively. Combined across the two populations, the major QTL on Chromosome 11 Mi-C11 had a LOD score of 19.21 (9.69 and 9.61 for Pop1 and Pop2, respectively) and accounted for 63.7% (52.6 and 65.56% for Pop1 and Pop2, respectively) of the total phenotypic variation. The minor QTL locus on Chromosome 7 Mi ₁ -C07 had a LOD score of 3.48 and accounted for 7.7% of the total phenotypic variation in the combined dataset but was detected in only one population. The allele from the M-120 RNR parent contributed to increased resistance in the Mi-C11 locus, but surprisingly, the Pima S-6 allele contributed to increased resistance in the Mi-C07 locus. The M-120 RNR allele in the Mi-C11 locus, derived from the Auburn 623 RNR, is likely to have originated from the Clevewilt 6 cultivar. Results from this study indicated that the SSR marker CIR316 may replace the laborious greenhouse screening in breeding programs to identify genotypes resistant to M. incognita.     

<Emphasis Type="Italic">Vr</Emphasis><Subscript><Emphasis Type="Italic">2</Emphasis></Subscript>: a new apple scab resistance gene

Reports from several European countries of the breakdown of the Vf resistance, the most frequently used source of resistance in breeding programs against apple scab, emphasize the urgency of diversifying the basis of apple scab resistance and pyramiding different apple scab resistances with the use of their associated molecular markers. GMAL 2473 is an apple scab resistant selection thought to carry the resistance gene Vr. We report the identification by BSA of three AFLP markers and one RAPD marker associated with the GMAL 2473 resistance gene. SSRs associated with the resistance gene were found by (1) identifying the linkage group carrying the apple scab resistance and (2) testing the SSRs previously mapped in the same region. One such SSR, CH02c02a, mapped on linkage group 2, co-segregates with the resistance gene. GMAL 2473 was tested with molecular markers associated with other apple scab resistance genes, and accessions carrying known apple scab resistance genes were tested with the SSR linked to the resistance gene found in GMAL 2473. The results indicate that GMAL 2473 does not carry Vr, and that a new apple scab resistance gene, named Vr ₂, has been identified.     

Assessing genetic diversity in <Emphasis Type="Italic">Gossypium arboreum</Emphasis> L. cultivars using genomic and EST-derived microsatellites

The cultivated diploid, Gossypium arboreum L., (A genome) is an invaluable genetic resource for improving modern tetraploid cotton (G. hirsutum L. and G. barbadense L.) cultivars. The objective of this research is to select a set of informative and robust microsatellites for studying genetic relationships among accessions of geographically diverse G. arboreum cultivars. From more than 1,500 previously developed simple sequence repeat (SSR) markers, 115 genomic (BNL) and EST-derived (MUCS and MUSS) markers were used to evaluate the allelic diversity of a core panel of G. arboreum accessions. These SSR data enabled advanced genome analyses. A set of 25 SSRs were selected based both upon their high level of informativeness (PIC ≥ 0.50) and the production of clear PCR bands on agarose gels. Subsequently, 96 accessions representing a wide spectrum of diversity of G. arboreum cultivars were analyzed with these markers. The 25 SSR loci revealed 75 allelic variants (polymorphisms) ranging from 2 to 4 alleles per locus. The Neighborjoining (NJ) method, based on genetic dissimilarities, revealed that cultivars from geographically adjacent countries tend to cluster together. Outcomes of this research should be useful in decreasing redundancy of effort and in constructing a core collection of G. arboreum, important for efficient use of this genetic resource in cotton breeding.     

Inheritance and molecular mapping of a downy mildew resistance gene, Pl 13 in cultivated sunflower (Helianthus annuus L.)

The inheritance of resistance to sunflower downy mildew (SDM) derived from HA-R5 conferring resistance to nine races of the pathogen has been determined and the new source has been designated as Pl ₁₃ . The F₂ individuals and F₃ families of the cross HA-R5 (resistant) × HA 821 (susceptible) were screened against the four predominant SDM races 300, 700, 730, and 770 in separate tests which indicated dominant control by a single locus or a cluster of tightly linked genes. Bulked segregant analysis (BSA) was carried out on 116 F₂ individuals with 500 SSR primer pairs that resulted in the identification of 10 SSR markers of linkage groups 1 (9 markers) and 10 (1 marker) of the genetic map (Tang et al. in Theor Appl Genet 105:1124–1136, 2002) that distinguished the bulks. Of these, the SSR marker ORS 1008 of linkage group 10 was tightly linked (0.9 cM) to the Pl ₁₃ gene. Genotyping the F₂ population and linkage analysis with 20 polymorphic primer pairs located on linkage group 10 failed to show linkage of the markers with downy mildew resistance and the ORS 1008 marker. Nevertheless, validation of polymorphic SSR markers of linkage group 1 along with six RFLP-based STS markers of linkage group 12 of the RFLP map of Jan et al. (Theor Appl Genet 96:15–22, 1998) corresponding to linkage group 1 of the SSR map, mapped seven SSR markers (ORS 965-1, ORS 965-2, ORS 959, ORS 371, ORS 716, and ORS 605) including ORS 1008 and one STS marker (STS10D6) to linkage group 1 covering a genetic distance of 65.0 cM. The Pl ₁₃ gene, as a different source with its location on linkage group 1, was flanked by ORS 1008 on one side at a distance of 0.9 cM and ORS 965-1 on another side at a distance of 5.8 cM. These closely linked markers to the Pl ₁₃ gene provide a valuable basis for marker-assisted selection in sunflower breeding programs.     

Inheritance and QTL mapping of Fusarium wilt race 4 resistance in cotton

Diseases such as Fusarium wilt [Fusarium oxysporum f.sp. vasinfectum (FOV) Atk. Sny & Hans] represent expanding threats to cotton production. Integrating disease resistance into high-yielding, high-fiber quality cotton (Gossypium spp.) cultivars is one of the most important objectives in cotton breeding programs worldwide. In this study, we conducted a comprehensive analysis of gene action in cotton governing FOV race 4 resistance by combining conventional inheritance and quantitative trait loci (QTL) mapping with molecular markers. A set of diverse cotton populations was generated from crosses encompassing multiple genetic backgrounds. FOV race 4 resistance was investigated using seven parents and their derived populations: three intraspecific (G. hirsutum × G. hirsutum L. and G. barbadense × G. barbadense L.) F₁ and F₂; five interspecific (G. hirsutum × G. barbadense) F₁ and F₂; and one RIL. Parents and populations were evaluated for disease severity index (DSI) of leaves, and vascular stem and root staining (VRS) in four greenhouse and two field experiments. Initially, a single resistance gene (Fov4) model was observed in F₂ populations based on inheritance of phenotypes. This single Fov4 gene had a major dominant gene action and conferred resistance to FOV race 4 in Pima-S6. The Fov4 gene appears to be located near a genome region on chromosome 14 marked with a QTL Fov4-C14 ₁ , which made the biggest contribution to the FOV race 4 resistance of the generated F₂ progeny. Additional genetic and QTL analyses also identified a set of 11 SSR markers that indicated the involvement of more than one gene and gene interactions across six linkage groups/chromosomes (3, 6, 8, 14, 17, and 25) in the inheritance of FOV race 4 resistance. QTLs detected with minor effects in these populations explained 5–19 % of the DSI or VRS variation. Identified SSR markers for the resistance QTLs with major and minor effects will facilitate for the first time marker-assisted selection for the introgression of FOV race 4 resistance into elite cultivars during the breeding process.     

Genetic linkage maps of Populus nigra L. including AFLPs, SSRs, SNPs, and sex trait

Black poplar (Populus nigra L.) is a tree of ecological and economic interest. A better knowledge of P. nigra genome is needed for an effective protection and use of its genetic resources. The main objective of this study is the construction of a highly informative genetic map of P. nigra species including genes of adaptive and economic interest. Two genotypes originated from contrasted natural Italian populations were crossed to generate a F₁ mapping pedigree of 165 individuals. Amplification fragment length polymorphism (AFLP), simple sequence repeat (SSR), and single nucleotide polymorphism (SNP) markers were used to genotype 92 F₁ individuals, and the pseudo-test-cross strategy was applied for linkage analysis. The female parent map included 368 markers (274 AFLPs, 91 SSRs, and 3 SNPs) and spanned 2,104 cM with 20 linkage groups, and the male parent map, including 317 markers (205 AFLPs, 106 SSRs, 5 SNPs, and sex trait), spanned 2,453 cM with 23 main linkage groups. The sex, as morphological trait, was mapped on the linkage group XIX of the male parent map. The generated maps are among the most informative in SSRs when compared to the Populus maps published so far and allow a complete alignment with the 19 haploid chromosomes of Populus sequence genome. These genetic maps provide informative tools for a better understanding of P. nigra genome structure and genetic improvement of this ecologically and economically important European tree species. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Molecular mapping of a nuclear male-sterility gene in sunflower (Helianthus annuus L.) using TRAP and SSR markers

A nuclear male-sterile mutant, NMS 360, induced by streptomycin from an inbred maintainer line HA 89, possesses a single recessive gene, ms9, controlling male sterility. The present study identified DNA markers linked to the ms9 gene in an F₂ population derived from the cross of NMS 360 × RHA 271 and maps the ms9 gene to an existing sunflower SSR linkage map. Bulked segregant analysis was performed using the target region amplification polymorphism (TRAP) marker technique and the simple sequence repeats (SSR) technique. From 444 primer combinations, six TRAP markers linked with the ms9 gene were amplified. Two markers, Ts4p03-202 and Tt3p09-529, cosegregated with the ms9 gene. The other four markers, To3d14-310, Tt3p17-390, Ts4p23-300, and Tt3p09-531, linked with ms9 at a distance of 1.2, 3.7, 10.3, and 22.3 cM, respectively. Thirty SSR primers from 17 linkage groups of a PHA × PHB cultivated sunflower linkage map were screened among the two parents and the F₂ population. SSR primer ORS 705 of linkage group 10 was tightly linked to ms9 at a distance of 1.2 cM. The ms9 gene was subsequently mapped to linkage group 10 of the public sunflower SSR linkage map. The markers that were tightly linked with the ms9 gene will be useful in marker-assisted selection of male-sterile plants among segregating populations, and will facilitate the isolation of the ms9 gene by map-based cloning.     

Identification of peanut (Arachis hypogaea L.) RAPD markers diagnostic of root-knot nematode (Meloidogyne arenaria (Neal) Chitwood) resistance

DNA markers linked to a root-knot nematode resistance gene derived from wild peanut species have been identified. The wild diploid peanut accessions K9484 (Arachis batizocoi Krapov. & W. C. Gregory), GKP10017, (A. cardenasii Krapov & W. C. Gregory), and GKP10602 (A. diogoi Hoehne) possess genes for ressitance to Meloidogyne arenaria. These three accessions and A. hypogaea cv. Florunner were crossed to generate the hybrid resistant breeding line TxAg-7. This line was used as donor parent to develop a BC₄F₂ population segregating for resistance. Three RAPD markers associated with nematode resistance were identified in this population by bulked segregant analysis. Linkage was confirmed by screening 21 segregatingh BC₄F₂ and 63 BC₅F₂ single plants. Recombination between marker RKN410 and resistance, and between marker RKN440 and resistance, was estimated to be 5.4±1.9% and 5.8±2.1%, respectively, on a per-generation basis. These two markers identified a resistance gene derived from either A. cardenasii or A. diogoi, and were closely linked to each other. Recombination between a third marker, RKN229, inherited from A. cardenasii or A. diogoi, and resistance was 9.0±3.2% per generation. Markers RKN410 and RKN229 appeared to be linked genetically and flank the same resistance gene. All markers were confirmed by hybridization of cloned or gel-purified marker DNA to blots of PCR-amplified DNA. Pooled data on the segregation of BC₅F₂ plants was consistent with the presence of one resistance gene in the advanced breeding lines. Different distributions of resistance in the BC₅F₂ progeny and TxAG-7 suggest the presence of additional resistance genes in TxAG-7.