Segregation Of Resistance Research Articles

Identification of major consensus and epistatic QTLs for late leaf spot resistance in a cultivated groundnut breeding line VG 9514

A late leaf spot resistant breeding line VG 9514 was bred through interspecific hybridization between Arachis hypogaea L. and Arachis cardenasii. Genetic study involving segregation for late leaf spot resistance in 164 F2 and recombinant inbred line populations derived from a cross between VG 9514 X TAG 24 revealed duplicate recessive resistance for the disease. Integration of newly developed SSR markers from A02 chromosome with existing linkage map generated a new genetic linkage map with 278 markers and 2679.1 cM map distances. QTL mapping involving this genetic linkage map and phenotypic field score of late leaf spot identified two major consensus additive QTLs in the A02 chromosome of cultivated groundnut. Epistatic interaction between these two major QTLs was also noticed through an epistatic QTL analysis in Ici-Mapping 4.1. In all the fourteen epistatic QTLs, a common component locus was remained within the major additive QTL at 90 cM in chromosome A02. Sequence analysis within the major additive QTL peaks revealed open reading frame of genes that code LRR domain containing proteins which are involved in disease resistance of crop plants.

Cryptic introgressions contribute to transgressive segregation for early blight resistance in tomato.

We identified cryptic early blight resistance introgressions in tomato breeding lines and demonstrated efficient genotypic selection for resistance in the context of a tomato breeding program. Early blight is a widespread and problematic disease affecting tomatoes (Solanum lycopersicum). Caused by the fungal pathogen Alternaria linariae (syn. A. tomatophila), symptoms include lesions on tomato stems, fruit, and foliage, often resulting in yield losses. Breeding tomatoes with genetic resistance would enhance production sustainability. Using cross-market breeding populations, we identified several quantitative trait loci (QTL) associated with early blight resistance. Early blight resistance putatively derived from 'Campbell 1943' was confirmed in modern fresh market tomato breeding lines. This resistance offered substantial protection against early blight stem lesions (collar rot) and moderate protection from defoliation. A distinctive and potentially novel form of early blight foliar resistance was discovered in a processing tomato breeding line and is probably derived from S. pimpinellifolium via 'Hawaii 7998'. Additional field trials validated the three most promising large-effect QTL, EB-1.2, EB-5, and EB-9. Resistance effects for EB-5 and EB-9 were consistent across breeding populations and environments, while EB-1.2's effect was population specific. Using genome-wide marker-assisted backcrossing, we developed fresh market tomato lines that were near-isogenic for early blight QTL. Resistance in these lines was largely mediated by just two QTL, EB-5 and EB-9, that together captured 49.0 and 68.7% of the defoliation and stem lesion variance, respectively. Our work showcases the value of mining cryptic introgressions in tomato lines, and across market classes, for use as additional sources of disease resistance.

Identification of QTLs for Spot Blotch Resistance in Two Bi-Parental Mapping Populations of Wheat.

Spot blotch (SB) disease caused by the hemibiotrophic pathogen Bipolaris sorokiniana inflicting major losses to the wheat grown in warm and highly humid areas of the Indian subcontinent, including Bangladesh, necessitates identification of QTLs stably expressing in Indian subcontinent conditions. Thus, two RIL mapping populations, i.e., WC (WUYA × CIANO T79) and KC (KATH × CIANO T79), were phenotyped at Dinajpur, Bangladesh for three consecutive years (2013-2015) and genotyped on a DArTseq genotyping by sequencing (GBS) platform at CIMMYT, Mexico. In both populations, quantitative inheritance along with transgressive segregation for SB resistance was identified. The identified QTLs were mostly minor and were detected on 10 chromosomes, i.e., 1A, 1B, 2A, 2B, 2D, 4B, 4D, 5A, 5D, and 7B. The phenotypic variation explained by the identified QTLs ranged from 2.3–15.0%, whereby QTLs on 4B (13.7%) and 5D (15.0%) were the largest in effect. The identified QTLs upon stacking showed an additive effect in lowering the SB score in both populations. The probable presence of newly identified Sb4 and durable resistance gene Lr46 in the identified QTL regions indicates the importance of these genes in breeding for SB resistance in Bangladesh and the whole of South Asia.

Inheritance of Antibiosis Resistance to the Dectes Stem Borer, Dectes texanus, in Soybean PI165673

Dectes texanus larvae girdles the stems of soybean and cultivated sunflowers causing significant yield losses in North America. The soybean Plant Introduction (PI) 165673 exhibits antibiosis resistance to the larval stage. The objectives of this study were: (1) to determine the inheritance of D. texanus resistance in PI165673; (2) evaluate PI165673 antibiosis resistance before 21 d post infestation; (3) evaluate girdling damage in PI16563 at the end of the season. K07-1544/PI165673 F2 and F2:3 populations were tested for resistance to D. texanus in 2011 and 2012, and PI165673 antibiosis resistance and girdling damage were evaluated in 2014. Segregation for resistance to D. texanus and heritability estimates in the F2 and F2:3 populations indicated that resistance was controlled by two genes with dominant and recessive epistasis. Antibiosis evaluations indicated: (1) PI165673 contained lower number of larvae and eggs relative to the number of oviposition punctures at 15 d post infestation; (2) the proportion of first-instar larvae was higher in PI165673 at 15 d post infestation; (3) larvae reach the sixth-instar stage in PI165673. None of the PI165673 plants were girdled at the end of the season. Identification of additional sources of D. texanus resistance is required to impair larval development in the stem.

Inheritance and allelic relationship of foliar blast resistance gene(s) in pearl millet

Foliar blast disease in pearl millet (Pennisetum glaucum (L.) R. Br.) caused by Magnaporthe grisea once considered as a minor disease, has become major threat for pearl millet cultivation during recent decade in almost all pearl millet growing areas. The present study is an attempt to understand the inheritance of foliar blast resistance in pearl millet against the natural inoculum of Magnaporthe grisea and also against artificial inoculum of its devastating strain PMg_DI. The experiment was conducted at ICRISAT, Hyderabad and IARI, New Delhi during 2017-19. F2 and backcross populations derived from a susceptible genotype ICMB 95444 and two resistant lines PPMI-10018 and PPMI-2464 were used. Screening of individual plant in F1s, F2s, BCP1s (susceptible parent × F1) and BCP2s (resistant parent × F1) under field and controlled conditions indicated that the resistance is governed by a single dominant gene. The test of allelism was conducted by crossing the resistant parents. No segregation for resistance was observed in F2 generation when screened against the natural inoculum of Magnaporthe grisea and its strain PMg_DI. This indicates that both the resistant parents possess the same gene.

Heritability and segregation of resistance to brown rot in peach fruits

Heritability and segregation of resistance to brown rot in peach fruits

Adult Plant Leaf Rust Resistance in AC Taber Wheat Maps to Chromosomes 2BS and 3BS.

AC Taber is a hard red spring wheat cultivar that has had long-lasting resistance to the leaf rust fungus Puccinia triticina. The objective of this study was to determine the chromosome location of the leaf rust resistance genes in AC Taber. The leaf rust-susceptible cultivar Thatcher was crossed with AC Taber to develop an F6 recombinant inbred line (RIL) population. The RILs and parents were evaluated for segregation of leaf rust resistance in five field plot tests and in two seedling tests to race BBBDB of P. triticina. A genetic map of the RIL population was developed using 90,000 single nucleotide polymorphism markers with the Illumina Infinium iSelect 90K wheat bead array. Quantitative trait loci (QTLs) with significant effects for lower leaf rust severity in the field plot tests were found on chromosomes 2BS and 3BS. The same QTLs also had significant effects for lower infection type in seedlings to leaf rust race BBBDB. The gene on 2BS was the adult plant resistance gene Lr13, and the gene on 3BS mapped to the same region as the adult plant resistance gene Lr74 and other QTLs for leaf rust resistance. Kompetitive allele-specific PCR assay markers linked to the 2BS and 3BS regions were developed and should be useful for marker-based selection of these genes.

Inheritance of resistance against Peronospora destructor in onion cv. ‘Regia’

Onion downy mildew (DM) caused by Peronospora destructor is a very destructive leaf disease. Genetic resistance is an economic and environmentally friendly way of disease control. In this research, the segregation of DM resistance was analysed in six offspring from crosses between national cultivars and the cv. ‘Regia’, a partial resistance source. F1 plants were self-fertilised, and F1S1 derived progenies were evaluated. The segregations in DM severity were skewed towards susceptibility, with transgressive segregation in five of six progenies. The recessive inheritance could be associated with loss of susceptibility mechanisms. In another experiment, the response of advanced F1S2 lines ‘Regia’ x ‘Pantanoso del Sauce’ were evaluated two times (August and November 2017) in DM disease severity, DM histological quantitative differences, and agronomic traits. F1S2 lines, which have had an earlier selection process, presented an intermediate DM severity between the parents. Disease severity was positively correlated with histological differences in the proportion of infected stomata. ‘Regia’ presented the lowest DM severity and the highest percentage of healthy stomata. Most resistant F1S2 lines did not differ from control cultivars in bulb yield, bulb quality traits and postharvest behaviour, and could be the basis to develop a DM resistant cultivar in comparison to currently grown cultivars

A Two-Locus System with Strong Epistasis Underlies Rapid Parasite-Mediated Evolution of Host Resistance.

Parasites are a major evolutionary force, driving adaptive responses in host populations. Although the link between phenotypic response to parasite-mediated natural selection and the underlying genetic architecture often remains obscure, this link is crucial for understanding the evolution of resistance and predicting associated allele frequency changes in the population. To close this gap, we monitored the response to selection during epidemics of a virulent bacterial pathogen, Pasteuria ramosa, in a natural host population of Daphnia magna. Across two epidemics, we observed a strong increase in the proportion of resistant phenotypes as the epidemics progressed. Field and laboratory experiments confirmed that this increase in resistance was caused by selection from the local parasite. Using a genome-wide association study, we built a genetic model in which two genomic regions with dominance and epistasis control resistance polymorphism in the host. We verified this model by selfing host genotypes with different resistance phenotypes and scoring their F1 for segregation of resistance and associated genetic markers. Such epistatic effects with strong fitness consequences in host–parasite coevolution are believed to be crucial in the Red Queen model for the evolution of genetic recombination.

Distribution and validation of genotypic and phenotypic glyphosate and PPO-inhibitor resistance in Palmer amaranth (Amaranthus palmeri) from southwestern Nebraska – ERRATUM

Failure to control Palmer amaranth with glyphosate and protoporphyrinogen IX oxidase (PPO)-inhibitor herbicides was reported across southwestern Nebraska in 2017. The objectives of this study were to 1) confirm and 2) validate glyphosate and PPO-inhibitor (fomesafen and lactofen) resistance in 51 Palmer amaranth accessions from southwestern Nebraska using genotypic and whole-plant phenotypic assay correlations and cluster analysis, and 3) determine which agronomic practices might be influencing glyphosate resistance in Palmer amaranth accessions in that location. Based on genotypic assay, 88% of 51 accessions contained at least one individual with amplification (>2 copies) of the 5-enolypyruvyl-shikimate-3-phosphate synthase (EPSPS) gene, which confers glyphosate resistance; and/or a mutation in the PPX2 gene, either ΔG210 or R128G, which endows PPO-inhibitor resistance in Palmer amaranth. Cluster analysis and high correlation (0.83) between genotypic and phenotypic assays demonstrated that EPSPS gene amplification is the main glyphosate resistance mechanism in Palmer amaranth accessions from southwestern Nebraska. In contrast, there was poor association between genotypic and phenotypic responses for PPO-inhibitor resistance, which was attributed to segregation for PPO-inhibitor resistance within these accessions and/or the methodology that was adopted herein. Genotypic assays can expedite the process of confirming known glyphosate and PPO-inhibitor resistance mechanisms in Palmer amaranth from southwestern Nebraska and other locations. Phenotypic assays are also a robust method for confirming glyphosate resistance but not necessarily PPO-inhibitor resistance in Palmer amaranth. Moreover, random forest analysis of glyphosate resistance in Palmer amaranth indicated that EPSPS gene amplification, county, and current and previous crops are the main factors influencing glyphosate resistance within that geographic area. Most glyphosate-susceptible Palmer amaranth accessions were found in a few counties in areas with high crop diversity. Results presented here confirm the spread of glyphosate resistance and PPO-inhibitor resistance in Palmer amaranth accessions from southwestern Nebraska and demonstrate that less diverse cropping systems are an important driver of herbicide resistance evolution in Palmer amaranth.Nomenclature: fomesafen; glyphosate; lactofen; Palmer amaranth, Amaranthus palmeri S. Watson

Inheritance of resistance to the newly identified Plasmodiophora brassicae pathotypes in Brassica napus L.

Clubroot disease caused by Plasmodiophora brassicae Woronin is one of the most devastating diseases of canola (Brassica napus L.). Pathotype 3H has been the most prevalent and virulent pathotype in western Canada. Many resistant cultivars have been developed in Canada in recent years. However, large-scale cultivation of these cultivars resulted in the identification of several new virulent pathotypes including 3A and 5X that overcame this resistance. We studied two doubled haploid (DH) populations, developed by crossing a canola line carrying resistance to 3H, 3A, and 5X to two canola lines carrying resistance to 3H, for resistance to these three pathotypes. Furthermore, two clubroot-resistant populations derived from the first and second cycles of breeding were also investigated for resistance to 10 pathotypes to understand their inheritance. A 3R:1S segregation for resistance to 3H confirmed the involvement of two independent major gene loci in these DH populations. In contrast, a 1R:7S and 1R:15S segregation, respectively, for resistance to 3A and 5X indicated that more than two gene loci are involved in the control of resistance to these pathotypes. A significant positive correlation for resistance to 3A and 5X indicated that some common genes are involved in the control of resistance to both pathotypes. About 4% of the DH lines were resistant to all three pathotypes, suggesting that pyramiding of multiple resistance genes in canola is possible. Overall, the resistance observed in the population derived from the first cycle of breeding remained effective in the population derived from the second cycle of breeding.

Genetic mapping of Fusarium wilt resistance in a wild banana Musa acuminata ssp. malaccensis accession

Banana is an important fruit and food crop, but is threatened by Fusarium wilt, one of the most devastating soil-borne fungal diseases. Only host resistance facilitates banana cultivation in infested soils around the world, but the genetic basis of Fusarium wilt of banana (FWB) is unknown. We selfed a heterozygous wild banana accession Musa acuminata ssp. malaccensis (Mam, AA, 2n = 22) to generate a mapping population and to investigate the inheritance of resistance to Race 1 and tropical race 4 (TR4) that cause FWB. Phenotyping (N = 217) revealed segregation for resistance, and genotyping by sequencing resulted in 2802 high-quality single-nucleotide polymorphic markers (SNPs) that were used for genetic mapping. Combined analyses of these data showed that a single dominant resistance locus controls resistance to Race 1 and maps near the distal part of chromosome 10. Recombinants, together with the position of the putative resistance gene, were further analysed using graphical genotyping, which retrieved markers flanking a 360 kb genetic region that associates with Race 1 resistance. The region contains 165 putative genes on the reference genome, including 19 leucine-rich repeat receptor-like kinase-like genes. At the same position and phase, we also identified a QTL for TR4 resistance, showing that the locus for resistance against Race 1 provided partial resistance to TR4. However, this effect was far less significant and hence not included in the mapping. These data support the breeding of new banana varieties with resistance to Fusarium wilt.

Inheritance and metabolomics of the resistance of two F2 populations of Phaseolus spp. to Acanthoscelides obtectus

The bean weevil Acanthoscelides obtectus is one of the major storage pests of dry bean. Tepary bean (Phaseolus acutifolius) is a great source of resistance against A. obtectus to improve dry bean production. We developed two F2 populations of Phaseolus spp. by crossing bean genotypes resistant to A. obtectus × susceptible, to (1) search for metabolites related to bean weevil resistance using an untargeted metabolomics approach and (2) understand the genetics of resistance to A. obtectus attack using the number of adults emerged from seeds as resistance criteria. The segregation of the number of adults emerged from seeds showed that the genetics of resistance (0 emerged adults = resistant, 1 or more emerged adults = susceptible) was controlled by a single dominant gene in two (resistant × susceptible) F2 populations. From nine metabolites detected, pipecolic acid concentration was significantly higher in the testa of resistant individuals than in the testa of susceptible individuals. From a breeding point of view, (P. Saltillo × T. amarillo) F2 is of special interest because it is derived from a cultivated tepary × cultivated common bean cross and showed 3:1 resistant vs susceptible segregation. Therefore, this F2 is a suitable base material for breeding programmes to obtain new bean lines with good agronomic performance, seed quality and resistance to insect attack. Pipecolic acid plays a role in resistance against A. obtectus.

Evaluation of resistance to stemphylium blight in interspecific recombinant inbred lines derived from Lens culinaris × Lens ervoides

AbstractCultivated lentil suffers yield loss from stemphylium blight, caused by Stemphylium botryosum Wallr. Identification of sources of stemphylium blight resistance and knowledge of the mode of inheritance of resistance are important for developing resistant cultivars. The interspecific recombinant inbred line (RIL) population developed from a cross between the moderately resistant parent Lens culinaris cv. ‘Eston’ and the resistant parent L. ervoides (Brign.) Grande accession IG 72815 was evaluated for stemphylium blight resistance under controlled conditions at the University of Saskatchewan, Saskatoon, Canada, and under field conditions at the Pulses Research Centre (PRC), Ishurdi, Bangladesh. We hypothesized that resistance from both parents will lead to transgressive segregation indicative of pyramiding of resistance genes from the same. However, no resistant transgressive segregants were observed in the RIL population. A large proportion (50%) of the RILs had disease severity levels similar to the resistant parent IG 72815 in experiments conducted under natural disease pressure in Bangladesh. Under controlled conditions in Saskatoon, 38% of RILs had resistance levels similar to IG 72815. Across all environments, 14 RILs consistently had resistance levels similar to IG 72815. The distribution of disease severity scores for all RILs indicated polygenic inheritance of stemphylium blight resistance in the population. RILs with consistent resistant reactions should prove useful for lentil improvement programmes. This will contribute to increasing the productivity of lentil crops in North America and the Indo-Gangetic region, which account for more than 68% of world lentil production.

Two Clubroot-Resistance Genes, Rcr3 and Rcr9wa, Mapped in Brassica rapa Using Bulk Segregant RNA Sequencing.

Genetic resistance is widely used to manage clubroot (Plasmodiophora brassicae) in brassica crops, but new pathotypes have recently been identified on canola (Brassica napus) on the Canadian prairies. Resistance effective against both the most prevalent pathotype (3H, based on the Canadian Clubroot Differential system) and the new pathotypes is needed. BC1 plants of Brassica rapa from a cross of line 96-6990-2 (clubroot resistance originating from turnip cultivar ‘Waaslander’) and a susceptible doubled-haploid line, ACDC, exhibited a 1:1 segregation for resistance against pathotypes 3H and 5X. A resistance gene designated as Rcr3 was mapped initially based on the percentage of polymorphic variants using bulked segregant RNA sequencing (BSR-Seq) and further mapped using Kompetitive Allele Specific PCR. DNA variants were identified by assembling short reads against a reference genome of B. rapa. Rcr3 was mapped into chromosome A08. It was flanked by single nucleotide polymorphisms (SNP) markers (A90_A08_SNP_M12 and M16) between 10.00 and 10.23 Mb, in an interval of 231.6 Kb. There were 32 genes in the Rcr3 interval. Three genes (Bra020951, Bra020974, and Bra020979) were annotated with disease resistance mechanisms, which are potential candidates for Rcr3. Another resistance gene, designated as Rcr9wa, for resistance to pathotype 5X was mapped, with the flanking markers (A90_A08_SNP_M28 and M79) between 10.85 and 11.17 Mb using the SNP sites identified through BSR-Seq for Rcr3. There were 44 genes in the Rcr9wa interval, three of which (Bra020827, Bra020828, Bra020814) were annotated as immune-system-process related genes, which are potential candidates for Rcr9wa.

Distribution and validation of genotypic and phenotypic glyphosate and PPO-inhibitor resistance in Palmer amaranth (Amaranthus palmeri) from southwestern Nebraska

AbstractFailure to control Palmer amaranth with glyphosate and protoporphyrinogen IX oxidase (PPO)-inhibitor herbicides was reported across southwestern Nebraska in 2017. The objectives of this study were to 1) confirm and 2) validate glyphosate and PPO-inhibitor (fomesafen and lactofen) resistance in 51 Palmer amaranth accessions from southwestern Nebraska using genotypic and whole-plant phenotypic assay correlations and cluster analysis, and 3) determine which agronomic practices might be influencing glyphosate resistance in Palmer amaranth accessions in that location. Based on genotypic assay, 88% of 51 accessions contained at least one individual with amplification (>2 copies) of the 5-enolypyruvyl-shikimate-3-phosphate synthase (EPSPS) gene, which confers glyphosate resistance; and/or a mutation in the PPX2 gene, either ΔG210 or R128G, which endows PPO-inhibitor resistance in Palmer amaranth. Cluster analysis and high correlation (0.83) between genotypic and phenotypic assays demonstrated that EPSPS gene amplification is the main glyphosate resistance mechanism in Palmer amaranth accessions from southwestern Nebraska. In contrast, there was poor association between genotypic and phenotypic responses for PPO-inhibitor resistance, which was attributed to segregation for PPO-inhibitor resistance within these accessions and/or the methodology that was adopted herein. Genotypic assays can expedite the process of confirming known glyphosate and PPO-inhibitor resistance mechanisms in Palmer amaranth from southwestern Nebraska and other locations. Phenotypic assays are also a robust method for confirming glyphosate resistance but not necessarily PPO-inhibitor resistance in Palmer amaranth. Moreover, random forest analysis of glyphosate resistance in Palmer amaranth indicated that EPSPS gene amplification, county, and current and previous crops are the main factors influencing glyphosate resistance within that geographic area. Most glyphosate-susceptible Palmer amaranth accessions were found in a few counties in areas with high crop diversity. Results presented here confirm the spread of glyphosate resistance and PPO-inhibitor resistance in Palmer amaranth accessions from southwestern Nebraska and demonstrate that less diverse cropping systems are an important driver of herbicide resistance evolution in Palmer amaranth.

Quantitative Trait Locus Mapping of Clubroot Resistance and Plasmodiophora brassicae Pathotype Banglim-Specific Marker Development in Brassica rapa.

Clubroot resistance is an economically important trait in Brassicaceae crops. Although many quantitative trait loci (QTLs) for clubroot resistance have been identified in Brassica, disease-related damage continues to occur owing to differences in host variety and constant pathogen variation. Here, we investigated the inheritance of clubroot resistance in a double haploid population developed by crossing clubroot resistant and susceptible lines “09CR500” and “09CR501”, respectively. The resistance of “09CR500” to Plasmodiophora brassicae pathotype “Banglim” was controlled as a single dominant gene, with the segregation of resistance and susceptibility being nearly 1:1. PbBrA08Banglim was identified as having a logarithm of odds value of 7.9–74.8, and a phenotypic variance of 26.0–97.1% with flanking marker “09CR.11390652” in A08. After aligning QTL regions to the B. rapa reference genome, 11 genes were selected as candidates. PbBrA08Banglim was located near Crr1, CRs, and Rcr9 loci, but differences were validated by marker analysis, gene structural variations, and gene expression levels, as well as phenotypic responses to the pathotype. Genotyping using the “09CR.11390652” marker accurately distinguished the Banglim-resistance phenotypes in the double haploid population. Thus, the developed marker will be useful in Brassica breeding programs, marker-assisted selection, and gene pyramiding to identify and develop resistant cultivars.

Fast-track breeding system to introduce CTV resistance of trifoliate orange into citrus germplasm, by integrating early flowering transgenic plants with marker-assisted selection

BackgroundGlobal warming will expand the range of new and invasive pathogens in orchards, and subsequently increase the risk of disease epidemics and economic losses. The development of new resistant plant varieties can help to reduce the impact of pathogens, however, the breeding speed can be extremely slow, due to the growth rates of the plants, and the availability of resistance genes. Citrus trees are suffering immense damage from serious diseases such as citrus canker (XCC), huanglongbing (HLB), and citrus tristeza virus (CTV). A fast-track breeding system, that aimed at shortening the duration for disease resistance breeding by incorporating the resistance genes from related species to commercial varieties, has been developed using the integration of precocious transgenic trifoliate orange with the overexpression of CiFT and MAS. It was applied here to incorporate CTV resistance of trifoliate orange into citrus germplasm.ResultsOne generation of backcrossed breeding, that would normally take at least 5 years, was achieved in a single year by fast-track breeding system. Linkage analysis using the corresponding DNA markers revealed that CTV resistance and T-DNA integrated regions were found in different linkage groups, and they were independently segregated in the BC progenies. The CTV resistant null segregants, in which the T-DNA integrated region was removed from their genome, were feasibly obtained by MAS in each generation of the BC progenies, and their CTV resistance was confirmed by immunological analysis. Several BC3 null segregants, whose genetic backgrounds had been substituted into citrus germplasm, except for the haplotype block of CTV resistance, were successfully obtained. CGH and NGS analyses revealed that the T-DNA integrated region was safely segregated out in null segregants.ConclusionFast-track breeding systems are expected to shorten the required breeding time by more than one-fifth in comparison with conventional cross breeding techniques. Using this system, we obtained BC3–8, whose genetic background was successfully substituted except for the CTV resistance locus, and could be a novel mandarin breeding material. The fast-track breeding system will be useful to introduce important traits from related species to citrus germplasm while also drastically reducing the time required for breeding.

QTL-seq for identification of loci associated with resistance to Phytophthora crown rot in squash

Phytophthora capsici Leonian, the causal agent of foliar blight, root rot, fruit rot and crown rot syndromes in squash (Cucurbita moschata), is a devastating pathogen worldwide. Resistance to Phytophthora crown rot in University of Florida breeding line #394-1-27-12 (C. moschata) is conferred by three independent dominant genes (R1R2R3). Availability of DNA markers linked to R1R2R3 genes would allow efficient breeding for Phytophthora crown rot resistance through marker-assisted selection (MAS). The goal of the current study was to identify quantitative trait loci (QTLs) associated with resistance to Phytophthora crown rot in an F2 population (n = 168) derived from a cross between #394-1-27-12 (R) and Butter Bush (S) using QTL-seq bulk segregant analysis. Whole-genome resequencing of the resistant (n = 20) and susceptible (n = 20) bulk segregants revealed ~900,000 single nucleotide polymorphisms distributed across C. moschata genome. Three QTLs significantly (P < 0.05) associated with resistance to Phytophthora crown rot were detected on chromosome 4 (QtlPC-C04), 11 (QtlPC-C11) and 14 (QtlPC-C14). Several markers linked to these QTLs are potential targets for MAS against Phytophthora crown rot in C. moschata. The present study reports the first QTLs associated with Phytophthora crown rot resistance in C. moschata.

Experimental Study on the Workability and Stability of Steel Slag Self-Compacting Concrete

There is important application value and economic value in exploring the potential use of steel slag to prepare self-compacting concrete (SCC) and make full use of solid waste resources. In this paper, steel slag self-compacting concrete (SSCC) with relatively ideal workability is prepared by using steel slag instead of natural fine aggregate based on mix proportion optimization and SSCC performance research. The filling ability, passing ability and resistance segregation were tested to evaluate the workability of SSCC. The results show that when the content of steel slag sand is 20%, the workability performance of SSCC is similar to that of SCC with natural aggregates. When the content of steel slag sand is less than 60%, the performance of SSCC can also meet the workability requirements after adjusting the amount of raw materials.